48
A3
Inferred pH Derived from
Differential Conductivity
Where cation resin columns are used to remove the effects on
the conductivity measurement of alkaline and hydrazine
chemical treatment on boilers, it is common practice to measure
both before- (specific conductivity) and after-cation conductivity.
The sensitivity of the conductivity measurement to chemical
contaminants resulting from condenser leaks or poor boiler-feed
make-up water is increased by passing the sample through the
cation column. Both measurements can be made on one dual
conductivity input analyzer.
If it is known that a sample contains only one impurity, e.g.
ammonia, the conductivity measurement now becomes an
indication of the concentration of that impurity. It is now possible
to calculate the pH of the sample from the concentration data
and the result is referred to as 'inferred pH'.
It is stressed that the inferred pH value is valid only if there are no
other impurities present. To ensure this, the chemist looks at the
after-cation conductivity (which is a sensitive method of
detecting impurities in the sample) and only after establishing
that it is low is the inferred pH value validated.
The dual input conductivity analyzer, when used to monitor
direct and after-cation conductivities on a sample, automatically
calculates the inferred pH for the most commonly used pH
correction chemicals when programmed to do so. The user-
configurable after-cation conductivity alarm is used to detect
other impurities in the sample and can thus inform the user of the
validity of the inferred pH value.
The maximum after-cation conductivity value is programmable
between 0.060 and 1.000µS cm
–1
dependent on local
conditions. Values above this level generate an
AFTER CAT.
HIGH
alarm and before-cation conductivity above 10.000µS
cm
–1
generates a
BEFORE CAT. HIGH
alarm.
✶
Note.
Both conductivity inputs must be configured
as µS cm
–1
in order to calculate inferred pH.
The inferred pH feature can be used only in the following
circumstances:
a) On steam raising plant.
b) For boiler chemical treatment such as ammonia, sodium
hydroxide, and/or hydrazine. For this application, either
NH3
or
NaOH
temperature compensation must be selected – see
Section 7.3.
✶
Note.
Inferred pH measurement is inappropriate
to chemical treatments such as phosphate, morpholine
and quinhydrone.
c) Where the after-cation conductivity value is an
insignificant value to the before-cation value, or is greater
than 1.0µS cm
–1
.
…APPENDIX A
Summary of Contents for AX411
Page 1: ...AX400 Series User Guide Model AX411 Dual Input Conductivity Analyzer ...
Page 53: ...51 NOTES ...
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