What could cause a Free Chlorine DPD measurement to continually increase in pink color over time?

Document ID

Document ID TE200

Version

Version 4.0

Status

Status Published

Published Date

Published Date 12/11/2018
Question
What could cause a Free Chlorine DPD measurement to continually increase in pink color over time?
Summary
Increasing Free Chlorine measurement
Answer
This gradual increase in pink color during a Free Chlorine DPD analysis indicates that your sample may have a high concentration of monochloramine, which interferes with the Free Chlorine DPD method at high concentrations. There is no way to get an accurate reading for Free Chlorine using DPD when there is a high amount of monochloramine in the sample.
The recommendation is to measure Total Chlorine using the Total DPD reagent, or utilize the Free Chlorine Indophenol chemistry. You may also find the information in the following articles helpful:
Why does DPD continue to develop color over time?
When is the Free Chlorine Indophenol reagent recommended over the Free Chlorine DPD reagent?
 
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