The objective of this study was to develop a specific analytic method for free residual chlorine with minimum sensitivity or, ideally, no response whatever to combined residual chlorine. The ability of the method to distinguish between the two forms of residual chlorine was particularly important, because in many cases both forms are present but only the free residual is effective as a disinfectant to assure that the water supply is safe. In addition, impurities interfere in one or more of the other methods for residual chlorine determinations. These include: dissolved oxygen, copper, nitrite, iron and manganese. As susceptibility to interferences of this kind seriously limits the application of any method under field conditions, their effect upon the p‐amino diethylaniline or N,N‐diethyl‐p‐phenylene diamine (DPD) method were evaluated as a part of this study.