Operator-Friendly Technique and Quality Control Considerations for Indigo Colorimetric Measurement of Ozone Residual

Drinking water ozone disinfection systems measure ozone residual concentration, C, for regulatory compliance reporting of concentration-times-time (CT), and the resultant log-inactivation of virus, Giardia and Cryptosporidium. The indigotrisulfonate (ITS) colorimetric procedure is the Standard Method for measuring ozone residual. Although the currently written Standard Method is relatively easy to implement, its accuracy depends on specific ITS quality control considerations. The Standard Method is based upon specific quantities of materials and sample volumes, which make it somewhat inflexible. Tests are often performed in plant surroundings by operating staff, as opposed to in certified laboratories by analytical chemists. In this paper, a more flexible, quality-assured and “operator-friendly” technique is presented for the ITS method. This manuscript investigates several aspects of the ITS method (e.g. effects of aging, storage temperature, ITS supplier, accuracy of ITS weight measurement, and manganese interference), which are intended to improve the quality assurance and quality control (QA/QC) of the dissolved ozone measurement. Results are presented that document the negative consequences for ignoring certain conditions. For example, ITS solution stored on a shelf for several days can cause an appreciable under-estimation of ozone residual, while storage for several weeks can cause a major under-estimation of ozone residual. Other special study results suggest that greater flexibility is possible for reagent strength, without negative consequence, as compared to the recommended strength that is currently outlined in Standard Methods.