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3.2 Calibration in Extended Measurement Range
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experimental uncertainty of the used instrument was ∼0.1 mV. Extremely erroneous points to
be excluded cannot be found. Perhaps, the first and the last points of the strong acid – strong
base titration curve might be excluded with the help of the operating panel. After omitting
these two points, the state of the work-window is shown in Figure 3.2. After clicking the
«save&quit» button, the program creates the result files and terminates. Three result files
have been created:
1. The kh2-12e.cin is the same as kh2-12e.tid except for two differences. (1) The .cin file
contains the fitted values of the parameters instead of the initial ones and (2) it includes
the changed weighting factors according to the modifications done via the operation
panel. The program uses the relative weighting factors for the exclusion/inclusion of
the experimental points. This topic is discussed in Subsection 2.2.1 where choosing the
titration point to be fitted is detailed.
The purpose of this result file is to save the modifications in the form of input data and
let the user carry out further evaluations by using this file. Since there are no further
calculations in the first example, this file has no significance at this moment.
2. The kh2-12e.res file contains the essential results of the fitting. The detailed discussion
of this file is given in Subsection 2.4.1 but for users familiar with curve fitting, the file itself
may seem self-explanatory. Maybe the names of the parameters inside pHCali should be
identified. These names can be found in the second part of Subsection 2.4.1 in detail.
3. The kh2-12e.crv file contains the measured and calculated curves. The interpretation of
the individual columns can be found in Subsection 2.4.2.
The calibration can be carried out with the pHm –Vm titration curves, as well. In this
case, the objective is the same but the values of Id and mp defined in equation (A.11) are
determined instead of E 0 and mE . The kh2-12ph.tid file (provided during the installation)
contains the input data needed. For this case, the look of the work-window is shown in
Figure 3.3. The files created during the calculation can be interpreted the same way as
potentials were measured.
3.2 Calibration in Extended Measurement Range
The second example demonstrates the capabilities of pHCali when the calibration also required outside of the Nernstian ranges. This example is the same as the reference example
throughout the guide so the experimental conditions of this measurement are described in
the introductory part of Chapter 2.
The program provides three methods for taking the deviation from the Nernstian behavior into account.
1. The user fits the values of JH and JOH together with the usual calibration parameters. It can
be done by changing the 0-s to 1-s in the 14th column of lines 6 and 7 in the kh1-13e.tid
file (see Listing 2.1). After saving these modifications, either (1) the command "PROGNAME
kh1-13e.tid" or clicking the icon of pHCali starts the program to calculate the required
values. If the program is started from icon then the appropriate file should be chosen from
the file-chooser window.
This method is the most simple one but it may most easily lead to wrong values so the
usage of this method is not suggested! The reasons are analyzed more deeply in Section 4.8
but as it can also be seen from the result of the fitting here (see the lower right graph of
Figure 3.4), this method leads to large and systematic deviations at high pH.
2. The calculations are carried out in several successive steps:
• The program should be started as in case of the first method. The experimental points in
the non-Nernstian ranges (typically the pHm .2 and pHm &12 values) should be omitted