What could cause the sample to turn orange color after adding the Chlorine DPD reagent?
Orange color using DPD chlorine reagent
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.