Acid–base constants of neutral bases in acetonitrile–water mixtures Original Research Article

The acid–base constants of several bases (aniline, pyridine, ammonia, ethanolamine, and triethylamine) in acetonitrile–water mixtures up to 60% of acetonitrile by volume have been potentiometrically determined. Two different pH scales have been used: the pH scale relative to each acetonitrile–water mixture as standard state solvent (sspH) and the absolute pH scale in reference to water as standard state solvent (wspH). The two sets of pK values are in a reasonable good agreement. In contrast with the variation of pKa values of neutral and anionic acids, the pKa values of the bases decrease with the increase of the acetonitrile contents in the solvent mixture. In the studied range of solvent composition, linear relationships have been obtained between the wspKa values of the bases and the volume fraction of acetonitrile in the mixture.