Ozone is a strong oxidizing agent. Ozonation is used as an alternative biocide and disinfectant to chlorination of drinking water. Ozone is used to remove odor, decolorize, and to control algae and other aquatic growths.  Ozone is also used in various disinfectant/sterilization processes in the food & beverage and pharmaceutical industries. Because ozone is unstable in water, monitoring ozone residuals is important to ensure that proper treatment levels are maintained.

The Colorimetric Methods. 
The DDPD chemistry employs a methyl substituted form of the DPD reagent. The A-7400 activator solution (potassium iodide) is added to the sample before analysis. Ozone reacts with the iodide to liberate iodine. The iodine then reacts with the reagent to give a blue-violet color.  Various free halogens and halogenating agents produce color with the reagent. Chromate in test samples below 25 ppm will not interfere with results.  Results are expressed as ppm (mg/L) O₃.

The indigo method employs the indigo trisulfonate reagent, which reacts instantly and quantitatively with ozone, bleaching the blue color in direct proportion to the amount of ozone present.  Malonic acid is included in the formulation to prevent interference from up to 3 ppm chlorine.  Results are expressed as ppm (mg/L) O₃.