Spectrophotometric Determination of Total Trihalomethanes in Finished Waters

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Although chlorine has been the principal disinfectant used in the United States due to its effectiveness and low cost, a problem arose when trihalomethanes (THMs) were found in chlorinated waters. Subsequently, there is an increasing need for a relatively inexpensive method of monitoring THMs, in contrast to the costly process of gas chromatography (GC) with electron capture detection. This article describes research into a spectrophotometric procedure for determining THMs in finished waters that uses solvent extraction and chemical quantification and is based on the classic Fujiwara reaction. The THMs are extracted from solution by pentane, and the extract is mixed with 50% NaOH and pyridine and slowly heated first to strip the solvent from the solution and then to develop a bluish-pink color. The procedure is characterized by the red color that develops when a halogen compound is heated with sodium hydroxide and pyridine. Color is measured with a visible spectrophotometer. The developed colorimetric method, when compared with GC with electron capture detection, is less sensitive and less specific. However, this method can provide a semiquantitative screening of THMs at water treatment plants that are not equipped with a GC.

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