Automated determination of the pKa values of 4-hydroxybenzoic acid in cosolvent-water mixtures and related solvent effects using a modified HPLC system.

An automated spectrophotometric method based on an HPLC system with a diode array detector was used to determine the pK(a) values of compounds with low water solubility in a universal buffer containing acetonitrile as cosolvent. The column of the system was replaced with a capillary connecting the injection system and the diode array detector. Specific solvent effects were corrected for using the dielectric constants of the mixed solvent and pure water. The method was tested using 4-hydroxybenzoic acid and the results were compared with those obtained with a spectrophotometer. Linear regression lines with different slopes were obtained from spectrophotometric measurements of different cosolvent-water mixtures. These effects were shown to depend upon the polarity of the solvent-water mixture, and they were explained by the solvatochromic behavior of the 4-hydroxybenzoic acid in the solvent-water mixture.