Residual ozone measurement: Indigo sensitivity coefficient adjustment

The determination of residual ozone using indigotrisulfonate (Standard Methods 4500-O3B) assumes a molar absorptivity of similar to20,000 M(-1)cm(-1) and a constant sensitivity coefficient of 0.42 L mg(-1)cm(-1). Data are presented showing large molar absorptivity differences and subsequent differences in the calculated sensitivity coefficients for various sources of indigo. The variation in molar absorptivity (up to 15%) emphasizes the need for measuring the sensitivity coefficient on a regular basis. A revised simple calculation that compensates for decomposing indigo reagents is proposed. A modified equation that can be used with the current Standard Method procedure is given. However, for the most accurate results, it is recommended that the dry indigo reagent be periodically calibrated.