Effect of temperature and solvent composition on acid dissociation equilibria, I: Sequenced p s s K a determination of compounds commonly used as buffers in high performance liquid chromatography coupled to mass spectroscopy detection

Abstract

A new automated and rapid potentiometric method for determining the effect of organic-solvent composition on pKa has been developed. It is based on the measurements of pH values of buffer solutions of variable solvent compositions using a combined glass electrode. Additions of small volumes of one precisely thermostated solution into another, both containing exactly the same analytical concentrations of the buffer components, can produce continuous changes in the solvent composition. Two sequences of potential measurements, one of increasing and the other of decreasing solvent content, are sufficient to obtain the pKa values of the acidic compound within the complete solvent-composition range in about 2 h. The experimental design, procedures, and calculations needed to convert the measured pH into the thermodynamic pKa values are thoroughly discussed. This rapid and automated method allows the systematic study of the effect of solvent compositions and temperatures on the pKa. It has been applied to study the dissociation constants of two monoprotic acids: formic acid and triethylamine:HCl in acetonitrile/water mixtures within the range from 0 to 90% (v/v) at temperatures between 20 °C and 60 °C. These volatile compounds are frequently used to control the pH of the mobile phase in HPLC, especially in methods coupled to mass-spectrometry detection. The obtained pKa values are in excellent agreement with those previously reported. The results were fitted to empirical functions between pKa and temperature and composition. These equations, which can be used to estimate the pKa of these substances at any composition and temperature, would be highly useful in practical work during chromatographic method development.