Why does DPD continue to develop color over time?
DPD chlorine reagent.
DPD (N,N-diethyl-p-phenylenediamine) will continue to oxidize due to contact with the atmosphere over time and it will eventually turn a very dark brown. This is why timing is important in Free and Total Chlorine tests. Free Chlorine must be measured within a minute of adding the reagent. Total chlorine must be measured within three to six minutes after adding the reagent.