Using indigo absorbance to calculate the indigo sensitivity coefficient

The standard method for measuring residual ozone (O 3 ) is based on the decolorization of indigotrisulfonate at 600 nm using a sensitivity coefficient of 0.42 L mg –1 cm –1 (ε ~ 20,000 M –1 cm –1 ). The sensitivity coefficient for a source of indigo is calculated following a time‐consuming and labor‐intensive O 3 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 O 3 calibration when routine measurements are made. The molar absorptivity for seven sources of indigo ranged from 15,800 to 20,355 M –1 cm –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 O 3 . The authors describe indigo solution decomposition and offer a simple initial absorbance method for validating the O 3 sensitivity coefficient.