References:  DPD method-USEPA Methods for Chemical Analysis of Water and Wastes, method 330.5 (1983). APHA Standard Methods, 20th ed., pp. 4-63, method 4500-Cl- G and pp. 4-76, Method 4500-ClO₂ D (1998).

Chlorine dioxide is used as an oxidizing microbiocide in industrial cooling water treatment, the dairy industry, the meat industry, and many other food and beverage industry applications.  It is used as a bleaching agent in the pulp and paper industry, and as a disinfectant in municipal water treatment. Industrial waste treatment facilities use chlorine dioxide because of its selectivity for certain compounds, including phenols, sulfides, cyanides, thiosulfates, and mercaptans.  The oil and gas industry uses chlorine dioxide for downhole applications, as well as a stimulation enhancement additive.  The Maximum Residual Disinfectant Level for chlorine dioxide is 0.8 mg/L in drinking water.

The Colorimetric Method.  In the standard DPD methodology, chlorine dioxide reacts with DPD (N,N-diethyl-p-phenylenediamine) to form a pink product. Interference from free chlorine is prevented (up to 6 ppm Cl₂) by the addition of glycine to the sample.Results are expressed as ppm (mg/L) chlorine dioxide.