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pH
By Erich L.Springer
©
2013
7
7
by T&C Technical
1.
.......................................................................................................................... 5
1.1
............................................................................................................. 7
2.
PH
................................................................................................................ 8
3.
PH
......................................................................................................................... 12
3.1
........................................................................................................ 12
3.2 PH
...................................................................................................... 16
3.2.1
........................................................................................................... 16
3.2.1.1
................................................................................................. 18
3.2.1.2
..................................................................................................... 19
3.2.2
........................................................................................................... 21
3.2.3
........................................................................................................... 22
3.2.4
KCl
3.2.5
(
(
)
............................................ 23
)
.......................... 24
3.2.5.1
3.2.6
...................................................................................... 25
pH
............................................................................. 26
3.3 PH
...................................................................................................... 27
3.3.1
...................................................................................... 28
3.3.2
...................................................................................... 29
3.3.3
........................................................................................................ 31
3.3.4
pH
....................................................... 32
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3.3.5
3.4
.................................................................................................... 34
.......................................................................................................................... 36
3.4.1
........................................................................................................... 40
3.4.2
............................................................................... 42
3.4.4.1
(EVEREF).................................................... 42
3.4.4.2
-B
3.5
...................................................... 43
......................................................................................... 44
3.5.1
.................................................................................................... 44
3.5.2
............................................................................................................... 45
3.6
............................................................................................. 46
3.7
........................................................................................................ 48
3.8
.......................................................................................................................... 51
3.8.1
................................................................................................. 51
3.8.2
................................................................................................. 52
3.9
.......................................................................................................................... 54
3.9.1
pH
3.9.2
............................................................................. 54
........................................................................................................ 57
3.9.2.1
............................................................................ 58
3.9.2.2
pH
.......................... 61
3.9.3
............................................................................................................... 62
3.9.4
........................................................................................................... 62
3.9.5
/
..................................................................................... 64
3
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3.10
...................................................................................................... 65
4.
............................................................................................................. 66
4.1
5.
.......................................................................................................................... 66
4.1.1
................................................................................................. 67
4.1.2
................................................................................................. 68
4.1.3
................................................................................................. 68
4.2
................................................................................................. 69
4.3
........................................................................... 71
PH
PH
.................................................................................. 72
5.1
......................................................................................... 72
5.2
......................................................................................... 72
5.3
............................................................................................. 73
5.4
............................................................................................. 73
5.5
...................................................................................... 75
4
Page 3
1.
pH Measurement
utilising
pH Measurement
Glass Electrodes
utilising
Theory and Practice
Glass by
Electrodes
Erich K. Springer
Page 3
by T&C Technical
Theory and Practice
1
1
by Erich K. Springer
INTRODUCTION
pH sour and bitter when eating liquid or
Since the creation of man, he could pH
differentiate between
INTRODUCTION
solid foods. We say that vinegar and lemon juice taste sour and that soap water tastes bitter.
pH
This
characteristic
and thebetween
marked degree
of bitter
sour or
bittereating
is indicated
by the pH
Since the
creation
of man,ofhethese
couldliquids
differentiate
sour and
when
liquid or
value.
the pH
value
is today
to athat
great
portion
of tastes
our population
solid foods.
WeThe
saysignificance
that vinegarof and
lemon
juice
taste known
sour and
soap
water
bitter. alone
through
the
fact
that
it
is
accepted
that
the
pH
value
of
swimming-pool
water
has
to
bepH
kept within
This characteristic of these liquids and the marked degree of sour or bitter is indicated by the
certain
limits. The
food
eatiscontains
a lot toofa water
and ourofdrinks
are nothing
else than
value. The
significance
of the
pHwe
value
today known
great portion
our population
alone
andit flavoured
water,
andpH
thisvalue
waterofcan
be either sour
or bitter;
inkept
otherwithin
words, it will
through coloured
the fact that
is accepted
that the
swimming-pool
water
has toorbe
have aThe
certain
value.
certain limits.
foodpHwe
eat contains a lot of water and our drinks are nothing else than
pH
coloured and flavoured water, and this water can be either sour or bitter; or in other words, it will
In general we can say that water is the most
have a certain pH value.
important substance on earth. Without water there
will be
life on
planet.
People
living
in areas of
In general
wenocan
sayourthat
water
is the
most
1onhave
severe
drought
experienced
the importance
of
important
substance
earth.
Without water
there
water
for
their
existence.
Continuous
droughts
do
will be no life on our planet. People living in areas of
not onlyhave
affect
the farmers,
are a disaster
for
severe drought
experienced
thethey
importance
of
thetheir
entire
economyContinuous
of a nation.
It is therefore
of
water for
existence.
droughts
do
importance
treat
this precious
not onlyutmost
affect the
farmers,that
theyweare
a disaster
for liquid
with
care,
use
it
responsibly
and
try
to
save
the entire economy of a nation. It is therefore of every
drop of it. that we treat this precious liquid
utmost importance
with care, use it responsibly and try to save every
drop of it.
Water is everywhere! Three quarters of the
surface of the earth is covered with water in the
form
of oceans,Three
rivers,
lakes of
andtheswamps.
Water is
everywhere!
quarters
whiskey,
beer,with
wine,
cool-drinks,
fruit
surface Brandy,
of the earth
is covered
water
in the
coffee
and tea
consist
water containing
form ofjuices,
oceans,
rivers,
lakes
andof swamps.
certain
additives
which
change
the
and
Brandy, whiskey, beer, wine, cool-drinks, property
fruit
the taste
theconsist
water. of
Blood,
vital liquid of our
juices, coffee
andoftea
waterthe
containing
body is 1mainly
water, the
in fact
our entire
1 change
certain additives
which
property
and body
of approximately
65%liquid
water.
the tasteconsists
of the water.
Blood, the vital
of our
body is Water
mainlyis water,
fact our
entire body
used ininindustry
as cooling
water, wash
consists water,
of approximately
65%
water.
boiler water, steam, condensate and solvent. Dirty water is usually disposed of as effluent.
2/3
Notably
chemical
industrywater,
especially
Water is used inthe
industry
as cooling
wash makes use of this universal solvent, because most
substances
are somewhat
soluble
water. Dirty water is usually disposed of as effluent.
water, boiler
water, steam,
condensate
andinsolvent.
Notably the chemical industry especially makes use of this universal solvent, because most
substances are somewhat soluble in water.
5
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65%
pH
pH
pH
pH
pH
pH
pH
pH
6
by T&C Technical
1.1
pH
pH
pH
pH
pH
pH
pH
pH
pH
pH
Dr.Hannes Bühler, Andreas Brügger, Dirk Tillich,
and thank to my loving wife
7
by T&C Technical
2.
pH
1(100)
10-7
Page 5
10-14mol/L
2 THE pH SCALE
If we express the hydrogen ion concentration of an aqueous solution in relation to its molecular
value we derive a scale of 1 (100) via 10-7 to 10-14 mole/litre.
0
14
pH
This scale is impractical but if written as a function of its negative logarithm a real and simple
scale of 0 – 14 has been created:
the pH scale.
pH
H+ concentration
(mole/litre)
mol/L
OH- concentration
(mole/litre)
pH
mol/L
1
0.1
0.01
0.001
0.0001
0.00001
0.000001
0.00000000000001
0.0000000000001
0.000000000001
0.00000000001
0.0000000001
0.000000001
0.00000001
0
1
2
3
4
5
6
0.0000001
0.0000001
7
0.00000001
0.000000001
0.0000000001
0.00000000001
0.000000000001
0.0000000000001
0.00000000000001
0.000001
0.00001
0.0001
0.001
0.01
0.1
1
8
9
10
11
12
13
14
hydrogenii
1909term
pH is pH
the abbreviationpondus
of pondus
hydrogenii and means the weight of hydrogen. This
was introduced in 1909 by the Danish biochemist S. P. L. Sørensen (1868 – 1939).
S.P.L. SØrensen (1868
1939
)
The pH scale covers the active concentration of the H+ ions and OH¯ ions and therefore the pH
value is defined as the negative + common logarithm
of the active hydrogen ion
pH
H
OHconcentration in an aqueous solution.
pH
1
pH = log hydrogen ion concentration ( mole/litre)
If the H+ ion concentration changes by a factor of ten, the pH value changes by one unit. This
illustrates how important it is to be able to measure the pH value to a tenth of a unit or even a
hundredth of a unit in special applications.
The pH definition refers to the active hydrogen ion concentration and not just to the hydrogen ion
concentration. It is important to understand this difference. Only in dilute solutions are all anions
and all cations so far apart that they are able to produce the maximum of the chemical energy,
8
i.e. the H+ ion concentration and the H+ ion activity are identical. For instance 0,01 mole
hydrochloric acid is still classified as a dilute solution which dissociates completely and therefore
concentration equals activity.
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1
pH = Log
(mol/L)
H+
10
pH
1
0.1
0.01
pH
H+
H+
0.01
0.01 mol HCl:
(H+)
1 mol HCl:
pH
9
(OH-)
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pH
pH
pH
pH
pH0
HCl + NaOH
pH0 + pH14
pH
1mol/L
pH14
1mol/L
→
HOH + NaCl
→
H2O + NaCl
→
pH7
NaCl
pH
pH
=7
pH
7
pH
10
8
by T&C Technical
Page 7
pH
The pH Scale
-1
mol/L HCl
-1 mole
1010
HCl
pH 0
1
2
3
-1
10 mol/L NaOH
10-1
mole NaOH
HOH
HOH
4 5
6
7
8 9 10 11 12 13 14
1mol/L HCl
1mol/L NaOH
1 mole/l HCl
1 mole/l NaOH
increasing acidity
pH 0
1
2
3 4
5
neutral
6
7
8
increasing alkalinity
9 10 11 12 13 14
Gastric
Milk Well
Sour
Juices
Milk Coffee
Water
Household
Yoghurt
Soap
Ammonia
Suds
Beer
Blood
Milk
Lemon
of
Juice
Magnesia
(30%wt)
3 THE pH MEASUREMENT
3.1 The NERNST Equation
To determine the active hydrogen ion concentration, a pH measurement is necessary.
Three methods are generally used for the direct determination of the pH value in aqueous
solutions:
11
1) The visual method, a colour comparison with pH sensitive indicator paper
(litmus) to a standard colour scale.
by T&C Technical
3.
pH
3.1
pH
pH
3
1
pH
2
pH
3
pH
pH
pH
(
)
(
)
12
If two hydrogen electrodes (each a thin plate of polished platinum) are immersed in two solutions,
having different hydrogen ion concentrations, each electrode will generate a potential which
depends on the active hydrogen concentration of the solution in which the electrode is immersed.
To be able to measure this potential, the two solutions are connected by a salt bridge (electrolyte
bridge) and the two electrodes are connected to a high impedance voltmeter. Both solutions are
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saturated with pure hydrogen gas.
Since the current passing during such a
measurement is negligible, the chemical
composition of the sample solution is not altered.
The bridge acts as a phase boundary between
solution C1 and solution C2 and closes the
electric circuit.
High impedance mV meter
0
-1500
+1500
mV
A potential difference will be generated between
C1,C2
the two platinum electrodes by the different active
hydrogen-ion concentrations in the solutions. The
relationship is expressed by the NERNST
equation:
Electrolyte Bridge
Pt
Pt
E
C2
C1
RxT
C
x log 1
nxF
C2
where:
E = potential difference (mV)
R = gas constant (8,31439 J x mol-1 x K-1)
R
T constant (96495,7
C1 C x mol-1)
F = Faraday
E=
Log
n
F
C2
E=
mV
R=
(8.31439J×mol-1×K-1)
(96495.7C×mol-1)
F=
T=
(K)
n=
(
C1 =
C1
C2 =
C2
nH = 1)
13
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R
T
E=
C1
C1
C2
10:1
Log
n
F
C2
E
R
T
E=
10
UN
Log
n
F
UN
10
R
1
n
R
T
T
E=
℃
2.303585
n
F
T = 273.15 + 20 = 293.15 K
UN = 58.16mV
14
F
The values of R and F are constant. The charge number n is known for each kind of ion and the
temperature T can be calculated from the measured value in °C.
The values of R and F are constant. The charge number n is known for each kind of ion and the
If we assume the temperature of the solutions to be 20 °C, then:
temperature T can be calculated from the measured value in °C.
T of
= 273,15
+ 20 =to293,15
If we assume the temperature
the solutions
be 20 Kelvin
°C, then:
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This will give us a NERNST potential
T =of273,15 + 20 = 293,15 Kelvin
Page 7
pH
This will give us a NERNST potential
UNof= 58,16 mV
The pH Scale
UN =
58,16 mV
HOH
HOH
-1
mol/L HCl
1010-1 mole
HCl
-1
-1 mol/L NaOH
1010
mole NaOH
The pH Scale and the Related NERNST Potential
-1
-1
10 mole
HCl
HOH
10 mole NaOH
The
pH Scale
and the
Related NERNST
Potential
pH 0 1 2 3 4 5
10-1 mole HCl
1mol/L HCl
1pH
mole/l
HCl2
0 1
3
4 5
6
7
8 9 10 11 12 13 14
HOH
10-1 mole NaOH
6
7
1 mole/l
8 9 10 11 12
13 14NaOH
increasing
pH
0 1 2acidity
3 4 5neutral
6 7
1 mole/l
HCl
1mol/L NaOH
alkalinity
8increasing
9 10 11
12 13NaOH
14
1 mole/l
1 mole/l HCl
1 mole/l NaOH
0
0 1 2 3 4 5 6 mV
7 8 9 10 11 12 13 407,12
14
+pH
407,12
0
mV
mV
mV
407,12
+ 407,12
Gastric
mV
Milk Well
Sour
Juices
pH
MilkpH CoffeepH Water
Household
+58,16
2x
Yoghurt
Soap-58,16
mV
mVAmmonia
mV at 20 oC +58,16mV=
pH
pH
Suds
116,32
mVpH
Beer
Blood
+58,16
-58,16
Milk
Lemon
2x
o
mV
mV at 20
C +58,16mV=
of mV
Juice
116,32 mV
Magnesia
mV
As the ion activity is temperature dependent, so is the NERNST potential (refer to the
NERNST equation). The following table illustrates the temperature dependency:
As the ion activity is temperature dependent, so is the NERNST potential (refer to the
NERNST equation). The
table
UN
UN the temperature
UN dependency:
T following
T illustrates
T
o
o
o
C
C
C
mV
mV
mV
UN
UN
U
T
T
T
0o 54.20
35o 61.14
70o 68.08 N
5 C 55.19mV
40 C 62.13mV
75 C 69.08mV
056.18
54.20 45 3563.12
61.14 80 7070.07
68.08
557.17
55.19 50 4064.12
62.13 85 7571.06
69.08
1058.16
56.18 55 4565.11
63.12 90 8072.05
70.07
1559.16
57.17 60 5066.10
64.12 95 8573.04
71.06
2060.15
58.16 65 5567.09
65.11 100 9074.04
72.05
25 59.16
60 66.10
95 73.04
Temperature Dependency of the NERNST Potential
30 60.15
65 67.09
100 74.04
10
15
20
25
30
Temperature Dependency of the NERNST Potential
3 THE pH MEASUREMENT
3.1 The NERNST Equation
To determine the active hydrogen ion concentration, a pH measurement is necessary.
Three methods are generally used for the direct determination of the pH value in aqueous
solutions:
15
1) The visual method, a colour comparison with pH sensitive indicator paper
(litmus) to a standard colour scale.
by T&C Technical
Page 11
3.2 The pH Measuring System
3.2
pH
A basic pHpH
measuring system consists of
0
-1500
mV
+1500
(1) the measuring electrode,
a pH sensitive electrode,
(3)
High impedance
voltmeter
Electrode
assembly
(1)
Measuring
electrode
1
(2)
and
Reference
electrode
(2) the reference
pH electrode
(3) a high impedance voltmeter.
2
3
3.2.1 The Measuring Electrode
3.2.1
The purpose of the measuringpH
electrode is to determine the pH value of an /aqueous solution.
The platinum/hydrogen electrode was originally used to measure the hydrogen ion
1897
concentration
in aqueous solutions (since 1897) and today still serves as a reference standard
for the electrometric pH determination. The hydrogen electrode consists of a platinised
pH with platinum black), subjected to a flow of gaseous hydrogen. A
platinum plate or rod (coated
silver wire coated with silver chloride serves as a reference electrode.
The basic theory, when employing a hydrogen electrode, is as follows: If a metal rod (electrode)
is immersed into an aqueous solution containing its own salt (silver electrode in silver nitrate), the
atoms on the surface of that metal rod will ionise. The water molecules will attract the positively
charged metal ions from the surface of the rod, which leaves the metal rod negatively charged.
This charge exchange develops a potential difference at the phase boundary metal/solution. The
potential depends on the ion concentration in the (solution and is known as the galvanic
potential.
)
Today the hydrogen
electrode still serves as a reference standard especially as its measuring
results are extremely accurate. However for practical reasons the hydrogen electrode has lost its
/
importance because of its difficult and complicated handling.
Only the antimony electrode has survived out of various metal electrodes. Antimony is chemically
resistant to hydrofluoric acid and can therefore be used for the pH measurement in solutions
containing hydrofluoric acid. However, the accuracy and reproducibility of the measuring result
incorporates large tolerances.
16
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pH
17
3.2.1.1 The Glass Electrode
It was not until the development of the glass electrode that pH measurement became a simple
and reliable tool for all kinds of applications. In recent years the glass electrode has outgrown all
other types of indicator electrodes for pH measurements. The pH determination of an aqueous
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solution is today as common as temperature and pressure measurements,
thanks to the
reliability and accuracy of the glass electrode in combination with extreme stable electronic
amplification.
However, the successful application of the glass electrode requires some
3.2.1.1
knowledge about its functionality and its maintenance, which this booklet will provide.
The Construction of a Glass Electrode
pH cable
pH
Electrode head
Electrode glass shaft
A glass electrode consists of a shaft pH
made from glass
which should be highly resistant to hot alkaline
solutions and its electrical resistance must be several
times greater than that of the membrane glass. The pH
sensitive part of the glass electrode is the normally
cylindrically shaped electrode tip, the glass membrane.
The membrane is made from special hydrogen ion
sensitive glass and is fused to the electrode shaft. The
pH
glass electrode
is partly filled with a buffer solution,
normally having a pH value of 7.
A defined amount of potassium chloride (KCl) is added
to this internal buffer.
Screen
Internal conductor
electrode (Ag/AgCl)
Inner buffer+KCl(pH7)
+ KCl (pH 7)
A silver wire, coated with silver chloride (Ag/ AgCl) is
inserted into the glass electrode right down into the
internal buffer and serves as a conducting electrode.
Via the core of the coaxial pH cable, the Ag/AgCl wire
is connected to one terminal of a pH meter.
pH sensitive
pH membrane
glass
3.2.1.2 The Glass Membrane
All types of glasses possess the property of producing a potential difference relative to the
hydrogen ion concentration in aqueous solutions. However only special types,
such as the
pH
conventional Mc-Innes glass (Corning 015) produce galvanic potentials which satisfy the
NERNST equation over a wide range of the pH scale.
Every manufacturer of pH electrodes is constantly researching for better pH sensitive glasses.
Through constant development HAMILTON has achieved results which have not previously been
available without unsatisfactory compromises.
pH7
KCl
When the membrane glass of a measuring electrode comes into contact with an aqueous
solution, it forms a thin gel layer of approximately 10-4 mm thickness between the glass
surface and the solution. The thickness of the gel layer depends on the quality and composition
of the membrane glass, the temperature and the pH value of the measured
solution. As the
pH
internal side of the glass membrane is in contact with the inner buffer (an aqueous solution of pH
7) a gel layer is also
pH formed on the inside of the glass membrane.
18
by T&C Technical
3.2.1.2
Mc-Innes
(Corning 015)
pH
pH
pH
0.1µm
pH
H+
H+
H+
H+
0mV
pH
Li+
19
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A continuous exchange of H+ ions in the gel layers and H+ ions of the solutions takes
both sides of the membrane. This ion exchange is1 controlled
by the H+ ion concentrati
pH
2
solutions.
24 ion concentration of each solution is identical on both sides of
If the hydrogen
membrane, the ion exchange stops after an equilibrium has been reached between the
the solutions and the H+ ions in the gel layers. Therefore, both sides of the membrane g
the same potential and the potential difference is 0 mV.
The Glass Membrane
Ag/AgCl
electrode
/
Glass membrane
1mm
appr. 0.5mm
0,5 to 1mm
thick
Inner gel layer
approx. 10 mm
thick
+
+ Li+ pH
+
+
+
+
++ Li +
H+
< pH 7
7
H+= const.
Li
Li+
Li +
Outer gel0.1
layerm
approx. 10 mm thick
H+
> pH 7
Measured medium
If a difference of a hydrogen ion conexists between the inner buffer and
solution, a potential difference develops
the inner and outer sides of the membr
0.1 m
which is proportional to the differen
between the inner buffer and the outer s
be able to measure the membrane pot
membrane itself has to be conductiv
achieved by the mobility of the alkaline
membrane glass (Li+ ions in most glasse
Na+ ions in older membrane glasses).
The thickness and composition of the
determine
the
response
time
characteristic slope of the glass
Therefore the gel layer is of critical im
to the electrode performance.
Without the gel layer there can be no pH measurement. Unfortunately it takes approxim
to two days until a gel layer is fully developed. Therefore a measuring electrode ne
hydrated (immersed into normal clean tap water) for at least 24 hours prior to
manufacturers deliver their electrodes already hydrated (the membrane is kept wet w
solution in a plastic cap) which renders the electrode ready for immediate use.
3.2.2 The Reference Electrode
The reference electrode represents a defined electrical connection between the
medium and the pH meter. The accuracy of the pH measurement is often determin
reference electrode and therefore the choice of the reference electrode is of s
importance. An ideal reference electrode should produce a predictable potential, wh
respond only in accordance with the NERNST equation. A good and stable reference
should also have a low temperature coefficient and possess no temperature hysteresis.
A reference electrode consists of an internal electrode (similar to the measuring electro
is immersed into a defined electrolyte. This electrolyte must be in contact with the
medium. Over the years various reference systems have been employed, but only two
the mercury-mercurous
20 chloride (calomel) and the silver-silver chloride
electrodes were found reliable with respect to an accurate and stable potential.
HAMILTON applies exclusively the silver-silver chloride reference system. (refer als
EVEREF reference system” on page 25).
Page 14
by T&C Technical
At low and stable temperatures (max. 80 °C) the calomel electrode has a high potential stability
3.2.2and a high accuracy down to a hundredth of a millivolt.
The Construction
of a
Reference Electrode
pH
pHcable
Electrode head
Electrode
glass shaft
Refill opening
Internal
reference
system
33mol/L
mole KCl
KCl
But today the silver/silver chloride electrode has gained
pH
practical acceptance and is by far the most frequently
employed reference system. It is easy to manufacture, its
pH
potential rapidly attains equilibrium between –30 °C and 135 °C,
and is very reproducible. The Ag-AgCl reference electrode
remains stable and accurate especially with wide temperature
fluctuations and at high temperatures up to 135 °C.
The internal electrode of an Ag-AgCl reference electrode consists
of a silver chloride coated silver wire which is immersed into
potassium chloride of 3 mole concentration situated in a large
chamber formed bypH
the glass body of the reference electrode.
A diaphragm (normally a small porous ceramic rod) is fitted at the
bottom of this chamber to permit the potassium chloride to diffuse
or leak into the measured medium. To complete the electric circuit
the silver-silver chloride wire is connected via a coaxial cable to
the pH meter.
The Construction
of a
Combination Electrode
Diaphragm
pH cable
3.2.3 The Combination (Single Rod) Electrode
Electrode head
Since 1947 electrode manufacturers have combined the
measuring electrode and the reference electrode into one unit,
hence the name combination electrode. Today, the
combination electrode is almost exclusively employed in
laboratories/ and industrial plants. Only when the life expectancy
is significantly different for the measuring and the reference
EVEREF
(
electrode, is the use
of a pH) measuring system consisting of
two separate electrodes recommended.
Electrode
glass shaft
Refill opening
3 mole KCl
Reference
system
/
In a combination electrode the concentric space surrounding the
measuring electrode is filled with the reference electrolyte and
1)
(80℃)
1/100mV
contains the internal reference system. A diaphragm near the
bottom of the electrolyte chamber serves as the junction
/
between the KCl solution and the measured medium. As the
reference electrolyte is a conductive medium, it acts as a screen
2) -30℃
135℃
to the measuring electrode.
Internal conductor
electrode (Ag/AgCl)
Inner Buffer + KCl
(pH 7)
pH sensitive
glass membrane
Diaphragm
21
by T&C Technical
calomel electrode has a high potential stability
3)
/
volt.
135℃
lver/silver chloride electrode has gained
nce and is by far the most frequently
ce system. It is easy to manufacture, its
ains equilibrium between –30 °C and 135 °C,
ducible. The Ag-AgCl reference electrode
d accurate especially with wide temperature
high temperatures up to 135 °C.
KCl
de of an Ag-AgCl reference electrode consists
coated silver wire which is immersed/ into
of 3 mole concentration situated in a large
the glass body of the reference electrode.
pH
ally a small porous ceramic rod) is fitted at the
ber to permit the potassium chloride to diffuse
sured medium. To complete the electric circuit
3.2.3
oride wire is connected
via a coaxial cable to
1947
The Construction
of a
Combination Electrode
1
pHcable
pH
ode
Electrode head
ombined the
into one unit,
Today, the
employed in
e expectancy
he reference
consisting of
Electrode
glass shaft
Refill opening
3 mole KCl
3mol/L KCl
Reference
system
KCl
rrounding the
lectrolyte and
agm near the
the junction
dium. As the
s as a screen
Internal conductor
electrode
(Ag/AgCl)(Ag/AgCl)
Inner Buffer +KCl(pH7)
+ KCl
(pH 7)
pH
pHsensitive
glass membrane
Diaphragm
22
by T&C Technical
3.2.4
KCl
(
)
Page 15
3.2.4 Combination Electrode filled with KCl-Gel as a Reference Electrolyte
(
Reference electrodes incorporating a liquid reference
electrolyte are maintenance intensive as their electrolyte level
has to be controlled and regularly topped up. The search for a
3mol/L assembly
KCl
maintenance-free electrode
led to the development
of the gel reference electrode.
pH
3mol/L
KCl
(Ag/AgCl)
+KCl
(pH7)
Diaphragm
pH
)
The gel electrode is a low-maintenance electrode. The
reference electrolyte chamber is filled with 3 mol/l KCl
electrolyte in gel form. The diaphragm is normally made
from ceramic. The glass shaft is often reinforced with an outer
plastic sleeve or the electrode shaft is made completely out of
plastic (Epoxy).
The gel electrode is mostly used in combination with portable
pH meters or laboratory pH meters for simple measuring
applications, e.g. public swimming pools. This electrode does
not need to be topped-up with reference electrolyte
– which
pH
reduces maintenance time. However it has to be taken into
account that the gel electrode has
pHa reduced accuracy and
a shorter life span than its counterpart with a liquid reference
electrolyte.
The response time of a gel electrode is somewhat slower than
an electrode filled with a liquid electrolyte.
3.2.5 Reference Electrode with POLISOLVE Electrolyte (Polymer)
In the early 1980s electrode manufacturers created the polymer electrode. The polymer
electrode is either a standard combination glass electrode or a separate reference electrode. In
both cases the reference electrolyte chamber is completely filled with a semi-solid
polymerised plastic material into which the KCl is embedded. No diaphragm junction is
required. Therefore the KCl saturated polymer has direct contact with the measured
solution. The contact is established through an aperture, which could either be one or more
holes near the bottom of the reference electrolyte chamber or a fissure separating the bottom
electrolyte chamber from the measuring electrode.
- What does not exist can not clog up! –
23
maintenance-free
electrode assembly led to the developm
maintenance-free electrode assembly
led to the development
of
the
gel
reference
electrode.
of the gel reference electrode.
3.2.5
The gel electrode.
electrode is
The gel electrode is a low-maintenance
Thea low-maintenance electrode.
reference
electrolyte
chamber is filled with 3 mol/l
reference electrolyte chamber is filled with 3 mol/l KCl
electrolyte
in gel form.
diaphragm is normally m
electrolyte in gel form. The diaphragm
is normally
made
by TThe
&C Technical
from
ceramic.with
Theanglass
from ceramic. The glass shaft is often
reinforced
outershaft is often reinforced with an o
or the out
electrode
shaft is made completely o
plastic sleeve or the electrode shaft plastic
is madesleeve
completely
of
(
)
plastic
(Epoxy).
plastic (Epoxy).
gel electrode
is mostly used in combination with port
The gel electrode is mostly used in The
combination
with portable
pH for
meters
or measuring
laboratory pH meters for simple measu
pH meters or laboratory pH meters
simple
applications,
e.g. public
swimming
applications, e.g. public swimming pools.
This electrode
does
1980 pools. This electrode d
not need
to be topped-up
not need to be topped-up with reference
electrolyte
– which with reference electrolyte – w
reduces
maintenance
time. However it has to be taken
reduces maintenance
time.
However it has to be taken
(
)
pHinto
thataccuracy
the gel electrode
has a reduced accuracy
account that the gel electrode has account
a reduced
and
a shorter
span
than its counterpart with a liquid refere
a shorter life span than its counterpart
with a life
liquid
reference
electrolyte.
electrolyte.
response slower
time of than
a gel electrode is somewhat slower
The response time of a gel electrodeThe
is somewhat
Diaphragm
an electrode filled with a liquid electrolyte.
an electrode filled with a liquid electrolyte.
hragm
1
KCl
2 with3.2.5
Reference
Electrode
with POLISOLVE Electrolyte (Polymer)
Reference Electrode
POLISOLVE
Electrolyte
(Polymer)
In the earlycreated
1980s the
electrode
manufacturers
created
the polymer electrode. The poly
early 1980s electrode manufacturers
polymer
electrode. The
polymer
electrode
is
either
a
standard
combination
glass
electrode
ode is either a standard combination glass electrode or a separate reference electrode. Inor a separate reference electrod
both cases
the isreference
electrolyte
chamber
is completely filled with a semi-s
cases the reference3 electrolyte
chamber
completely
filled with
a semi-solid
2
plastic
material into
the KCl
is embedded.
No diaphragm junctio
merised plastic material intopolymerised
which the KCl
is embedded.
No which
diaphragm
junction
is
required.
Therefore
KClcontact
saturated
has direct contact with the measu
ed. Therefore the KCl saturated
polymer
has the
direct
withpolymer
the measured
solution.
The
contact
is
established
through
an
aperture,
on. The contact is established through an aperture, which could either be one or more which could either be one or m
holeselectrolyte
near the chamber
bottom oforthe
electrolyte
near the bottom of the reference
a reference
fissure separating
thechamber
bottom or a fissure separating the bo
electrolyte
chamber from the measuring electrode.
olyte chamber from the measuring
electrode.
What
- What does not exist can not clog-up!
– does not exist can not clog up! –
Hole
in electrolyte chamber
Hole
in electrolyte chamber
1
2
24
by T&C Technical
3
4
pH2
90℃
5
3.2.5.1
(POLISOLVE)
1
pH0
2
3
4
130℃
600kPa
5
6
2µS/cm
pH
pH
SIP
pH
25
pH
As the KCl saturated polymer is free of AgCl, there is no possibility of silver sulphide
contamination when measuring the pH value of solutions containing sulphides.
The high pressure rating of 600 kPa, its extended temperature rating of up to 130°C and its maintenance-free operation should make the POLISOLVE reference system always a first choice.
Electrodes, utilizing the POLISOLVE polymer electrolyte, are even suitable for steam sterilisation
by T&C Technical
in biotechnology applications.
3.2.6
pH
3.2.6 The Measurement
of the Potential
Difference
The pH measuring
pH electrode and the pH reference
electrode form a so-called pH measuring chain
within the measured medium. This chain can
pH be
compared to a battery of which the voltage
produced depends on the measured medium.
High impedance
voltmeter
0
-1500
mV
+1500
pH
Electrode
assembly
Reference
The difference in potential between the measuring
electrode
Measuring
2
electrode and the reference electrode is a function
electrode
of the pH value of the measured medium. In
theory the voltage changes by 58,16 mV per
The
pH
pH unit at 20 °C according to the NERNST
measuring
pH
chain
equation. The voltage produced by the pH
measuring chain is large enough not to present
any problem for a measurement. But the
Measured medium
measuring chain is a voltage source from which no
pH
current can be drawn, not even the low current
which a moving coil of a DC voltmeter draws. The potential difference of the measuring chain has
20℃
58.16mV
pH the voltage
to be measured without drawing any current
from1pH
the voltage
source, otherwise
would be reduced and the pH measurement would be drastically falsified. The reason is the high
electrical resistance of the glass electrode which is mainly determined by the resistance of the
glass membrane.
pH
The resistance values of a glass membrane vary between 10 M and 1 000 M at 25 °C and
increase 10 times at a temperature decrease of 25 °C. The lowest possible operating
pH
temperature of a pH electrode is often determined by the resistance increase of the glass
membrane at low temperatures, the internal resistance of the measuring instrument, the required
accuracy of the pH measurement and the freezing point of the electrolyte.
25℃
25℃
10
pH
26
1000MΩ
pH
pH
±0.1pH
10MΩ
by T&C Technical
1/100
5000MΩ
1012Ω
3.3
pH
pH
27
3.3 The Characteristics of a pH Measuring Chain
The characteristics of a pH measuring chain are the result of the individual properties of the
measuring and the reference electrode. Most of the electrode assemblies in use today are
combination electrodes. For this reason we will refer to the combination electrode when
examining the different properties of a measuring chain. However everything
that is said about
by T&C Technical
the properties of a combination electrode may be applied to the individual measuring and
reference electrode as well.
3.3.1
3.3.1 The Different Potentials of a Combination Electrode
Etotal
E total
EE11 :
EE22 : Asymmetry potential of
the glass membrane.
0
-1500
mV
Potential developing on
the outer gel layer.
+1500
developing on
EE33 : Potential
the inner gel layer.
developing on
EE44 : Potential
the internal conductor
electrode.
developing on
EE55 : Potential
the reference electrode.
E5
E6
potential
E6 : Diffusion
the diaphragm.
E4
E3
E6
of
he pH
to ta l
otal
E total :EtTmeasured
p ot e nt i a l
by a pH
E
E6
electrode1 assembly
is
the sum of potentials
E1 to E6.
E2
E1
pH
pH
mV
6
E2
E1
E6
pH
E1
28
pH
by T&C Technical
E2
pH
0mV
E3
E3
E4
E5
/
E4
E6
E1
E2
E2
E6
E6
3.3.2
0mV
29
E5
by T&C Technical
pH
pH7
pH
0mV
Etotal
E1
7
E6
pH
-30mV
+30mV
pH
(
pH
(
pH6.8)
)
±0.02pH(1.16mV)
pH
pH
pH
a)
b)
pH
c)
30
/
by T&C Technical
3.3.3
pH
(
pH7)
0mV
±47mV(±pH0.8)
pH
31
by T&C Technical
3.3.4
pH
pH
1pH
pH
=
U
pH
58.16mV
0
pH7
pH7
-58.16mV
99.8%
pH
pH
32
pH14
slope potentiometer of the pH meter/transmitter. As with the zero point adjustment, the slope
adjustment has to be performed at regular intervals.
The slope of a pH electrode assembly is temperature dependent in accordance with the
NERNST equation. The slope increases with the rise in temperature of the measured solution, as
by T&lines
C Technical
can be seen from the diagram below. In theory, all temperature dependent slope
intersect
the theoretical zero point (pH 7).
pH
450
400
350
300
The Slope or Sensitivity of a pH Electrode Assembly
+
Slope =
pH
200
150
100
50
U
1
2
3
pH
4 5
6
mV
50
100
150
200
300
350
400
450
7
8
9
pH
Theoretical Values
o
Slope at 0 C = 54,20 mV
o
Slope at 20 C = 58,16 mV
o
Slope at 50 C = 64,12 mV
pH
a)
b)
10 11 12 13 14
pH
c)
d)
e)
33
0 oC
o
20 C
50 oC
d) The electrode assembly must be symmetrical, i.e. measuring and
reference electrode must have identical conducting systems in
order to neutralise their galvanic potentials.
e) The diffusion potential of the diaphragm should be as small and as
constant as possible.
by T&C Technical
Slope
and Asymmetry Potential
pH
of a
pH Electrode Assembly
450
400
350
300
+
200
150
100
50
U
pH
Slope =
pHas
U
1
2
3
pH
4 5
6
7
8
9
mV
pH
50
100
150
200
Asymmetry Potential
Theoretical Values
Slope at 0 oC = 54,20 mV
300
350
400
450
Slope at 20 oC = 58,16 mV
Slope at 50 oC = 64,12 mV
3.3.5
(0mV pH7)
pH
Uis(Uis=
10 11 12 13 14
)
34
o
0 C
20 oC
50 oC
Page 22
3.3.5 The Isotherm Intersection Point
by T&C Technical
Again we have a disagreement between theory and practice. In theory all temperature dependent
slope lines intersect the theoretical zero point (0 mV/pH 7). When an asymmetry potential is
present – and
pH that is always the case – this intersection shifts either to the right or to the left of
the zero point, as can be seen from the diagram below.
2 of a pH electrode assembly vary with temperature. The temperature dependency of
All potentials
each individual potential cannot be accurately defined, but it will shift the resultant intersection
pH away from the theoretical
pH zero point and away
pH from the
point of all temperature slope lines
asymmetry potential. This intersection point is then known as the isotherm intersection point
mV
pH
(Uis = isotherm potential).
In order to perform an accurate pH measurement, the position of the isotherm intersection point
Uis
Uis
has to be established. Two buffer solutions are required. The position of the isotherm intersection
point can then be determined by measuring the potential difference of the pH electrode assembly
pH
against various temperatures (heated buffer solutions). The mV outputs of the electrode
assembly are then plotted against their pH value on graph paper and thereby the position of the
pH
Uis
isotherm intersection point is established. The voltage and polarity of the established isotherm
potential Uis can only be compensated for if the pH meter/transmitter used is equipped with an Uis
potentiometer. Nowadays, only the microprocessor based pH meters/transmitters have the
capacity to compensate for the Uis potential.
pH
25℃
60℃
New electrodes from HAMILTON show a maximum compensation error of 0,1 pH when
0.1pH in a solution having 60 °C.
calibrated at 25 °C and thereafter measuring
pH
Slope and Isotherm Potential
of a
pH electrode Assembly
450
400
350
300
+
200
U
150
100
50
mV
50
100
150
200
300
350
400
450
Isotherm
Intersection
Point
pH
1
2 3
Uis
4
5
6
7
8
9
Slope =
U
pH
10 11 12 13 14
pH
Theoretical Values
Slope at 0 oC = 54,20 mV
o
Slope at 20 C = 58,16 mV
o
Slope at 50 C = 64,12 mV
35
o
0 C
o
20 C
o
50 C
by T&C Technical
3.4
Page 23
3.4 The Diaphragm
(
The diaphragm is a very important and critical part of the reference electrode. It provides an
electrolytic interface
between the silver/silver chloride conducting system and the measured
)
solution. In most cases the diaphragm consists of a porous ceramic plug fused into the glass wall
at the lower end of a reference electrode (porous ceramic diaphragm).
Various diaphragm types (differing in construction and shape) are available, each type has its
advantages and limitations. It is normally the measurement application which determines the use
of a specific diaphragm.
a) Porous Ceramic Diaphragm
a)
The porous ceramic diaphragm is probably the most frequently used today. It possess a high chemical resistance
and it is easy to manufacture.
This junction provides a reproducible electrolyte flow but
because of its large surface it is very vulnerable to
contamination.
b) Platinum Fibre Diaphragm
b) Platinum fibre diaphragms consist of very fine platinum
wires which are spun loosely together and fused into the
glass.
This type of junction resist contamination to a certain extent
but their electrolyte flow is less reproducible than ceramic
diaphragms.
c) SINGLE PORE (Trademark of HAMILTON) Diaphragm
The SINGLE PORE diaphragm is strictly speaking not a
diaphragm at all. It is a very small glass capillary which
allows a larger leakage rate than a ceramic or platinum
diaphragm. A constant and very reproducible electrolyte
flow is assured. Clogging or contamination is barely
possible. It gives the most accurate and repeatable results.
In combination with a polymeric electrolyte the SINGLE
PORE principle is adapted for industrial electrodes. Due to
the lack of36contamination and maintenance it has a lot of
advantages.
The German Federal Physical Technical Institute (PTB) decided during a traceability test in 1997
Platinum fibre diaphragms consist of very fine platinum
wires which are spun loosely together and fused into the
glass.
This type of junction resist contamination to a certain extent
but their electrolyte flow is less reproducible
than ceramic
by T&C Technical
diaphragms.
c)
(
)
c) SINGLE PORE (Trademark of HAMILTON) Diaphragm
The SINGLE PORE diaphragm is strictly speaking not a
diaphragm at all. It is a very small glass capillary which
allows a larger leakage rate than a ceramic or platinum
diaphragm. A constant and very reproducible electrolyte
flow is assured. Clogging or contamination is barely
possible. It gives the most accurate and repeatable results.
In combination with a polymeric electrolyte the SINGLE
PORE principle is adapted for industrial electrodes. Due to
the lack of contamination and maintenance it has a lot of
advantages.
The German Federal Physical Technical Institute (PTB) decided during a traceability test in 1997
that the SINGLE PORE pH electrode is the most accurate laboratory electrode. (“Traceability of
pH measurement” by Petra Spitzer; ISBN 3-89429-877-4 or ISSN 0947-7063)
(PTB)
pH
1997
(“Tracebility of pH measurement” by Petra Spitzer; ISBN
3-89429-877-4
37
ISSN 0947-7063)
Page 24
Page 24
d) Annular Ceramic Diaphragm
by T&C Technical
d) Annular Ceramic Diaphragm
d) annular ceramic diaphragm is formed by a porous
The
ceramic
layer ceramic
between two
glass tubes.
The direction
of the
The
annular
diaphragm
is formed
by a porous
measured
medium
is
not
critical
due
to
the
annular
shape
ceramic layer between two glass tubes. The direction of the
of this junction.
The
electrolyte
flow tois the
notannular
reproducible.
measured
medium
is not
critical due
shape
Hence
is mainlyThe
applied
in gel-type
of
this itjunction.
electrolyte
flowelectrodes.
is not reproducible.
Hence it is mainly applied in gel-type electrodes.
e) Ground Sleeve Diaphragm
Electrolyte
Electrolyte
e) Ground Sleeve Diaphragm
Ground sleeve diaphragms are ideally suited for
applications
in suspensions
as these
Ground
sleeve
diaphragmsandareemulsions,
ideally suited
for
e)
diaphragms
can
be
cleaned
easily
by
only
pulling
up
the
applications in suspensions and emulsions, as these
glass sleeve.
successful
diaphragms
can Another
be cleaned
easily byapplication
only pullingofup this
the
diaphragm
is theAnother
pH measurement
in low
ionic solutions
or
glass
sleeve.
successful
application
of this
in non-aqueous
The electrolyte
flowrate
diaphragm
is themedia.
pH measurement
in low
ionic depends
solutions on
or
the
roughness
of
the
ground
glass
surface
of
the
sleeve
in non-aqueous media. The electrolyte flowrate depends on
and roughness
the tightness
theground
sleeveglass
fit. However
the
of of
the
surfacethis
of diaphragm
the sleeve
is not
the this
pH diaphragm
electrode
and
thesuitable
tightnessfor
of applications
the sleeve fit.where
However
assembly
is subjected
to vibrationwhere
as thisthe
might
the
is
not suitable
for applications
pHloosen
electrode
diaphragmissleeve.
assembly
subjected to vibration as this might loosen the
diaphragm sleeve.
The selection of the right diaphragm for a measuring application is of utmost importance but not
always
easy. Very
the experimental
“trial and
error” method
will lead
to a successful
The
selection
of theoften
right only
diaphragm
for a measuring
application
is of utmost
importance
but not
application
of Very
a certain
type. For detailed
one haswill
to consult
technical
always
easy.
oftendiaphragm
only the experimental
“trial information
and error” method
lead to the
a successful
pH
data
sheets
of
the
electrode
manufacturers.
application of a certain diaphragm type. For detailed information one has to consult the technical
data
sheets ofprovides
the electrode
manufacturers.
A diaphragm
a deliberate
leak of the electrolyte solution into the measured medium
whilst
preventing
unrestricted
mixing
ofthe
both
solutionssolution
within into
the the
reference
electrode.
A diaphragm provides a deliberate leak of
electrolyte
measured
medium
Penetration
of the unrestricted
measured solution
intoofthe
reference
electrolyte,
thus poisoning
of the
whilst
preventing
mixing
both
solutions
within and
the reference
electrode.
reference conducting
system solution
occurs frequently
during pHelectrolyte,
measurements,
especially
when
Penetration
of the measured
into the reference
and thus
poisoning
of the
measured conducting
solution is pressurised.
reference
system occurs frequently during pH measurements, especially when the
measured
is pressurised.
There are solution
pH electrodes
on the market where the electrolyte storage vessel can be pressurised
in order
to pH
counteract
theon
penetration
of where
the measured
solutionstorage
throughvessel
the diaphragm.
As a rule
There
are
electrodes
the market
the electrolyte
can be pressurised
of
thumb
a
pressure
of
100
kPa
above
the
pressure
of
the
measured
solution
will
normally
in order to counteract the penetration of the measured solution through the diaphragm. As
a rule
suffice.
result aofsmall
of electrolyte
solution
penetrate
into the
of
thumbAsa apressure
100 amount
kPa above
the pressure
of thewill
measured
solution
willmeasured
normally
solution As
which
is generally
no significance
to the
process.
However this
the
suffice.
a result
a small of
amount
of electrolyte
solution
will penetrate
into decreases
the measured
resistance
of theisreference
to between 0,1
k and
2 k , improves
of
solution
which
generallyelectrode
of no significance
to the
process.
However the
thisreproducibility
decreases the
the
measurement
and
prevents
the
diaphragm
from
clogging
up.
resistance of the reference electrode to between 0,1 k and 2 k , improves the reproducibility of
the
measurement
and prevents
theisdiaphragm
up.
Before
the electrode
assembly
immersedfrom
intoclogging
the measured
medium the stopper which
closes the electrode
refill opening
must be
removed. Periodic
the electrolyte
level should
Before
assembly
is immersed
into the inspection
measuredofmedium
the stopper
which
be
part
of
the
electrode
maintenance
programme.
closes the refill opening must be removed. Periodic inspection of the electrolyte level should
be part of the electrode maintenance programme.
38
by T&C Technical
pH
pH
100kPa
0.1
2kΩ
1
39
by T&C Technical
3.4.1
Page 25
3.4.1 The1Diffusion Potential
(E6)
Another disturbing factor of the diaphragm is its diffusion potential (E6). This potential always
develops at the phase boundary between two electrolytes of different concentration or
composition. The diffusion potential can be contributed to the different migration velocities of
ions, which again depends on the polarity and size of the ion type.
The illustration below explains the
diffusion potential between two HCl solutions of different
HCl
concentration:
H+ H+ ions ClThe
diffuse nearly5 five times faster to the right
than the Cl¯ ions. This creates a potential across the
boundary of the two solutions. In order to keep the
diffusion potential at the diaphragm of a reference
electrode as small as possible, the different ions in the
reference electrolyte should have identical ionic mobility.
With a 3 mole KCl solution this ideal condition is nearly
reached.
Diffusion Potential
Potential
Cl
H+
H+
Cl
Cl
H+
Cl
H Cl
H+
H+
+
H
+
+
Cl
Cl
Cl
H+
Cl
Cl
H+
H
Cl
H+
H+
Cl
H+
In general it can be said:
Cl
Distance
1. The higher the KCl concentration of the reference
electrolyte, the lower the diffusion potential.
3mol/L
KCl
2. The larger the flowrate of the reference electrolyte
through the diaphragm, the smaller the diffusion
potential.
1.
The more the pH value of the measured solution differs from pH 7, the larger the diffusion
potential.
KCl
2.
3.
pH7
Diffusion Potentials
which develop between various solutions
and a saturated KCl electrolyte
pH
1,0
mole HCl
0,1
mole HCl
0,01
mole HCl
0,1
mole KCl
Buffer pH 1,68
Buffer pH 4,01
Buffer pH 4,65
Buffer pH 7,00
Buffer pH 10,01
0,01
mole NaOH
0,1
mole NaOH
1,0
mole NaOH
=
=
=
=
=
=
=
=
=
=
=
=
14,1 mV
4,6 mV
3,0 mV
1,8 mV
3,3 mV
2,6 mV
3,1 mV
1,9 mV
1,8 mV
2,3 mV
-0,4 mV
-8,6 mV
40
From the above it can be seen that different measured solutions will create different diffusion
potentials at the diaphragm of a reference electrode.
Distance
electrolyte, the lower the diffusion potential.
2. The larger the flowrate of the reference electrolyte
through the diaphragm, the smaller the diffusion
potential.
by T&C Technical
The more the pH value of the measured solution differs from pH 7, the larger the diffusion
potential.
Diffusion Potentials
which develop between various solutions
and a saturated KCl electrolyte
1,0
mole HCl
0,1
mole HCl
0,01
mole HCl
0,1
mole KCl
Buffer pH 1,68
Buffer pH 4,01
Buffer pH 4,65
Buffer pH 7,00
Buffer pH 10,01
0,01
mole NaOH
0,1
mole NaOH
1,0
mole NaOH
=
=
=
=
=
=
=
=
=
=
=
=
14,1 mV
4,6 mV
3,0 mV
1,8 mV
3,3 mV
2,6 mV
3,1 mV
1,9 mV
1,8 mV
2,3 mV
-0,4 mV
-8,6 mV
From the above pH7
it can be seen pH
that different measured solutions will create different diffusion
potentials at the diaphragm of a reference electrode.
41
by T&C Technical
3.4.2
Page 26
3.4.2 Diaphragm Contamination through Chemical Reaction
pH
Chemical reaction at the diaphragm between the reference electrolyte and the measured solution
must be avoided at all costs. This reaction will lead to diaphragm contamination, increase of
resistance across the diaphragm and falsified measuring results. The reference electrolyte
contains silver chloride which is prone to chemical reactions, especially with sulphides. For this
reason great care must be taken when measuring pH in solutions containing sulphides, as the
diaphragm may be contaminated with silver sulphide deposits.
Silver sulphide contamination can easily be identified by a blackened diaphragm. As a result, the
response time of an electrode assembly increases substantially, the diaphragm resistance
increases radically and it may be impossible to calibrate such a contaminated electrode
assembly.
3.4.4.1
(EVEREF)
3.4.2.1 The EVEREF Reference System
Reference conductor
Silver wire
Reference electrolyte
with constant
chloride concentration,
but free of AgCl
Silver chloride
reservoir
Diffusion pipe
filled with cotton wool
Diaphragm
In order to counteract silver sulphide
contamination at the diaphragm HAMILTON
have invented the EVEREF reference
system.
This system consists of a silver
(EVEREF)
chloride reservoir from which the silver
reference wire leads to the electrode plug.
The reservoir is separated from the
reference electrolyte by a diffusion barrier
consisting of densely packed cotton wool in
a glass tube. The barrier prevents the loss of
silver chloride into the reference electrolyte
induced by temperature variations.
The EVEREF reference system enhances the stability of the reference potential and extends the
life of the combination electrode considerably.
3.4.2.2 The EVEREF-B Double Liquid Junction
If a chemical reaction at the diaphragm is unavoidable, the application of a reference electrode
having an inter-mediate electrolyte provides a solution.
42
The EVEREF-B double liquid junction system from HAMILTON with its intermediate
electrolyte reservoir provides such a solution. The reference electrolyte is completely separated
from the EVEREF reference system by an internal diaphragm situated in a second reservoir filled
with an intermediate electrolyte. The life of the pH electrode is greatly prolonged as the reference
by T&C Technical
Page 27
3.5 Alkaline and Acid Error
3.5.1 Alkaline Error
3.4.4.2
-B
pH Electrode
pH
“Chemotrode
Bridge”
with Intermediate Electrolyte
Electrode Head
(Variopin or SMEK)
VP
SMEK
Refill opening
Intermediate
electrolyte
Reference
system
EVERREF-B -B
Internal
diaphragm
3 mole
KCl
3mol/L
KCl
Internal
conductor
electrode (Ag/AgCl)
Internal buffer KCl
plus KCl
At a value above pH 10 the gel layer structure at the
membrane of a measuring electrode is subject to certain
changes which lead to a measuring inaccuracy, the alkaline
error. This alkaline error is caused by the presence of a high
concentration of alkaline ions, especially sodium ions (Na+).
These ions replace, partly or completely, the hydrogen ions
in the outer gel layer of the glass membrane, and by doing
so, contribute to the potential
at the outer phase boundary.
-B(EVEREF-B)
As a result a lower pH value will be measured than the actual
pH value of the measured solution.
In earlier days the alkaline error of glass electrodes already
developed between pH 9 and pH 10. Today, where the glass
membranes contain lithium instead of sodium, the alkaline
error is only noticeable from between pH 12 and pH 13.
The alkaline error increases with increasing pH value,
with higher alkaline concentration and with rising
pH
temperature.
In order to counteract the alkaline error, electrode
manufacturers use special membrane glasses with low
alkaline errors for electrodes which are used to measure high
-B
pH values.
pH
50
1
External
diaphragm
2
3
4
5
6
7
8
9
10 11 12 13 14
0
50
100
150
mV
200
250
300
350
400
43
Alkaline
Error
3.5 Alkaline and Acid Error
3.5.1 Alkaline Error
pH Electrode
“Chemotrode Bridge”
with Intermediate Electrolyte
3.5
Electrode Head
(Variopin or SMEK)
3.5.1
pH10
Refill opening
At a value above pH 10 the gel layer structure at the
membrane of a measuring electrode is subject
certain
by T&CtoTechnical
changes which lead to a measuring inaccuracy, the alkaline
error. This alkaline error is caused by the presence of a high
concentration of alkaline ions, especially sodium ions (Na+).
These ions replace, partly or completely, the hydrogen ions
in the outer gel layer of the glass membrane, and by doing
so, contribute to the potential at the outer phase boundary.
As a result a lower pH value will be measured than the actual
pH value of the measured solution.
In earlier days the alkaline error of glass electrodes already
developed between pH 9 and pH 10. Today, where the glass
membranes contain lithium instead of sodium, the alkaline
error is only noticeable from between pH 12 and pH 13.
Intermediate
electrolyte
Reference
system
EVERREF-B
Internal
diaphragm
The alkaline error increases with increasing pH value,
pH with rising
with higher alkaline concentration and
temperature.
3 mole KCl
Internal
conductor
electrode (Ag/AgCl)
13
pH9
10
In order to counteract the alkaline error, electrode
manufacturers use special membrane glasses pH12
with low
alkaline errors for electrodes which are used to measure high
pH values.
Internal buffer
plus KCl
pH
50
1
External
diaphragm
2
3
4
5
6
7
8
9
10 11 12 13 14
0
50
100
150
mV
200
250
300
350
400
44
Alkaline
Error
by T&C Technical
Page 28
3.5.2 Acid Error
3.5.2
400
At low
pH pH values (< pH 2) the potential difference
between measuring and reference electrode will not
conform exactly to the NERNST equation. Through
experiments it has been proven that the gel layer of
the membrane will absorb acid molecules at very low
pH values.
Acid Error
350
300
+
mV
250
200
150
pH the activity of the H+ ions
This absorption decreases
and results in a lower potential at the outer
membrane phase boundary. The pH measurement
shows a higher pH value than the actual pH value of
H+
the measured
solution. This effect is known as the
acid error.
100
50
1
2
3
4
5
6
7
8
9
10 11 12 13 14
0
50
pH
pH
As with the alkaline error, manufacturers supply measuring electrodes with membrane glasses
having specially low acid errors.
HAMILTON membrane glasses show no acid error above pH 1.
3.6 Temperature Influence and Temperature Compensation
pH1
The pH measurement is temperature dependent. Three temperature factors have to be
considered in order to perform a nearly perfect pH measurement:
1. the temperature dependency of the NERNST equation
2. the position of the isotherm intersection point
3. the pH/temperature dependency of the measured solution
450
400
350
300
+
Measuring Error
without
Temperature Compensation
31.72 mV
Calibration at 25 oC
o
Measurement at 65 C
200
150
100
50
mV
50
100
150
200
300
350
400
450
The temperature dependency of the
Nernst equation and with it the
temperature dependency of the theoretical
slope of a pH electrode assembly has
already been discussed in paragraph 3.1.
The slope of a pH electrode assembly is temperature
dependent in accordance with the NERNST equation.
0,47 pH
1
2
3
4
5
6
7
8
9
10 11 12 13 14
pH
Theoretical Values
Slope at 25 oC = 59,16 mV
o
25 C
65 C
o
o
Slope at 65 C = 67,09 mV
45
The temperature dependency of the
NERNST equation is easily calculated, and
as a rule, only this temperature influence is
considered by instrument manufacturers
when they incorporate conventional manual
or automatic temperature compensation in
their pH measuring products. The adjacent
graph illustrates the theoretical error which
is compensated for by conventional
temperature compensation.
100
50
1
2
3
4
5
6
7
8
9
10 11 12 13 14
0
50
and results in a lower potential at the outer
membrane phase boundary. The pH measurement
shows a higher pH value than the actual pH value of
the measured solution. This effect is known as the
acid error.
by T&C Technical
As with the alkaline error, manufacturers supply measuring electrodes with membrane glasses
having specially low acid errors.
3.6
HAMILTON membrane glasses show no acid error above pH 1.
pH
pH
3.6 Temperature Influence and Temperature Compensation
The pH measurement is temperature dependent. Three temperature factors have to be
considered
1 in order to perform a nearly perfect pH measurement:
1. the temperature dependency of the NERNST equation
2
2. the position of the isotherm intersection point
3
450
400
350
300
+
pH/
3. the pH/temperature
dependency of the measured solution
Measuring Error
without
Temperature Compensation
31.72 mV
25 Calibration at 25 oC
o
Measurement at 65 C
65
200
150
100
50
0,47 pH
1
2
3
mV
4
5
6
7
8
9
10 11 12 13 14
pH
50
100
150
200
300
350
400
450
The temperature dependencypHof the
Nernst equation and with it the
temperature dependency of the theoretical
slope of a pH electrode assembly has
already been
3.1discussed in paragraph 3.1.
The slope of a pH electrode assembly is temperature
pH
dependent in accordance with the NERNST equation.
Theoretical Values
25
65
o
Slope at 25
C = 59,16 mV
59.16mV
67.09mV
o
Slope at 65 C = 67,09 mV
o
25 C
65 oC
The temperature dependency of the
NERNST equation is easily calculated, and
as a rule, only this temperature influence is
considered by instrument manufacturers
when they incorporate conventional manual
or automatic temperature compensation in
their pH measuring products. The adjacent
graph illustrates the theoretical error which
is compensated for by conventional
pH
temperature compensation.
3.3.5
pH7
pH
pH
Uis
pH
46
Uis
by T&C Technical
3
pH/
pH
pH
pH/
pH
pH
47
by T&C Technical
3.7
pH
Page 30
Page 30
3.7 Various Electrode Shapes
Various
Electrode
Shapes
It is 3.7
not possible
to use
one electrode
shape for every application. More often in the laboratory
different electrode shapes are required as there are numerous pH measurement applications.
It is not possible to use one electrode shape for every application. More often in the laboratory
Electrode manufacturers try to cover most of these applications by offering varying electrode
different electrode shapes are required as there are numerous pH measurement applications.
constructions.
a) manufacturers try to cover most of these applications by offering varying electrode
Electrode
Form
a)
constructions.
Form a)
This is the most common electrode shape and a wide field of applications can be covered with
this type of electrode, both in the laboratory and in process control. Measuring, reference and
This is the most common electrode shape and a wide field of applications can be covered with
combination electrodes are manufactured using this construction.
this type of electrode, both in the laboratory and in process control. Measuring, reference and
combination electrodes are manufactured using this construction.
Form b)
Form b)
b)
A standard combination electrode construction with a ground sleeve diaphragm. This electrode is
mainly used in the laboratory where dirty or strongly contaminated solutions have to be
A standard combination electrode construction with a ground sleeve diaphragm. This electrode is
measured. Its application includes non-aqueous media as well. The diaphragm can easily be
mainly used in the laboratory where dirty or strongly contaminated solutions have to be
cleaned by pushing the sleeve upwards. There is a limited use for this construction in process
measured. Its application includes non-aqueous media as well. The diaphragm can easily be
control (be aware of vibration).
cleaned by pushing the sleeve upwards. There is a limited use for this construction in process
control (be aware of vibration).
Form c)
Form c)
This construction example includes two electrode features: a ground sleeve diaphragm and a
pipe connector. In order to minimise maintenance time and to pressurise the reference electrolyte
This construction
two electrode
ground
sleeve
diaphragm
and a
an external
electrolyteexample
reservoirincludes
is connected
to the features:
electrode avia
the pipe
connector.
This
pipe
connector.
In
order
to
minimise
maintenance
time
and
to
pressurise
the
reference
electrolyte
electrode construction can be used in the laboratory and in process control, especially for high
external
electrolyte
reservoir is connected to the electrode via the pipe connector. This
purityanwater
control
in power stations.
electrode construction can be used in the laboratory
and in process control, especially for high
48
purity water control in power stations.
A standard combination electrode construction with a ground sleeve diaphragm. This electrode is
mainly used in the laboratory where dirty or strongly contaminated solutions have to be
measured. Its application includes non-aqueous media as well. The diaphragm can easily be
cleaned by pushing the sleeve upwards. There is a limited use for this construction
process
by T&C in
Technical
control (be aware of vibration).
c)
Form c)
This construction example includes two electrode features: a ground sleeve diaphragm and a
pipe connector. In order to minimise maintenance time and to pressurise the reference electrolyte
an external electrolyte reservoir is connected to the electrode via the pipe connector. This
electrode construction can be used in the laboratory and in process control, especially for high
purity water control in power stations.
Page 31
Form d)
d)
Page 31
Form d)
This electrode construction is mainly used in the chemical industry and in biotechnological
processes. The electrode features a large electrolyte vessel which is sometimes combined with
an intermediate electrolyte vessel. The electrolyte can be pressurised and also sterilised with
SIP hot
steam. A special electrode holder is required for this electrode. No laboratory application.
This electrode construction is mainly used in the chemical industry and in biotechnological
processes. The electrode features a large electrolyte vessel which is sometimes combined with
Form e)
an intermediate electrolyte vessel. The electrolyte can be pressurised and also sterilised with hot
steam. A special electrode holder is required for this electrode. No laboratory application.
Form e)
e)
The above electrode construction is used in the laboratory where small samples have to be
measured.
Form f)
The above electrode construction is used in the laboratory where small samples have to be
measured.
Form f)
The feature of this construction is the flat membrane which enables the operator to measure the
pH of surfaces, e.g. skin, leather, paper etc. HAMILTON supplies this electrode with an
unbreakable plastic shaft, as it will often be carried
49 around and used with a portable pH meter.
The feature of this construction is the flat membrane which enables the operator to measure the
Form g)
pH of surfaces, e.g. skin, leather, paper etc. HAMILTON supplies this electrode with an
processes. The electrode features a large electrolyte vessel which is sometimes combined with
an intermediate electrolyte vessel. The electrolyte can be pressurised and also sterilised with hot
steam.
Form e)A special electrode holder is required for this electrode. No laboratory application.
The above electrode construction is used in the laboratory where small samples have to be
measured.
Form e)
by T&C Technical
The above
electrode construction is used in the laboratory where small samples have to be
Form
f)
f)
measured.
The above electrode construction is used in the laboratory where small samples have to be
measured.
Form f)
The feature of this construction
pHis the flat membrane which enables the operator to measure the
pH of surfaces, e.g. skin, leather, paper etc. HAMILTON supplies this electrode with an
Form f)
unbreakable plastic shaft, as it will often be carried around and used withpH
a portable pH meter.
The feature of this construction is the flat membrane which enables the operator to measure the
pH ofg)surfaces, e.g. skin, leather, paper etc. HAMILTON supplies this electrode with an
Form
unbreakable plastic shaft, as it will often be carried around and used with a portable pH meter.
The feature of this construction is the flat membrane which enables the operator to measure the
pH of surfaces, e.g. skin, leather, paper etc. HAMILTON supplies this electrode with an
unbreakable
plastic shaft, as it will often be carried around and used with a portable pH meter.
Form g)
g)
This construction is used exclusively for combination electrodes featuring a gel or polymer
reference
Form g) electrolyte. The shaft is made completely from plastic. This design makes the shaft
unbreakable as it is often used with a portable pH meter.
This construction is used exclusively for combination electrodes featuring a gel or polymer
reference
design makes the shaft
Form
H) electrolyte. The shaft is made completely from plastic. This
pH
unbreakable as it is often used with a portable pH meter.
This construction is used exclusively for combination electrodes featuring a gel or polymer
reference electrolyte. The shaft is made completely from plastic. This design makes the shaft
unbreakable
as it is often used with a portable pH meter.
Form H)
The above construction, pointed electrode, are normally applied in the food laboratory and in the
dairy
They facilitate the pH measurement in meat and cheese.
Formindustry.
H)
h)
The above construction, pointed electrode, are normally applied in the food laboratory and in the
dairy industry. They facilitate the pH measurement in meat and cheese.
The above construction, pointed electrode, are normally applied in the food laboratory and in the
dairy industry. They facilitate the pH measurement in meat and cheese.
pH
50
by T&C Technical
3.8
3.8.1
1
2
3
4
60℃
5
6
a)
b)
c)
d)
pH4
pH8
18
90℃
2
51
by T&C Technical
pH13
90℃
2
3
pH
3.8.2
3.4
(
(
200kPa)
)
KCl
60℃
52
by T&C Technical
pH2
25℃
6
(POLISOLVE)
130℃
pH0
90℃
130
12
53
12
by T&C Technical
3.9
pH
3.9.1
pH
pH
pH
(NIST)
pH
9
pH
NIST
pH
NIST
0℃
±0.005pH
60℃
95℃
±0.008pH
±0.02pH
1
CO2
54
NIST
60℃
by T&C Technical
5
pH9.21
pH10.01
pH
NIST
PTB
pH Values ofNIST
Standard Buffer
Solutions
pH
ac cording to NIST
NBS
NBS
A
Code
Temp. Potassium
tetraC
oxalate
1,668
1,670
1,672
1,675
1,679
1,683
1,688
1,691
1,694
1,700
1,707
1,715
1,723
1,743
1,766
1,792
1,806
0
5
10
15
20
25
30
35
38
40
45
50
55
60
70
80
90
95
B
Potassium
hydrogen
tartrate
H
C
Potassium Potassium
dihydrogen hydrogen
phthalate
citrate
3,863
3,840
3,820
3,802
3,788
3,776
3,766
3,759
3,755
3,753
3,750
3,749
3,750
3,753
3,763
3,780
3,802
3,815
3,557
3,552
3,549
3,548
3,547
3,547
3,549
3,554
3,560
3,580
3,609
3,650
3,674
4,010
4,004
4,000
3,999
4,001
4,006
4,012
4,021
4,027
4,031
4,043
4,057
4,071
4,087
4,126
4,164
4,205
4,227
D
F
Phosphate
Borax
6,984
6,951
6,923
6,900
6,881
6,865
6,853
6,844
6,840
6,838
6,834
6,833
6,834
6,836
6,845
6,859
6,877
6,886
7,534
7,500
7,472
7,448
7,429
7,413
7,400
7,389
7,384
7,380
7,373
7,367
-
9,464
9,395
9,332
9,276
9,225
9,180
9,139
9,102
9,081
9,068
9,038
9,011
8,985
8,962
8,921
8,885
8,850
8,833
B
H
C
D
E
I
G
55
I
G
Sodium carbonate/ Calcium
sodium hydrogen hydroxide
carbonate
10,317
10,245
10,179
10,118
10,062
10,012
9,966
9,925
9,903
9,889
9,856
9,828
-
13,423
13,207
13,003
12,810
12,627
12,454
12,289
12,133
12,043
11,984
11,841
11,705
11,574
11,449
-
Page 35
A
F
E
Phosphate
/
by T&C Technical
Page 36
Stability Comparison of HAMILTON Alkaline Buffer to Conventional Alkaline Buffer
Both buffer solutions have been exposed to blowing air
9,3
9,2
Page 36
pH
Stability Comparison
of HAMILTON Alkaline Buffer to Conventional Alkaline Buffer
9,1
Both buffer solutions have been exposed to blowing air
9,3
9,0
HAMILTON DURACAL
Buffer 9,2
pH9.2
9,2
Conventional Buffer
pH
8,9
-12
9,1 0
9,0
12
24
36
Hours
48
60
72
HAMILTON DURACAL Buffer 9,2
Conventional Buffer
Used buffer solutions should always
be discarded
and never be returned to their original storage
8,9
60 storage
12
24
36
48
72
bottle. For this reason HAMILTON -12
have0 developed
a unique
bottle which includes a
Hours
calibration compartment with a non-return valve at the bottom, preventing the used buffer
(CALPAK)
solution to be returned into the storage
bottle. This CALPACK bottle is practical: no additional
Used
buffer
solutions
should
be discarded andonly
neverthe
be returned
to their
original
calibration container is required, andalways
it is economical:
required
amount
ofstorage
buffer solution
bottle. For this reason HAMILTON have developed a unique storage bottle which includes a
1
is used.
calibration compartment with a non-return valve at the bottom, preventing the used buffer
solution to be returned into the storage bottle. This CALPACK bottle is practical: no additional
The Practicality of the HAMILTON CALPACK Calibration Bottle
calibration
container is required, and it is economical: only the required amount of buffer solution
2
is used.
The Practicality of the HAMILTON CALPACK Calibration Bottle
56
by T&C Technical
pH
pH
(
(NIST/PTB)
1.09
±0.02pH
60
1.68
±0.02pH
60
2.00
±0.02pH
60
3.06
±0.02pH
60
4.01
±0.01pH
60
5.00
±0.02pH
60
6.00
±0.02pH
60
7.00
±0.01pH
60
8.00
±0.02pH
60
9.21
±0.02pH
60
10.01
±0.02pH
60
11.00
±0.05pH
24
12.00
±0.05pH
24
3.9.2
(
)
pH
pH
pH
pH
57
)
by T&C Technical
mV
pH
pH
pH
3.9.2.1
1)
2
pH
1
pH
Page 38
pH7
3.9.2.1 Calibration of Analogue pH Meters
1
pH
1) Two buffer solutions of different pH values have to be selected. The pH value of one buffer
solution should be as close as possible to the zero point of the electrode assembly, which is,
under normal circumstances, pH 7.
2pH should have a pH value which lies as near as possible at the end
The second buffer solution
of the anticipated measuring range and the value should differ as much as possible from the
zero3buffer solution, at least 2 pH units. 2
3
If the measuring range covers the acidity and the alkalinity range then a third buffer solution is
2pHsolution should lie in the acid region and the third
required. The pH value of the second buffer
buffer solution should lie in the alkaline region of the desired measuring range. Both values
should differ as much as possible.
pHpH Scale
0
1
2
3
4
5
6
1 I
measuring range
7
8
9
10
11 12
13 14
measuring range
II
2
Slope
Calibration
Zero
Calibration
Verifying
HAMILTON Buffer Solutions
2) The pH value of any buffer solution is temperature dependent. Therefore the temperature of
the applied buffer solution has to be measured in order to establish the correct pH value of the
buffer solution.
58
The temperature adjustment potentiometer (manual temperature compensation) must be set
to the buffer temperature. With automatic temperature compensation the temperature sensor,
connected to the pH measuring instrument, must be immersed into the buffer solution.
Slope
Slope
Calibration
Calibration
2)
Zero
Zero
Calibration
Calibration
Verifying
Verifying
by T&C Technical
HAMILTON
Buffer
Solutions
HAMILTON
Buffer
Solutions
pH
pH
The
value
of any
buffer
solution
is temperature
dependent.
Therefore
temperature
2) 2)
The
pHpH
value
of any
buffer
solution
is temperature
dependent.
Therefore
thethe
temperature
of of
(
)
the
applied
buffer
solution
has
to
be
measured
in
order
to
establish
the
correct
pH
value
of
the applied buffer solution has to be measured in order to establish the correct pH value of thethe
buffer
solution.
buffer
solution.
The
temperature
adjustment
potentiometer
(manual
temperature
compensation)
must
The
temperature
adjustment
potentiometer
(manual
temperature
compensation)
must
be be
setset
to the
buffer
temperature.
With
automatic
temperature
compensation
temperature
sensor,
to the
buffer
temperature.
With
automatic
temperature
compensation
thethe
temperature
sensor,
connected
to the
measuring
instrument,
must
immersed
buffer
solution.
connected
to the
pHpH
measuring
instrument,
must
be be
immersed
intointo
thethe
buffer
solution.
Manual
Temperature
Compensation
Manual
Temperature
Compensation
Temperature
Dependency
Temperature
Dependency
of of
Technical
buffer
Solutions
Technical
buffer
Solutions
7,00
pH pH
7,00
Uis
Uis
zero
pH
7,00
oC
zero
o
C
o
C
slope
pH
4,01
pH
7,00
5
10
15
18
20
22
25
30
35
40
45
50
slope
pH
4,01
pH
pH 10,01
10,01
3)
oC
HAMILTON
Buffer
Solutions
HAMILTON
Buffer
Solutions
54,014,01
104,004,00
154,004,00
184,004,00
204,004,00
224,004,00
254,014,01
304,014,01
354,024,02
404,034,03
454,044,04
504,054,05
7,097,09
7,067,06
7,047,04
7,037,03
7,027,02
7,017,01
7,007,00
6,996,99
6,986,98
6,976,97
6,976,97
6,976,97
10,19
10,19
10,15
10,15
10,11
10,11
10,08
10,08
10,06
10,06
10,04
10,04
10,01
10,01
9,979,97
9,929,92
9,869,86
9,839,83
9,799,79
pH7(0mV)
1
Page 39
3) After rinsing the electrode assembly with de-ionised water, it is immersed into the first buffer
solution for the zero point compensation (normally pH 7 = 0 mV). The electrode assembly has
@pH
pHas
to stay in the buffer solution for a short time, at least for one minute, till the indication of the pH
meter settles near the pH value of the buffer solution (pH 7) and becomes stable. Thereafter
pH
the indicated value has to be adjusted with the zero potentiometer to the exact value of the
buffer solution. The zero potentiometer is sometimes marked @pH or pHas.
For analogue pH meters it is important that the zero point calibration is always
performed
4) before the slope calibration.
4) After the zero point calibration, the electrode
assembly has to be rinsed again with deionised water and dried with tissue paper.
When drying the electrode, care must be
taken not to rub the membrane, i.e. only dab
the electrode with tissue paper. Under no
circumstances must an electrode be rubbed.
This might introduce static electricity into the
glass shaft of the electrode and could make
an accurate pH measurement impossible for
hours.
No rubbing!
59 second buffer solution (acid or base –
5) The electrode assembly is then immersed into the
according to the measuring range). After the indication of the pH meter has stabilised, the
indicated value has to be adjusted with the slope potentiometer to the exact pH value of the
second buffer solution. The slope potentiometer is sometimes marked mV/ pH.
Page 39
by T&C Technical
3) After rinsing the electrode assembly with de-ionised water, it is immersed into the first buffer
solution for the zero point compensation (normally pH 7 = 0 mV). The electrode assembly has
to stay in the buffer solution for a short time, at least for one minute, till the indication of the pH
meter settles near the pH value of the buffer solution (pH 7) and becomes stable. Thereafter
the indicated value has to be adjusted with the zero potentiometer to the exact value of the
buffer solution. The zero potentiometer is sometimes marked @pH or pHas.
5)
( zero point calibration is always
For analogue
pH meters it is important that the
performed before the slope calibration.
)
pH
4) After the zero point calibration, the electrode
assembly has to be rinsed again with deionised water and dried with tissue paper.
When drying the electrode, care must be
taken not to rub the membrane, i.e. only dab
the electrode with tissue paper. Under no
circumstances must an electrode be rubbed.
This might introduce static electricity into the
glass shaft of the electrode and could make
an accurate pH measurement impossible for
hours.
ΔmV/ΔpH
No rubbing!
6)
5) The electrode
assembly is then immersed into the second buffer solution (acid or base –
according to the measuring range). After the indication of the pH meter has stabilised, the
indicated value has to be adjusted with the slope potentiometer to the exact pH value of the
second buffer solution. The slope potentiometer is sometimes marked mV/ pH.
It is advisable to re-check the zero point after the slope calibration has been performed.
It is of utmost importance that the above calibration sequence must be adhered to,
otherwise no valid calibration will be obtained.
7)
6) If the measuring
range covers both the acidity and the alkalinity range the third buffer solution
serves as a verifying solution. After the zero point (with the first buffer) and the slope
calibration (with the second buffer) has been accomplished, the calibration over the entire
measuring range has to be verified with the third buffer solution (be aware of acid and alkaline
errors). 8)
7) In order to keep the temperature error as low as possible, especially
errors due to the diffusion potential and the isotherm intersection point,
it is recommended that the calibration be executed at the temperature
at which the actual pH measurement will be performed.
8) After the successful calibration the used buffer solution should be
discarded. Never re-use buffer solution and never return it to its
original storage container.
6)
7)
Technic
al Buffer Solution
pH 4,0 1
60
by T&C Technical
3.9.2.2
pH
pH
pH
pH7
1
pH
2
pH
pH
pH
3.9.2.1
61
by T&C Technical
3.9.3
pH
a)
pH
b)
c)
d)
e)
Page 41
The Frequency of re-calibration has to be established experimentally
Ma y
2002
C alib
ra
pH ! te
Calib
ra
pH ! te
Calib
ra
pH ! te
Calib
ra
pH ! te Calibr
a
pH ! te
Calib
ra
pH ! te
Calib
ra
pH ! te
3.9.4
3.9.4 Response Time
pH
pH
If a pH electrode
assembly is immersed into a buffer solution,
it does not show the pH value of
that buffer solution instantaneously. A response time of approximately 30 seconds is normal in
pH the buffer value with a tolerance
30 of 0,01
0.01pH(0.6mV)
order to reach
pH units (0,6 mV). For this reason it is
very important to wait long enough during the calibration cycle for the indication of the pH meter
to become stable. Only then can the indicated value
62 be adjusted to the buffer value. A premature
interruption of the electrode adaptation to the buffer value is often a major source for incorrect pH
measurements.
The response time of an electrode assembly is especially slow if the electrode temperature and
by T&C Technical
pH
pH
pH
10℃
3mol/L
pH
0.01pH
63
KCl
by T&C Technical
3.9.5
/
pH
3
1
2
3
(3
)
a)
b)
c)
d)
(
)
e)
f)
g)
10℃
h) pH
pH
pH7
0mV
pH7
i)
pH
64
by T&C Technical
3.10
pH
pH
pH
1
pH
pH
pH
pH
100kPa
65
by T&C Technical
1012Ω
pH
±0.03pH
±0.05pH
±0.02pH
4.
pH
pH
pH
4.1
pH
pH
)
66
(
by T&C Technical
4.1.1
48
67
Page 44
If a measuring electrode has to be stored for short periods between measurements it should be
immersed in a container filled with tap water or be fitted with a watertight plastic cap filled with tap
water. Therefore one should not throw the supplied plastic caps away. They should
kept for
by T&C be
Technical
re-use.
4.1.2Storage of Reference Electrodes
4.1.2 The
Reference electrodes should always be stored wet,
i.e. the diaphragm has to be covered with the same
reference electrolyte with which the reference
electrode has been filled. Wet storage must also be
applied when the electrode is stored for a long time.
It is not advisable to store reference electrodes dry as
3mol/L
the reference electrolyte will
slowly KCl
penetrate through
the diaphragm and crystallise and subsequently cover
the complete bottom of the electrode. The
crystallisation itself does not cause a problem but the
reference electrode might dry out completely which
will result in a substantial increase in the diaphragm
resistance. Even when the reference electrode is
refilled with its respective electrolyte, the high
diaphragm resistance will not disappear immediately
and will result in large measuring errors or even make
a measurement totally impossible.
Closed refill aperture
KCl
3 mol/l
or
(KClRubber
3mol/L)
plastic cap
filled with
reference electrolyte ( 3 mol/l KCl)
Therefore for short or long periods of time it is essential to store reference electrodes in their
respective reference electrolyte, with the refill aperture closed by a suitable stopper.
Storage in tap water or in distilled water should be avoided. Any penetration of these liquids
through the diaphragm will increase the diaphragm potential considerably and will significantly
falsify the subsequent pH measurement.
4.1.3 The Storage of Combination Electrodes
A combination electrode consists of a measuring electrode and a reference electrode combined
into one single-rod electrode. The storage conditions must therefore be suitable for a
measuring and for a reference electrode. As every reference electrolyte
is an aqueous
pH
solution it has been found that the optimum storage liquid is the respective reference electrolyte
of that combination electrode. The refill aperture has to be closed during storage time.
All that has been stated about the storage of a reference electrode applies equally to the storage
of a combination electrode.
Gel-filled combination electrodes are an exception to the rule. These electrodes have no refill
4.1.3
aperture and the drying-out of their diaphragm has to be avoided at all costs. Therefore gel-filled
combination electrodes must be stored wet in a 3 mol/l KCl solution.
This statement applies to polymer electrodes as well.
68
by T&C Technical
3mol/L
KCl
4.2
pH
a)
b)
c)
pH
pH
pH
1
0.1mol/L
HCl
69
by T&C Technical
1%
0.1mol/L
HCl
(
)
/
0.1mol/L HCl
0.1mol/L NaOH
2
(3mol/L KCl)
1
70
12
by T&C Technical
PN 238290
A
(<3%)
B
4%
500mL
500mL
3mol/L KCl
500mL
4.3
71
by T&C Technical
5.
pH
pH
5.1
5×109Ω
pH
pH
pH
(1012Ω)
1014Ω
1m
1017Ω
pH
150pF/m
200pF/m
pH
50
64pF/m
50m
pH
102pF/m
pH
pH
+80℃
70℃
pH
130℃
5.2
72
-30℃
by T&C Technical
5.3
5.4
pH
107Ω
pH
pH
73
5.3 Combination
Electrode
Connection
Cable
voltages which
might be created
when moving
the coaxial cable. It is of importance that this black
layer must be carefully removed when preparing the cable ends for connection to the electrode
What has been said about the cable requirements for a separate measuring electrode must also
plug and/or to the measuring instrument. If the black layer is not removed, it will cause a
be observed for the connection of a combination electrode. It is advisable to use a double
short circuit between the internal conductor and the copper screen. When stripping the
screened coaxial cable (triax cable). To save costs it is normal practice to connect the inner
coaxial cable, tools and hands of the technician should always be dry and clean. After stripping,
screen of the coaxial cable to the reference electrode part of the combination electrode.
the cable ends should be cleaned with alcohol or ether, which can be
with a cloth or a
by done
T&C Technical
brush, both of which should be absolutely clean. Touching the stripped cable ends with wet or
5.4 Cable
Preparation
and
Cable
fat-stained
fingers will
reduce
the Routing
insulation resistance down to, or below, 107 ohms, which will
result in a short circuit of the high resistance measuring chain – a pH measurement becomes
The insulation of the internal conductor of a pH connection cable is not only screened with
impossible.
copper wire mesh but also with an additional black semiconductor layer. This layer suppresses
voltages which
might
be created
moving the
coaxial
is taken
of importance
that this
When
routing
the pHwhen
connection
cable
carecable.
must Itbe
not to route
theblack
pH connection
layer must be
carefully
removed
when cables.
preparingParallel
the cable
ends
connection
cable
parallel
to power
situated
power
cables tointhe
theelectrode
vicinity of the pH
pHfor
plug and/orconnection
to the measuring
theelectro-magnetic
black layer is not
removed,(induction),
it will cause
a must be
cables instrument.
would leadIf to
interference
which
short circuit
between
internal
conductor
screen. When
stripping
the always be
avoided
at
allthe
cost.
The outer
screen ofand
the the
triaxcopper
cable
electrode)
should
50m
pH (combination
coaxial cable,
tools on
andone
hands
the In
technician
always
be dry and
After
stripping,
earthed
sideof
only.
principle,should
every pH
connection
cableclean.
should
be as
short as possible,
the cable ends
shouldunder
be cleaned
with alcohol
or ether,
which
can be done with a cloth or a
but should
no circumstance
be longer
than
50 metres.
brush, both of which should be absolutely clean. Touching the stripped cable ends with wet or
pH connection cables cannot be buried straight into the ground. If this has to be done these
fat-stained fingers will reduce the insulation resistance down to, or below, 107 ohms, which will
cables must be installed in a metal or plastic conduit.
result in a short circuit of the high resistance measuring chain – a pH measurement becomes
impossible.
Triax cable
Coaxial cable
PVC
PVCexternal
PVC
PVC external
insulation
insulation
When routing the pH connection cable care
must be taken not to route the pH
connection
insulation
screenParallel situated power cables inscreen
cable parallel to power cables.
the 1vicinity of the pH
insulation
connection cables PE
would
lead to electro-magnetic interference
(induction), which must be
PE insulation
avoided at all cost. The outer screen of the triax cable (combination electrode) should always be
earthed on one side only. In principle, every pH connection cable should be as short as possible,
screen 2
black
internal
but should under no circumstance
be semiconductor
longer than 50 metres.
layer
conductor
internal
black semiconductor
conductor
pH connection cables cannot be buried straight into the ground.
If this has tolayer
be done these
cables must be installed in a metal or plastic conduit.
Coaxial cable
PVC external
insulation
Triax cable
PVCPVC
1
screen
PE insulation
internal
conductor
insulation
PVC
PVCexternal
insulation
screen 1
PE insulation
black semiconductor
layer
screen 2 2
internal
conductor
74
black semiconductor
layer
by T&C Technical
5.5
pH
2
1
1m
Page 48
5.5 Plug or Cable?
pH
pH in two configurations: either with
Various electrode manufacturers supply their pH electrodes
an integrally installed cable (normally 1 metre in length), or with an electrode plug
connection. Both configurations have their advantages and disadvantages.
Page 48
If the cable is installed to the electrode by the manufacturer, the customer can be assured that
the cable connection is water tight and measuring faultspH
attributed normally to the electrode/cable
Various
electrode
manufacturers
supply their
pH electrodes
two configurations:
either hand
with however, when
connection
(short
circuit, moisture
ingress)
can beinruled
out. On the other
anthe
integrally
installed
cable
(normally
1
metre
in
length),
or
with
an
electrode
plug
pH electrode assembly has to be replaced (remember: a pH electrode is a consumable item
connection. Both configurations have their advantages and disadvantages.
with a certain life expectancy), the cable also has to be re-purchased.
5.5 Plug or Cable?
If the cable is installed to the electrode by the manufacturer, the customer can be assured that
An
electrode
connection
eases faults
the attributed
electrode
removal
or replacement during the
the
cable
connectionplug
is water
tight and measuring
normally
to the electrode/cable
maintenance
periodmoisture
and is ingress)
more economical
in the
runhand
(saving
of cable
connection
(short circuit,
can be ruled out.
On long
the other
however,
when cost). However,
the
pH electrode
assembly
has
toalways
be replaced
(remember:
aelectrode
pH electrodesocket
is a consumable
item
care
must
be
taken
to
connect
the
firmly
to the cable plug,
pH
pH
with a certain life expectancy), the cable also has to be re-purchased.
otherwise moisture, the biggest enemy to the electrode/cable connection, might penetrate
Anthe
electrode
plug connection
electrode
replacement
during pH
the measurement is
socket/plug
coupling.eases
Once the
moisture
hasremoval
enteredorthis
joint, a reliable
maintenance
period and is more economical in the long run (saving of cable cost). However,
no longer possible.
care must be taken to always connect the electrode socket firmly to the cable plug,
otherwise moisture, the biggest enemy to the electrode/cable connection, might penetrate
the socket/plug coupling. Once moisture has entered this joint, a reliable pH measurement is
Electrode socket – cable plug
no longer possible. Electrode head
with integrated
cable
pH
and
Electrode head
different instrument plugs
connection
Electrode socket – cable plug
connection
with integrated cable
and
different instrument plugs
75
by T&C Technical
pH
pH
pH
pH
pH
Theory is necessary, Experience is essential.
By Hamilton, pH Electrode system R&D team
76
by T&C Technical
pH
77
pH 測定と電極の選び方
6．参考資料
pH
付属参考文書
150322_E-J
by T&C Technical
水素イオン濃度
pH という用語を定義する前に、その化学的物理的構造について述べておく必要があります。その中心的な
存在は二つの水素原子（H2）と、1 つの酸素原子で構成される水についてです。
6.1
原子の構造
pH
pH
古代ギリシャの哲学において、原子という言葉の意味は、物質のこれ以上小さくできない大きさとされて
いました。この基本的な微粒子、現代においてこの概念に当たる用語として用いられるもの、はこれ以上
分解しないものとして考えられていました。事実、ギリシャ語の原子は「これ以上分けられないもの」と
いう意味を持ちます。
その寸法と原子というものについての知識は、数世紀にわかりゆっくりと思索され続けましたが、その内
容は、まだ単にそれを推測するだけにとどまっていました。
pH
それに変化が現れるのは、16 世紀から 17 世紀における実験的科学の発展により、原子理論の進歩が急速
に進むようになりました。その中で化学者は、すべての液体、気体そして固体は、究極の構成元素あるい
は元素に分析されると認識しだしました。
pH
例えば塩は二つの異なった違いのある元素で構成されていることを見つけました。ナトリウムそして塩素
Theory is necessary, Experience is essential.
は、現代では化学合成として知られるように、相互に深い絆で結ばれています。
By Hamilton, pH Electrode system R&D team
原子は、1 つの元素の建築ブロック
あらゆる元素のすべての原子は化学的に同じ振る舞いをします。このように、化学視点から、原子とは考
慮される最も小さな実体ということになります。元素の化学的特徴は大きな違いがあります。それらの原
子は、多数の化学合成物を構成するための多数の異なる方法により結合しています。それは今日において、
112 の元素として知られています。
1911 年、イギリスの物理学者アーネスト・ラザフォード（1837-1937）は原子構造の理論を組織的にまと
め上げました。それは原子を、電子の雲によって囲まれる高密度原子核として最初に視覚化したものでし
た。
ラザフォードは、
「原子の質量はその原子核に集中している。原子核には電気の正電荷がある。一方
電子に
は、おのおの負電荷がある。」としました。電子により生じる変化の量は、原子核にある正電荷と同じ電気
の量になります。このため通常の原子の電気的状態は中性になります。ラザフォードは原子核を構成する
その微粒子を陽子と呼びました。このラザフォードの原子の捉え方は、1913 年、ニール・ボーア（1885-1962）
76
1
pH 測定と電極の選び方
付属参考文書
150322_E-J
by T&C Technical
により見直されることになります。（次章 ボーアの原子）
pH
1932 年、イギリスのジェームズ・チャドウイック（1891-1974）は、現在中性子として知られている原子
の原子核にある陽子以外の他の微粒子が存在することを発見しました。それは陽子と同じ質量を持ってい
るのですが、電荷を持っていません。この発見により、原子核は陽子と中性子で構成されていることを知
ることになりました。
「すべての原子において、陽子の数は電子の数と同じである。ゆえに、それは原子の
原子番号となる。」（元素の周期律表における原子の位置）
pH
6.2
pH
ボーアの原子
原子の構造を説明する前に、デンマークの物理学者、ニールズ・ヘンドリック・デビッド・ボーアにより
1913 年に「ボーア
原子の理論
The Bohr Theory of the Atom」として知られる仮説が提唱されたことを
知っておく必要があります。彼は、原子核からかなりの距離まで、電子は定められた殻、量子準位に配置
されていると仮定していました。これら電子の位置は「電子配置」と呼ばれます。
pH
ボーアの原子
pH
電子の数は原子の原子番号と等しい：水素は 1 つの殻に一つの電子、酸素は 8 つ、そしてウランは 92 を
Theory is necessary, Experience is essential.
持ちます。電子の殻は第 1 の殻から第 7 の殻まで規則正しく並んでいます。それぞれの核は収納できる電
子の数に上限があります。殻は内側の殻から外側の殻まで、K
殻、L 殻から Q 殻と名付けられています。
By
Hamilton, pH Electrode system R&D team
K 殻は二つの電子まで持てます。L 殻は最大 8 つの電子
まで満たすことができます。M 核は 18 の電子、そして
それを一般化すれば、n 殻は 2n の電子で満たすことが
2
できることになります。唯一外側のお殻における電子の
数は、原子の化学的な振る舞いにより決定されます。
原初の殻は、殻毎に
間なく電子で埋め尽くす必要はあ
りません。元素の周期律表にある最初から 18 番目の元
素までは、電子は規則正しく増加していき、それぞれの
殻は新しい殻が始まる前まで制限いっぱいの電子で満
たされています。
76
2
pH 測定と電極の選び方
付属参考文書
150322_E-J
by T&C Technical
19 番目の元素からは、最も外殻の電子はそれより内側の殻の電子が完全に満たされている状態で始まりま
pH
す。規則性はまだ維持されています。しかし電子は満たされている内側の殻との間を行ったり来たりしだ
します。この原子量を増加させるためのその化学的性質の定性的な繰り返しは、元素の周期律表の並び方
と一致する結果になります。
pH
pH
pH
pH
Theory is necessary, Experience is essential.
6.3
元素の周期律表
By Hamilton, pH Electrode system R&D team
1869 年、ロシアの化学者ディミトリ・イワノビッチ・メンデレーエフ（1834-1907）は当時知り得たすべ
ての元素の質量を表にまとめました。そうすることで、彼はある元素の性質はそれ自身周期的に繰り返す
ことを発見しました。それによりメンデレーエフは似たような化学的挙動を持つ元素をまとめ、それぞれ
順番に行の下にまとめていきました。この元素の配置表は周期律表と呼ばれています。
年数を重ねることでより多くの元素が発見され、周期律表は数度にわたり配置し直されていきました。そ
の表、今日私たちが知っているものは次のページに示されている通りです。元素は昇順の原子番号（原子
核にある陽子の数の順番）で水平に 7 行にわたり配置されています。それぞれの行は原子の 7 つの電子殻
1 つを表します。1 番目の行の一番目にある水素は最も軽い元素になります。目下の一番重い元素、表の最
後の元素はウンウンビウム（放射性超ウラン元素）で、原子質量 277、112 番目になります。なお表には
全部で 118 の元素が載っています。列のグループは、元素を化学的な動きにより 18 に分けています。こ
れは最も外側の殻の電子の数に従っています。
76
3
pH 測定と電極の選び方
付属参考文書
150322_E-J
by T&C Technical
pH
pH
pH
pH
pH
Theory is necessary, Experience is essential.
By Hamilton, pH Electrode system R&D team
76
4
pH 測定と電極の選び方
付属参考文書
150322_E-J
by T&C Technical
pH
6.4
分子
分子とは、ある物質が同じ化学的性質を持つ化合物の最も小さい単位です。
水の分子は 1 つの酸素原子と二つの水素原子を電気的な力、いわゆる化学結
合により構成されています。
フランスの化学者アントワーヌ＝ローラン・ド・ラヴォアジエ（1743-1794）
pH
pH
により、古代の哲学者が考えていた「水は基本的な元素」、がそうではないこ
とを証明しました。それは酸素と水素の化合物であり、現代の表現では H2O
という式で表されます。
分子はともに 2 つの電子を分かち合うことで保たれています（共有結合）。この結合を最大化するために、
原子は相互に特定の位置を取ります。例えばそれぞれの分子はそれ自身定められた形を持ち、単純に見る
と、水分子内の水素原子は酸素原子に対し
104.5 の角度でくっついています。そのため水は水素の電子が
pH
大きな酸素原子の原子核に向かってわずかに囚われている状態、双極子モーメントを持っています。それ
に対し、二酸化炭素 CO2 は酸素原子が直線に配置され、それゆえ双極子モーメントを持ちません。
pH
Theory is necessary, Experience
is essential.
またある元素は他の元素と化合することはありません。
これらは希ガス
のグループとなります。これらの原子は 2 つの電子（He）あるいは外
By Hamilton, pH Electrode system R&D team
殻に 8 つの電子（Ne、Ar、 Kr、Xe、Rn）を持ちます。8 個以下の電子
を外殻に持つ元素は、他の活性元素と結合することになります。
原子が作れる化合の数は原子価と呼ばれます。酸素は原子価が 2 となりますが、それは 6 つの電子をその
外殻に持ち、そこを 8 つにするために 2 つの電子を必要としていることを意味します。水素は 1 つの電子
をその唯一の外殻に持っています。そのため原子価は 1 となります。電子殻を充足させるため他の電子を
獲得するか、もしくは 1 つ電子が足りない原子に 1 つ電子を与えることができます。このため 2 つの電子
を必要とする酸素原子の場合、2 つの水素原子は酸素原子の足りない電子の需要を満たし、一緒になるこ
とで水分子を作ることになります。
76
5
pH 測定と電極の選び方
付属参考文書
150322_E-J
by T&C Technical
pH
ナトリウム元素と塩素元素がくっついて塩（NaCl：食卓塩）を作る場
合、この結合はイオン結合となります。中世のナトリウム原子、それ
は 1 つの電子を外殻に持ちますが、それを 7 つの電子を外殻に持つ塩
素原子と共有します。再びですが、この結合の場合、外殻は 8 つの電
子で満たされます。
pH
pH
この 1 つの電子の変化の過程は両方の原子の電気的平衡状態を放り出
すことになります。ナトリウム原子は正の電荷を持つようになり（1
つの電子を失う）Na となり、そして塩素は 1 つの電子を受け取り負の
+
電荷を持つようになります（Cl ）。新しいナトリウムイオンの殻の構造
-
は、ネオン原子のものに似ており、塩素イオンの新しい殻構造はアル
ゴン原子のものに似ています。両方のイオンは互いにそれらのクーロ
pH
ン引力により保たれます。
6.5
イオン
pH
1 つのイオンは中性の原子が
1 つあるいは複数の電子を獲得するあるいは失うときに作られる
1 つの粒子
Theory
is necessary, Experience
is essential.
です。1 つの電子を失った 1 つの原子は、カチオンと呼ばれる正の電荷を持つイオンになります。電子を 1
By Hamilton, pH Electrode system R&D team
つ得た 1 つの原子は、アニオンと呼ばれる負の電荷を持つイオンになります。イオンという単語はギリシ
ャ語から派生し、
「旅人」を意味します。電場（電界）の影響の元、イオンはそれとは逆の極性を持つもの
へと移動（旅）します。そしてその結果、気体や液体に導電率を発生させます。
もし NaCl のイオン結合が高温あるいは水に溶けることで壊されると、塩素原子は獲得した電子を保ち、そ
して負の電荷を持つイオンとして存在します。ナトリウムイオンは逆に正の電荷（電子を失う）を持つイ
オンとして存在します。
6.6
解離
水は 3 つの化合物のグループ、塩、酸、そして塩基にとって優れた 1 つの溶剤です。水にこれらの化合物
を入れると、これらの化合物はイオンへと解離します。NaCl を水に入れると、水分子の極の作用は負と正
の電荷を持つイオンの間で静電引力が減少し、それらのイオンを引き離すようになります。このためイオ
ンの再結合を妨げるように働き、それらのイオンは水の分子に囲まれた状態になります（水和される）。
HCl（塩酸）も同じように水の中で解離し、H と Cl イオンになります。NaOH（水酸化ナトリウム）も同
+
-
76
6
pH 測定と電極の選び方
付属参考文書
150322_E-J
by T&C Technical
じように解離し Na と、OH イオンになります。
+
-
pH
水の中の塩の解離、酸と塩基の解離は水に対し優れた導電性を生じます。このように発生した溶液を電解
液と呼びます。
pH
pH
pH
もし二つの電極が電解液に差し込まれ電解液と電位の違いがこれらの電極に発生すると、負の電荷を持つ
pH
イオンは正の電荷を持つ電極（アノード）により捕らわれ、そしてアノードに達することでそれらの電荷
Theory
is necessary,
Experience
is essential.
を失います
（電子を失う）
。これによりアニオンと名付けられています。
同じように負の電荷を持つ電極（カ
ソード）はその電荷は電子を受け取ることで失います（カチオン）。
By Hamilton, pH Electrode system R&D team
6.7
酸̶塩基̶塩
化学において、私たちは三つの電解液、酸、塩基そして塩に分類してきました。
酸：
水の中に溶かすと水素イオン濃度 H+（プロトン）を発生し、それは純水の持つ量よりも高く
なります。従って酸はプロトンドナー（陽子供与体）です（プロトン＝正の電荷を持つ水素イ
76
7
pH 測定と電極の選び方
pH
付属参考文書
150322_E-J
by T&C Technical
オン）。
酸はその味は酸っぱくそしてリトマス試験紙を赤くします。リトマス試験紙は溶液が酸なのか
塩基なのかを判別するために用いられる最も古い、そして最も使われるものです。それは地衣
類、ある真菌の作り出した植物性有機物、そしてそれと共生しているある藻から分離された桃
色の色素です。
最も知られている酸：
pH
pH
塩酸
HCl
消化液を構成している成分
硝酸
HNO3
色素と爆薬で使用されます。
酢酸
CH3COOH
酢
蟻酸
HCOOH
染色と製革で使用されます。
硫酸
pH
H2SO4
電池
リン酸
H3PO4
歯科用セメント、化学肥料
pH
塩基：
水の中に溶かすとヒドロキシルイオン濃度 OH-（プロトンアクセプター）を発生し、プロトン
Theory is necessary, Experience is essential.
を受け止めます。塩基は苦味を持ちリトマス試験紙を青に変色させます。塩基はぬるぬるした
感じになります。最も一般的な塩基は以下の通りです。
By Hamilton,
pH Electrode system R&D team
塩：
水酸化ナトリウム
NaOH
排水、オーブンクリーナ
水酸化カルシウム
Ca(OH)2
建築用モルタルに使われています。消石灰
水酸化アルミニウム
Al(OH)3
アルミニウム化合物を作る原料
水酸化カリウム
KOH
軟石鹸
水酸化マグネシウム
Mg(OH)2
マグネシア乳（緩下剤）
アンモニア
NH3
家庭用クリーナー
酸と塩基の水溶液が混ざった時、中性化する反応が発生します。この反応は瞬く間に進行し、
一般的には水と塩を作ります。例えば硫酸と水酸化ナトリウムは水そして硫酸ナトリウムにな
ります。
例：
76
8
pH 測定と電極の選び方
pH
6.8
硝酸ナトリウム
NaNO3
=
Na + NO3
硫化アルミニウム
Al2(SO4)3
=
2Al
リン酸カルシウム
Ca3(PO4)2 =
+
3+
3Ca
-
+ 3SO4
2+
付属参考文書
150322_E-J
by T&C Technical
2-
+ 2PO4
3-
モル（mol）
pH
モルは、0.012Kg（12g）の炭素pH中に存在する原子の数と等しい要素粒子を含む系の物質量の SI 単位で
12
す。構成する要素粒子は原子、分子、イオン、電子、あるいは他の微粒子と特定されなくてはならない。
本単位は国際的に用いられるために 1971 年に定められた。（Oxford Dictionary）
炭素 12（他の物質を測定する際の対象としての原子）の 12g に含まれる元素の粒子の数は 6.0221367 x 10
23
pH
です。この数字はアボガドロ数として知られ、イタリアの物理学者アメデオ・アボガドロ（1776-1856）
の功績を称え名付けられています。1811 年、アボガドロは、温度、圧力の等しい体積のガスは、同数の分
子含むと仮定しました。1 モルはある物質の原子量が等しいもので、グラム重量で表されます。
pH
1mol の H2
Theory is necessary, Experience is essential.
1mol の H2O
＝
2g
＝
18g
By Hamilton, pH Electrode system R&D
team
1mol
の Cl2 ＝ 71g
1mol の Rn
＝
222g
1mol の HCl
＝
36.5g
1mol の NaOH
6.9
＝
40g
水の中の水素イオン濃度
水は電解質を溶解、分離するだけでなく、それ自身、わずかな量の水分子は正の電荷を持つ水素イオン H+
と、負の電荷を持つ水酸化イオン OH-に分離しています。
+
H2O = H + OH
-
H+
＝酸の反応を持つ正の電荷を持つ水素イオン
OH-
＝アルカリ反応を持つ負の電荷を持つ水酸化イオン
76
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pH 測定と電極の選び方
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もし水素イオンの量が水酸化イオンの量と同じ場合、水は中性と呼ばれます。清浄な中性の水は 10 の水
7
pH
分子の 1 つだけが解離した状態です。
現実には水素イオンは 1 つのプロトンであり、1 つの電子を持つ水素イオンが水素イオン同士結合してい
ます。そのため一般化された酸と塩基の理論においては、最も単純な酸と塩基の中性反応としてみること
になります。
+
2H + 2e =H2
pH
pH
しかし実際は水素イオン、あるいはプロトンは水の中で自由に存在せず、水分子を伴っています。水のイ
オン化はそのため正確には以下のように書かれる必要があります。
+
2HOH = H3O + OH
-
水の中にある H3O はヒドロニウムイオンと呼ばれ、そのイオンは酸の性質を持ちます。この解離状態を単
+
pH
純化にするため、この等式は通常 H を用いて書かれていることになります。
+
pH
1 つの酸を中性の水に入れることで、H
イオン濃度は酸の解離により作られる H イオンによりその濃度は
+
+
上昇します。その水はその性質を変化させ、あたかも酢あるいはレモンジュースのように酸っぱくなりま
Theory is necessary, Experience is essential.
す。そしてそれは腐食性となり金属を分解します。
By Hamilton, pH Electrode system R&D team
1 つの塩基を中性の水に入れると、OH イオン濃度は塩基の解離により作られる OH イオンによりその濃度
-
-
は上昇します。それに対応する量の H イオンがありますが、それは減ることになります。水はその性質が
+
変わり、苦味があり濡れた石鹸のようにぬるぬるした感触を持ちます。
両方の場合において、私たちはそれを水と呼ぶことはできなくなり、それを「水溶液」として扱うことに
なります。
76
10
pH 測定と電極の選び方
付属参考文書
150322_E-J
by T&C Technical
pH
すべての酸と塩基の水溶液は、加えられた酸、塩基の化学的反応により、それらに対応した水素イオン H
+
と水酸化イオン OH の濃度に変化を与えます。水溶液内の水素イオン濃度は 1 つの水素イオンに対応する
-
解離していない水分子量により表現されます。
例
もし 1 つの水素イオン H が 100 の水分子のなかに存在した場合、それは 1:100 あるいは 1/10
+
pH
2
pH
あるいは 10 と書かれます。
-2
もし 1 つの水素イオンが 10,000,000 の水分子のなかに存在した場合、それは 1:10,000,000 あ
るいは 1/10 あるいは 10 と書かれます。
7
-7
さらにもし 1 つの水素イオンが 1000,000,000 の水分子のなかに存在した場合、それは
1:1000,000,000 あるいは 1/10 あるいは 10 と書かれます。
9
-9
pH
水の中では H イオンと OH イオンに解離したイオン物質が 22℃、10 (mol/L)では一定の状態で存在しま
+
-
-14
す。そのため、純水中の
H イオンと OH イオンの濃度が等しい時、H イオン濃度は 10 に必ずなり、OH
pH
+
-
+
-7
-
イオン濃度は同じように 10 になります。この状況は自動的に pH の定義へと導かれ、その結果 pH は以下
-7
Theory is necessary, Experience is essential.
のように表現されます。
By Hamilton, pH Electrode
system R&D team
水溶液中における活性水素イオン濃度のマイナスの常用対数
あるいは数学的表現では
1
pH = log
水素イオン濃度（mol/L）
76
11
pH 測定と電極の選び方
付属参考文書
150322_E-J
by T&C Technical
pH
最後に
pH 測定において大切な点は以上です。
「理論は必要です ‒ 経験は欠かせません」
Erich L.Springer
pH
pH
pH
pH
Theory is necessary, Experience is essential.
By Hamilton, pH Electrode system R&D team
76
12