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pH Value Calculation
pH value calculation by means of dual conductivity measurement
When monitoring boiler feed water in power plants, the pH value can be
calculated by means of a dual conductivity measurement. For that purpose,
the boiler feed water conductance is measured before and after the cation
exchanger. This commonly used method of indirect pH value measurement
does not require much maintenance and has the following advantage:
Normal pH measurement in ultrapure water is very critical. Boiler feed water
does not contain many ions. This requires the use of a special electrode,
which must be calibrated constantly and the service life of which is generally
rather short.
Function
Two sensors are used to measure the conductivity before and after the cation exchanger. The pH value is inferred from these two conductivity values
according to the calculation formulas shown below:
Cond measuring point A
Sensor A
TC: NH3 / NaOH
Cation
exchanger
Calculation Block
pH
Cond measuring point B
Sensor B
TC: HCl
H2O
Calculating the concentration of sodium hydroxide solution /
pH value:
c(NaOH) =
34
COND A – 1/3 COND B
243
pH = 11+log[c(NaOH)]