Alternate Reagents for the Amperometric Analysis of Residual Chlorine I. Background and Theory

Amperometric measurement of residual chlorine is widely practiced, especially for the continuous monitoring and control of the chlorination process. Developed more than fifty years ago, the technique has evolved somewhat, but the recommended practice is quite similar regardless of the instrumentation used. Most often a reagent is added, the principal purpose of which is to reduce the sample pH to below about 5, ostensibly to have the free chlorine in the HOCl form to assure an accurate measurement. The most common reagent used is an acetate buffer; alternatively, a phosphate buffer, acetic acid or CO2 has been used to lower the sample pH.Whereas the lowering of the sample pH to below 5 does permit accurate monitoring of free chlorine residuals, the reagents used have drawbacks. Most are nutrients that can facilitate the growth of microorganisms which can clog lines and portals in the monitors, and which can be a disposal problem if an adequate drain is not available. While CO2 is not a nutrient, its limited solubility and slow dissolution so limit its pH-reducing capability, especially in buffered waters, that its use often proves unsatisfactory. Cost and maintenance requirements are also concerns with all commonly used alternatives.A re-examination of the theory of amperometric chlorine measurement has resulted in the recognition that it is not necessary to adjust the sample pH into the range wherein the chlorine is in the HOCl form. It is necessary to provide protons required for the electroreduction of chlorine, but these protons may be the “free” protons associated with a low pH or protons bound to a proton donor but at a higher pH. It is the availability of sufficient protons that is critical to an accurate measurement, rather than a low pH per se. The theory leading to this conclusion is reviewed and experimental confirmation is presented. The constraints on reagent selection are lessened and new selection criteria for alternative reagents for the amperometric analysis of residual chlorine are delineated.