The method used for measuring chlorine residuals uses a chemical compound called DPD (N,N-Diethylparaphenylenediamine), either through the use of a chlorine comparator or photometric instrument (this provides the highest accuracy and precision for measuring chlorine residuals). This compound can be provided as a tablet or as a liquid. DPD reacts instantly to produce a red color in the presence of chlorine. The subsequent addition of potassium iodide evokes a rapid color response from the combined forms of chlorine (typically chloramines).
