Polarography of some sulphur-containing compounds: Part XVII. Polarographic and spectral investigation of acid-base equilibria in aqueous solutions of some amino and imino derivatives of 2-thiobarbituric acid

Abstract

A comparison of changes of u.v. absorption spectra and polarographic reduction waves with pH indicates that the reduction of 5-diethyl-amino-5′-ethyl-2-thiobarbituric (I) acid resembles in principle that of 5,5′-dialkyl-2-thiobarbiturates, but that an additional acid-base equilibrium involving the exocyclic amino group is part of the electrode process. The imino compound, 5-isobutyl-5′-alkyl-1-methyl-6-imino-2-thiobarbituric acid (II), in contrast, behaves in quite a different manner. The character of the reduction process indicates that the CNH + group rather than the heterocyclic ring is reduced. Nevertheless, species with corresponding protonation of the heterocycle show similar acid-base and spectral properties to other 2-thiobarbiturates. Approximate p K-values corresponding to equilibria (9)–(16) are given in Table 1. Protonation reactions take place at the DME as surface reactions. pH-regions for optimum analytical waves are given.