Determination of residual chlorine in tap water by flow-injection spectrophotometry

The method is based on the redox reaction between 4,4′-tetramethyldiaminothiobenzophenone (thio-Michler's ketone) and chlorine at pH 3.5; 2-methoxyethanol and Triton X-100 are used to keep the reagent in solution. The absorbance of the blue quinoidal product is measured at 640 nm. Linear calibration is obtained for 0.2?1.0 mg l^?1 chlorine. The method is suitable for tap waters.