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j. Soc. Cosmet. Chem., 33, 243-248 (August 1982) The stability of 2-pyridinethiol-l-oxide, sodium salt, as a function of pH ROBERTJ. FENN and DAVID A. CSEJKA, Olin Research Center, P.O. Box 30-275, New Haven, CT06511. Received December 9, 1981. Synopsis The hydrolysis of sodium-2-pyridinethiol-l-oxide has been studied in aqueous solutions buffered at pH 4.1, 7.1, and 10.0 at 5øC and 40øC. The sample concentration studied was approximately 100 ppm. Samples stored at 5øC showed very little loss of starting material over the 30 day duration of the study. Samples held at 40øC showed significant loss only at pH 10.0. The major product found at pH 4.1 and 7.1 was the disulfide oxidation product, while at pH 10.0 the major product was the sulfinate salt, formed by attack of hydroxide ion on the disulfide. Additional 30-day experiments run at pH 7, 8, 9, and 10 indicated a gradual shift of the product distribution from the disulfide to the sulfinate with increasing pH. The sulfinate salt was shown to be the end product of alkaline hydrolysis up to 30 days by its demonstrated stability at pH 10 and 40øC. An authentic sample of 2-pyridinesulfinic acid-l-oxide was prepared and characterized as an aid to product identification and quantitation. INTRODUCTION The sodium salt of 2-pyridinethiol-l-oxide (NaPT) is used as a preservative in some cosmetic formulations at typical use levels of 100 to 1000 ppm. Information was desired concerning the stability and possible degradation products of this compound in formulations at various pH values. Accordingly, studies were undertaken to monitor stability of aqueous buffer solutions of 100 and 500 ppm NaPT from pH 4 to 10 at 5 ø and 40øC. Degradation products were identified and quantitated during the course of the 30-day study. Based upon the results of the present study, recommendations can be made concerning the product pH range suitable for the applications of NaPT as a preservative. EXPERIMENTAL APPARATUS Polarograms were obtained using a Princeton Applied Research Model 174 Polaro- graphic Analyzer equipped with either a Model 172A Drop Timer or a Model 303 Static 243