<1. General Information>
1-4
IM 12Y18A03-01EN
1st Edition : Oct. 04, 2019-00
The above equation indicates that the measured potential varies with both temperature and
the concentration of the ion of the interest. In order to eliminate error caused by fluctuations in
sample temperature, the WE430 microprocessor constantly updates temperature corrections
from data supplied by the ATC probe.
From the Nernst equation, the theoretical response of a iodide ion selective electrode to a ten-
fold change in concentration at 25 °C is -59.16 mV. This is referred to as the electrode slope (S).
Most electrodes, however, do not exhibit a theoretical slope. Therefore, the analyzer is calibrated
to determine its actual value. Two standards are used to provide information necessary for the
microprocessor to compute the actual slope and E
0
for use during sample analysis.
Acid is added to the sample to prevent a competing reaction from occurring:
3I
2
+ H
2
O → 5I
-
+ IO
3
-
+ 6H
+
Maintaining acidic conditions prevents the release of iodide which would cause high apparent
reductant readings.
The pH adjustment and iodine addition are accomplished by the patented passive-diffusion
process wherein the sample passes through a length of tubing in the reagent bottle that contains
the acid reagent and iodine. The reagent and iodine diffuse through the tube wall and mix with
the sample. This passive method eliminates sample contamination problems due to reagent
contamination and makes the fluid handling apparatus simple and reliable.
1.2
Principles of Calibration
Calibration procedures for analytical instruments are important and must be performed carefully.
The WE430 Hydrazine analyzer uses a dynamic two point calibration that utilizes advanced
electrode technology in combination with a dynamic calibrator.
1.2.1
Dynamic Two Point Calibration (DYN)
The dynamic two point calibration procedure provides maximum calibration accuracy and
requires the use of the dynamic calibrator. In addition to calculating the E
o
value of the electrode,
as is done in an offline calibration procedure, this procedure determines electrode slope. To
perform a dynamic two point calibration, the WE430 analyzer is first connected to a reductant-
free sample stream. An appropriate diluted standard is prepared and then a syringe is filled
with the diluted standard and mounted on the calibrator. The calibrator is then mounted near
the analyzer and the syringe tubing is connected to the standard injection port. By adjusting the
calibrator pump settings, two different flow rates produce two known standards that are diluted
into the sample background. By pressing the appropriate keys on the analyzer when prompted,
the analyzer’s microprocessor completes the calibration. After allowing approximately 30 minutes
for the calibration solution to be flushed from the system, the WE430 analyzer can begin sample
measurement again.
1.2.2
Offline Calibration
In addition to a dynamic two point calibration, an offline calibration can be performed with
some oxygen scavengers. Since the oxygen scavenger concentration in feedwater is normally
controlled over a rather narrow range, usually an offline calibration procedure is sufficient to
provide accurate results. The offline calibration procedure has the advantage of being quick and
easy to perform. If sample reductant concentrations are expected to vary widely from day to day
or if online verification of analyzer readings is desired, a dynamic two point calibration should be
performed.
The offline calibration feature of the WE430 analyzer allows the operator to adjust the analyzer to
values determined by alternate methods used in their laboratory such as a standard colorimetric
method.