See introduction for Chlorine, free & total.

The Colorimetric Method. CHEMetrics' method is applicable to the direct determination of hypochlorite concentrations in various cleaning preparations and disinfectants prior to their dilution. The DDPD methodology is a derivative of DPD. The DDPD compound reacts with hypochlorite ions to form a purple color. The intensity of the color is directly proportional to the concentration of hypochlorite present. Results are expressed as percent NaOCl.