Free chlorine reacts with diethyl-p-phenylene diamine (DPD) in buffered solution to produce a pink coloration. The intensity of the colour is proportional to the free chlorine concentration.Subsequent addition of excess potassium iodide induces a further reaction with any combined chlorine present. The colour intensity is now proportional to the total chlorine concentration; the increase in intensity represents the combined chlorine concentration. In this way, it is possible to differentiate between free and combined chlorine present in the sample. The colour intensities are measured using a Palintest Photometer.The DPD Oxystop tablet is added after measurement for free chlorine but before the DPD XT tablet. It prevents the reaction between shock treatment chemicals and potassium iodide which would give a positive response.