Ozone has become an important tool for the water quality engineer. As new more stringent requirements from regulators and customers must be met, ozone is often the oxidizer and disinfectant of choice for a wide range of process applications. These include viral, bacterial and parasitic disinfection, the removal of taste- and odor-causing compounds, the destruction of refractory/toxic organic matter and the coagulation or oxidation of inorganic impurities such as iron, manganese and sulfides1.

With the rising popularity of ozone in water treatment comes the need for a versatile analytical method for routinely measuring dissolved ozone in a wide variety of solution matrices. The method must be accurate and precise over a wide concentration range, insensitive to interferences and easy to use and dispose of. The indigo trisulfonate method fulfills these requirements. It uses a non-toxic blue dye that is instantly decolorized by ozone. Interference from the most common oxidizer, chorine, can be masked with malonic acid. The method is described in Standard Methods for the Examination of Water and Wastewater (Method 4500-O3B)2 and is offered as a product in various forms by a number of companies.

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