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

Document ID

Document ID TE200

Published Date

Published Date 27/05/2017
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.