AWWA JAW53072

The standard method for measuring residual ozone (O3) is based on the decolorization of indigotrisulfonate at 600 nm using a sensitivity coefficient of 0.42 L mg-1cm-1 (e ~ 20,000 M-1cm-1). The sensitivity coefficient for a source of indigo is calculated following a time-consuming and labor-intensive O3 calibration. In contrast, the data presented in this article show that a simple initial absorbance measurement can be used to calculate the indigo sensitivity coefficient, thereby eliminating the need for O3 calibration when routine measurements are made. The molar absorptivity for seven sources of indigo ranged from 15,800 to 20,355 M-1cm-1. These results indicate that indigo purity (and stability) is potentially a significant source of measurement error when unvalidated indigo stock solutions are used to measure O3. The authors describe indigo solution decomposition and offer a simple initial absorbance method for validating the O3 sensitivity coefficient. Includes 8 references, tables, figure.