Solvent effect on protonation constants of salicylic acid in mixed aqueous organic solutions of DMSO

The first protonation constant of salicylic acid in water-DMSO solutions containing 0-50% (v/v) DMSO has been determined by spectrophotometric and potentiometric methods at 25 degrees C and an ionic strength of 0.1 M sodium perchlorate. The results indicate that the pK(a) value of salicylic acid increases with increasing proportion of DMSO in the mixed solvent. The variation of pK(a) value over the media composition range was explained by a linear solvation energy relationship, and correlations between pK(a) values and the Kamlet-Taft parameters a (hydrogen bond donor acidity), pi* (dipolarity/polarizability), and beta (hydrogen bond acceptor basicity), and the dielectric constant were calculated by means of multiple linear regression. The equations obtained can be used to estimate the first protonation constant of salicylic acid in a water-DMSO solvent from the relationship between the pK(a) values and the Kamlet-Taft parameters of the solvent. Also, the pK(a) value in aqueous medium was calculated by use of the SPARC online calculator and was compared with the value determined experimentally.