Palintest AP056 Ozone DPD Method Reagent Tablets 250 per box. The Palintest Ozone test uses the DPD method now internationally recognised as the standard method of testing for disinfectant residuals. In the DPD method, the reagents are provided in tablet form for maximum convenience and simplicity of use. Ozone reacts with diethyl-p-phenylene diamine (DPD) in buffered solution in the presence of potassium iodide to produce a pink coloration. The intensity of the colour is proportional to the ozone concentration and is measured using a Palintest Photometer.