The acid-base equilibrium constants of some azine compounds in various aqueous-organic solvent mixtures

The acid dissociation constants of the protonated form of some azine compounds (acridine, acridine orange and neutral red, BH+) were determined pH-metrically at 25 degrees C and at the constant ionic strength I = 0.1 mol l(-1) (KNO3) in pure water as well as in various aqueous mixtures having different proportions (w/w%) of organic solvents. The organic solvents used are methanol, ethanol (as amphiprotic solvents), N,N-dimethylformamide, dimethylsulfoxide (as dipolar aprotic solvents) and acetonitrile (as a low basic solvent). The results obtained indicated that the pKa values decrease as the content of the organic solvent in the medium is increased. It is deduced that, the major effect responsible for this behaviour is the differences in stabilization of the free base (B) by dispersion forces and of the proton by its interaction with solvent and water molecules in aqueous-organic solvent mixtures (ion-solvent interaction). Moreover, it is concluded that the ability of the solvent to accept hydrogen bond from the protonated form (BH+) contributes significantly to the deprotonation process of the compounds.