Determination of acidity constants of low solubility compounds by potentiometry and capillary electrophoresis.

Abstract

The determination of acidity constants (pKa) is essential in drug discovery and pharmaceutical analysis because many drug candidates possess acid-base properties, and their ionization state strongly influences their physicochemical properties such as absorption, distribution and excretion. However, such compounds usually exhibit poor aqueous solubility, making pKa determination in hydro-organic media necessary. In this work, potentiometric titration and the internal standard capillary electrophoresis method (IS-CE) were applied to study determine pKa and pH in different hydro-organic solvent mixtures. Firstly, potentiometric determinations were carried out to establish reliable reference pKa values for several acidic and basic compounds in acetonitrile-water mixtures containing between 0% to 90% of organic solvent. The results showed that for acidic compounds, pKa values increased progressively when increasing the acetonitrile content, while bases exhibit a decrease in the values up to 70% of acetonitrile, followed by an increase at higher organic solvent proportions. These trends are consistent the relative permittivity, the basicity of the solvent and the specific solvation effects. Reliable potentiometric determinations for acids and protonated bases were only possible up to 60% of acetonitrile content because side reactions between the titrant (potassium hydroxide) and the organic solvent prevented accurate measurements. Secondly, the IS-CE method was evaluated for the determination of the pH of ammonium-acetate buffers prepared in different methanol-water mixtures containing between 10 and 90% (V/V) of the organic solvent. The method was applied using internal standards (IS) whose reference pKa values in water-methanol mixtures had previously been established by the research group. The pH values determined by this method showed great agreement with values previously measured for each of the buffers, yielding deviations equal or lower than 0.2 units. Overall, potentiometric results provide reference pKa values of many compounds in acetonitrile-water mixtures, which could help IS-CE method implementation in this media. Furthermore, the results obtained by the IS-CE method in water-methanol mixtures validate it as a fast, accurate and reliable alternative for pH determination in this media.