Titrations in non-aqueous media. Part X. Potentiometric and conductimetric titrations of amino acids with tetrabutylammonium hydroxide in pyridine and acetonitrile solvents
Of the 22 α-amino acids studied, all but three (tyrosine, cystine and cysteine) have been titrated potentiometrically using a standard solution of tetrabutylammonium hydroxide in two solvent systems, 90% pyridine-water and 90% acetonitrile-water. The half-neutralisation potentials and the corresponding pK′a1 and pK′a2 values of the amino acids in these solvent systems have been calculated. Potentiometric titrations of binary mixtures of the two dicarboxylic amino acids, glutamic and aspartic acid, with the other monocarboxylic amino acids have also been carried out. The end-points of the titrations performed in the 90% acetonitrile-water solvent system were found to be much sharper than those of the titrations carried out in the 90% pyridine-water solvent system. All the determinations of amino acids, either in single or in binary mixtures, have been shown to have an error of less than ±2%.
The α-amino acids and their binary mixtures with glutamic and aspartic acids have also been titrated conductimetrically in the same two solvent systems. Almost all the titrations gave either no end-points or non-stoicheiometric end-points. It has been shown that the solvent systems used in this work are suitable for potentiometric titrations of amino acids, but are not suitable for the conductimetric titrations.