Effect of Residual Chlorine on pH Measurements Using Indicator Reagents and Countermeasures

Abstract

The indicator method using bromothymol blue (BTB) and phenol red (PR) is occasionally employed for on-site pH measurements in places such as water treatment plants or public baths. However, large discrepancies between the values obtained by this method and those obtained by the electrode method have been confirmed. In this study, the effect of free residual chlorine on pH indicators, which has been rarely studied, was investigated by a quantitative method using xy color space. With free residual chlorine, the BTB and PR readings were biased significantly toward the acidic and alkaline sides, respectively. The likely reasons for these biases were considered to be oxidation in the case of BTB and chlorination leading to chlorophenol red from PR. In addition, the colors of the indicators were still within or close to the transition ranges even after the chemical changes, and thus the indicated values could easily be misread as valid, and this was considered to be the cause of the discrepancy from the pH value obtained by the electrode method. As a countermeasure, the effect of the removal of free residual chlorine using sodium thiosulfate in the pH test with BTB or PR was examined in the laboratory and on-site, and for a 0.07 mol L^-1 solution, no significant effect was observed when added up to 0.5% of the sample volume.