Effect of temperature on pH measurements and acid–base equilibria in methanol–water mixtures

The knowledge of the acid–base equilibria in water–solvent mixtures of both common buffers and analytes is necessary in order to predict their retention as function of pH, solvent composition and temperature. This paper describes the effect of temperature on acid–base equilibria in methanol–water solvent mixtures commonly used as HPLC mobile phases. We measured the δ-correction parameter (δ=swpH−sspH=Ēj−log swγH0) between two pH scales: pH measured in the solvent concerned and referred to the same standard state, sspH, and the pH measured in that solvent mixture but referred to water as standard state, swpH, for several methanol compositions in the temperature range of 20–50 °C. These determinations suggest that the δ-term depends only on composition of the mixture and on temperature. In water-rich mixtures, for which methanol is below 40% (w/w), δ-term seems to be independent of temperature, within the experimental uncertainties, whereas for methanol content larger than 50% (w/w) the δ-correction decreases as temperature increases. We have attributed this decrease to a large increase in the medium effect when mixtures have more than 50% methanol. The pKa of five weak electrolytes of different chemical nature in 50% methanol–water at 20–50 °C are presented: the effect of temperature on pKa was large for amines, pyridine and phenol, but almost no dependence was found for benzoic acid. This indicates that buffers can play a critical role in affecting retention and selectivity in HPLC at temperatures far from 25 °C, particularly for co-eluted solutes.