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8150710 
Meetings & Symposia 
Determining free chlorine in the presence of manganese without sodium arsenite pretreatment 
Wiese, PM 
2009 
San Diego, CA 
American Water Works Association Annual Conference and Exposition 2009, ACE 2009 
English 
Manganese interference in the determination of free chlorine using DPD indicator is an on-going problem for analysts in the drinking water industry. This problem will increase as additional source water supplies are required to comply with the Ground Water Rule. The positive interference caused by manganese often leads to the question, "Is the pink color in my sample really coming from chlorine?" The interference from manganese is not readily obvious to the analyst because the color forms immediately with the DPD indicator in the same manner as free chlorine. The level of interference can vary from sample to sample depending upon the relative concentration and reactivity of the manganese present. The impact of the manganese interference is that it will cause an over-estimation of the disinfectant level and microbial protection actually present in the waters tested. Utilities that are required to monitor free chlorine may not be maintaining a true detectable level throughout all points in their distribution systems. Pretreatment techniques are available for compensating for the manganese interference. The most common compensating technique uses sodium arsenite to remove chlorine in a sample followed by measurement of the color due only to the manganese present. This result is then subtracted from the original sample value which measured both free chlorine and manganese. This pretreatment technique requires running two analyses on each sample, generates a waste solution that is regulated for disposal due to the presence of arsenite, and is not user-friendly for field analysis. Work has been completed on the evaluation of a new method for determining free chlorine concentrations in the presence of manganese. No pretreatment steps are required and the reagents used are not regulated for disposal. The key to the development of the method was to design a reaction mechanism for free chlorine that does not require the oxidation of an indicator such as DPD. This automatically eliminates the oxidant effects of any manganese present in the samples. Analysts do not have to predetermine if manganese is present nor do they need to apply a correction factor to compensate for the manganese interference. © 2009 American Water Works Association.