Analytical experiments were conducted to improve the measurement accuracy of the official method used in Japan for the determination of nonionic surfactants (NIS) in tap water. Since color development involves extraction of a cobalt-NIS complex to an organic layer and cobalt back extraction with 4-(2-pyridylazo)-resorcinol (PAR) solution, the effect of the related experimental parameters (pH, haking time, cobalt concentration, PAR solution volume) were investigated. The optimum analytical condition was found to be the one in which the sample volume was reduced to half that used in the official method. We evaluated the validity of the improved analytical method based on the results of recovery test. The trueness and relative standard deviation of the calibration curve, and the accuracy (recovery ratio), repeatability and intermediate precision of the recovery test satisfied the criteria stated in the Japanese validation guideline concerning tap water analysis.

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