2. DPD Total Chlorine Reagent (DPD Method #4) for measuring total chlorine.
The operator should review and familiarize themselves with the DPD method, reagents, and devices to make sure
knowing how to perform this important calibration measurement.
PreparetheequipmentandDPDreagentsforafastandefficientmeasurement.
MakesuretheA420-FCLchlorineelectrodeisfullypolarizedandconditionedinzero-chlorinewaterand record the output
of zero point (the 1st point).
Make sure the electrode is installed properly inthe measurement system with a constant flowrate ranging from
30 L/hr to 60 L/hr (0.6 cm/s to 1.3 cm/s).
Add chlorine to the system, and preliminarily measure the chlorine concentration by DPDmethod.
Approximately adjust the chlorine concentration to the middle point of the measurement range by adding water
or chlorine if necessary.
Let the probe output be stable, and then take water samples from the measurement system for DPD
standardization. This sample should be close to the A420-FCL electrode location for best accuracy.
Record the output of A420-FCL (the 2nd point) as well as the chlorine reading from DPD test.
Calculate the slope as follows:
Once the slope is obtained, the electrode output including voltage (0-5V) or current (4-20 mA) can be converted into
“mg/L” or “ppm” in customers’ analyzer/controller/meter. The conversion equation for any of A420-FCL output is
below:
When the zero point of A420-FCL is not within specifications or there is no significant change in the measurement
environment, but the electrode reading is continuously increasing or decreasing (not fluctuating) within an hour, re-
polarization is probably needed. The root cause is that some interfering substances probably were brought to the
working electrode again, such as unsuccessfully polishing the surface of working electrode, changing the IFG, replacing
the membrane or soaking the electrode with the power off (unconnected) in chlorine or other strong oxidant solution
for a long time. The re- polarization process is mandatory until the electrode presents a stable output. If the zero point
of A420-FCL still can not meet the specifications after a reasonable amount of polarization time, changing the IFG or
polishing the working electrode again may be necessary, or adding tiny sodium thiosulfate in testing water for
removing the potential oxidizers.
If the zero point of the A420-FCL does not meet the listed specifications it may be necessary to change the IFG. The IFG
also needs be changed if the stainless-steel ring (counter electrode) is not submersed in the water when the electrode
is powered for more than 5 minutes. Also, when the slope is lower than 50% of the nominal value, changing the IFG
may correct this situation. It is important to disconnect/power off the electrode before removing it from the water.
Instructions for changing the IFG are as follows:
Calibration Preparation and Procedure
5. Maintenance
5.1. Electrode re-polarization
5.2. Changing the Internal Filled Gel (IFG)
