Titrations in non-aqueous media. Part XIII. Potentiometric and conductimetric titrations of alpha-amino acids with perchloric acid in acetic acid and acetonitrile-acetic acid solvents.

Abstract

Of the 22 α-amino acids studied, all but five (tyrosine, cystine, cysteine, glutamic acid and aspartic acid) have been titrated potentiometrically with perchloric acid in two solvents, glacial acetic acid and 90% acetonitrile-acetic acid. The half-neutralisation potentials and the pKa′ values of the amino acids in these solvents have been calculated. The half-neutralisation potentials of the monoaminomonocarboxylic acids were ca. 390 mV in acetic acid and ca. 290 mV in 90% acetonitrile-acetic acid. The titrations of binary mixtures of histidine with monoaminomonocarboxylic α-amino acids have also been carried out in these solvents. The titration curves of binary mixtures of histidine with diaminomonocarboxylic acids showed only one stoicheiometric end-point, corresponding to the sum of the four amino groups. The data obtained from the titrations in glacial acetic acid suggested that this solvent has a strong levelling effect on the basicity of the amino groups of the α-amino acids. All the amino acids titrated potentiometrically could be determined with an error of less than ±3%.Seventeen of the 22 α-amino acids have also been titrated conductimetrically in acetic acid. All the α-amino acids gave one stoicheiometric end-point except for histidine and arginine which gave two.No reliable conductimetric titration curves or results could be obtained in 90% acetonitrile-acetic acid. In those conductimetric determinations of α-amino acids that were possible, the relative errors never exceeded ±3%.