Inline chlorine analysis and measurement

Chlorine added to water in its elemental or hypochlorite form first undergoes hydrolysis to form free available chlorine consisting of aqueous molecular chlorine (Cl2), hypochlorous acid (HClO), and the hypochlorite ion (ClO-). Free chlorine reacts readily with ammonia and certain nitrogenous compounds to form combined available chlorine. Both free and combined chlorine can be present simultaneously and are referred to as total chlorine.

The sample is buffered with hydrogen dihydrogen phosphate buffer at pH 6 without iodide ions – combined chlorine is not detected under these conditions. DPD (NN-dimethyl-1,4-phenylenediamine) solution is then added to develop the color. For total chlorine, iodide ions are added. Combined chlorine is calculated as the difference between total and free chlorine. Calibration is performed using a chlorine-equivalent iodate concentration in an iodide-iodate solution. The developed color complex is measured at 524 nm. Under these conditions, linearity is provided up to 4 mg Cl2/L. For higher ranges, a cubic formula is used.