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Effect of the Structure of Some Mercaptans on Their Amperometric Titration with Silver Nitrate¹

Department of Food Technology, University of Illinois, Urbana

ABSTRACT

The mercaptans studied were L-cysteine hydrochloride, DL-homocysteine thiolactone hydrochloride, S-methyl-L-cysteine, thioglycolic amide, N-acetyl-DL-penicillamine, N-acetyl-DL-homocysteine thiolactone, L-cysteine methyl ester hydrochloride, 2-mercaptoethylamine hydrochloride, DL-penicillamine hydrochloride, -toluenethiol, and sodium thioglycolate. These mercaptans yielded theoretical titrations with silver nitrate, except cysteine hydrochloride and penicillamine, which were about 25% too high in titer. Cysteine has three ionized forms whereby the carboxyl group is completely ionized at the pH of the amperometric titration, while at pH 11 or higher, the –SRNHNO₂ form predominates, but at pH 9.0 it is negligible and the –SRNH⁺₃ form predominates. Ultra-violet absorption analyses indicated that silver ions react not only with the -SH groups, but at some other site in the molecule, probably involving the ionic forms, –SRNH⁺₃, –SRNH₂, and complex formation.

¹ This work was supported by U. S. Public Health Grant 1947.