22 | Calibration and Validation
7070iX TRO Analyzer
Calibration and Validation
Each Orion 7070iX Total Residual Oxidant Analyzer gets
thoroughly wet-tested and calibrated at the factory, so the
system is ready to begin analyzing sample streams. The
results of this activity are recorded in a Quality Control Report
shipped to you with the analyzer.
Since the elapsed time between factory calibration and your
startup process is unpredictable, it is important that you
calibrate the analyzer again before you place it in service for
the first time.
The reported result of a calibration is a number that the
analyzer uses to determine the sample concentration. The
particular value of this number is not important, but it should
be within ±5% of the corresponding value in the Quality
Control Report. If so, you can be confident that all
subsystems of your analyzer are working properly.
Calibration Standards
The analyzer calibrates using standards with a known chlorine
equivalent and setting the monitor to the known value. During
calibration, the three-way valve is used to switch flow from the
sample to the standards / grab sample line. Two standards
are used to adjust the electronics for the actual slope of the
electrode in the same manner that two buffers of different pH
are required to calibrate a pH meter.
Chlorine solutions are unstable and are not suitable for use as
standards. Therefore, stock solutions of potassium
permanganate, which are stable and react with iodide to
produce iodine are used. Standards prepared are
stoichiometrically equivalent to the chlorine concentrations
indicated.
Figure 21. 3-Way Valve
Standards should be chosen to bracket the expected sample
concentration and can be prepared by serial dilution of the
stock iodate solution described in the Preparing Standards
section.
To Prepare Each Standard
1. In a 1000 mL volumetric flask, pipette 0.2 mL of standard
solution (UM-272700-A01).
2. Dilute to the mark with distilled water.
3. Ensure complete mixing by inverting the flask at least
20 times.
4. Label the flask: 0.2 ppm Cl
2.
5. To prepare the 2.0 ppm Cl
2
standard, repeat steps
1 through 4, instead pipetting 2 mL standard solution and
label this flask: 2.0 ppm Cl
2
.
You now have two standards, one equivalent to 0.2 ppm and
the other equivalent to 2.0 ppm total residual chlorine. For the
best accuracy, calibration should be started immediately after
the standards have been prepared. Even carefully stoppered
standards should be discarded after one hour.
