New Methods for the Colorimetric Determination of Halogen Residuals. Part II. Free and Total Chlorine

Abstract

This article discusses a method for the determination of free chlorine using leuco crystal violet as the indicator. It provides accurate and reproducible results with minimal interference from combined chlorine at relatively high water temperatures. The method is very rapid, taking only a matter of seconds to develop the test color, and the stability of the developed color allows ample time for accurate color matching with standards. The effect of sample temperature on the accuracy of the method is negligible over a wide range and constituents normally present in water do not interfere. The oxidized forms of manganese produce the only serious interference found, and in the relatively few waters presenting this problem, it can be solved by using a suitable arsenite modification. Application of the test method in field studies involving swimming pool and treatment plant waters gave results very similar to those obtained by amperometric titration. All reagents were easily prepared and demonstrate exceptional stability in solution form. The test method is successfully used with free‐chlorine concentrations of 2 mg/l or less, but may be extended greater than this concentration by appropriate dilution methods. A very sensitive method for total chlorine is presented and when used in conjunction with the free‐chlorine test, will permit the determination of a combined chlorine residual by difference.