What could cause the sample to turn orange color after adding the Chlorine DPD reagent?

Document ID

Document ID TE204

Published Date

Published Date 27/05/2017
Question
What could cause the sample to turn orange color after adding the Chlorine DPD reagent?
Summary
Orange color using DPD chlorine reagent
Answer
Check the pH of the sample. At a low pH, nitrites interfere with the indicator in the DPD, causing an orange color. Sample pH should be above 4 for these tests. Nitrite and chlorine will not exist in a sample at the same time. If there is a large amount of nitrite, one can add sulfamic acid to destroy the nitrite, then adjust pH to ~6 before adding DPD.