DPD Test
DPD (N,N-diethyl-p-phenylenediamine) is the standard reagent used in liquid test kits to measure free chlorine and total chlorine.
Definition
DPD (N,N-diethyl-p-phenylenediamine) is the standard reagent used in liquid test kits to measure free chlorine and total chlorine.
Typical Values: DPD-1 measures free chlorine (FC); DPD-3 measures total chlorine (TC)
In Plain Language
DPD-1 reagent reacts with free chlorine only, producing a pink colour proportional to FC concentration. DPD-3 (or DPD-1 plus a second reagent) reacts with total chlorine. The difference between the two readings is combined chlorine. DPD tests are more accurate than OTO tests because they specifically measure free chlorine rather than total chlorine. Always compare DPD colour tubes in natural or bright white light.
Why It Matters
DPD is the gold standard for free chlorine measurement. Using OTO tests instead gives total chlorine, which overstates effective sanitiser levels.
Typical Values
DPD-1 measures free chlorine (FC); DPD-3 measures total chlorine (TC)
Last reviewed: 2026-06-01