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

Abstract

Summary 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, α -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 −SRNHNO 2 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 , −SRNH 2 , and complex formation.