What would cause a sample with Chlorine to start pink and then become clear when running a DPD Free Chlorine test?

Document ID

Document ID TE203

Published Date

Published Date 10/09/2019
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Question
What would cause a sample with Chlorine to start pink and then become clear when running a DPD Free Chlorine test?
Summary
DPD pink color turns clear
Answer
When Free Chlorine reacts with DPD, a magenta colored complex, known as the Würster Dye, is formed. This is proportional to the Free Chlorine concentration. When Free Chlorine concentrations greatly exceed the concentration range of the test, a colorless Imine complex will be formed, which will bleach the color from the sample. Samples that are high in Chlorine concentrations will show a brief, flash of pink color, before turning clear. If the smell of Chlorine is present in the sample, then Chlorine concentrations would be very high.




Try diluting the sample with deionized water and then repeat the test. Keep diluting the sample until the results are within the acceptable range of the method. Multiply the results by the dilution factor.

Additional information can be found in the Chlorination, Chloramination And Chlorine Measurement, LIT2129, document.

HIgh Chlorine concentrations can also affect the DPD Total Chlorine test. The following article addresses this: Why did the sample turn a dark brown after adding the powder pillow when running a DPD Total Chlorine test?

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