Deprotonation of salicylic acid and 5-nitrosalicylic acid in aqueous solutions of ethanol

Abstract

The protonation constant values of two hydroxybenzoic acids (salicylic and 5-nitrosalicylic acid) were studied in some water-ethanol solutions using spectrophotometric and potentiometric methods at 25 °C and in an ionic strength of 0.1 M sodium perchlorate. The results indicated that the pK a values increase with increasing proportion of ethanol in mixed solvent. The dependence of the protonation constants on the variation of the solvent were correlated by the dielectric constants of the media. Furthermore, for a better understanding of the solvent influence, the obtained results were explained in terms of the Kamlet-Taft parameters α (hydrogen-bond donor acidity), π* (dipolarity/polarizability) and β (hydrogen-bond acceptor basicity) by means of linear solvation energy relationships. The multiple linear regression analysis indicated that the pKa values of salicylic acid were strongly dependent on the non-specific electrostatic solvent-solute interactions, whereas specific hydrogen bonding interactions controlled the protonation of nitrosalicylic acid in water-ethanol binary mixtures.