Thermodynamics ofp-nitroanilinium ion and the acidity function in ethylene carbonate-water mixtures at 25°C

Hydrochloric acid solutions (0.05–1.18m) containing p-nitroaniline (8×10^–5M) as indicator base have H_o values which increase with increase of ethylene carbonate (EC) content in the solvent mixture for a fixed molality of the acid. This behavior confirms similar results obtained in AN-H₂O, MCS-H₂O, DMSO-H₂O and NMF-H₂O systems. This trend indicates that the solvent basicity increases by successive addition of the organic solvent. The partition of the proton between water and EC has been calculated using Sadek's theory. It is clear that [H₃O⁺] decreases gradually with increase of EC content. The change in the dissociation constant of p-nitroanilinium ion, decreases sharply with increasing dielectric constant. The free energy change of transfer, G_t^o, has been attributed to an electrostatic, G_t(el)^o, and a nonelectrostatic, electrostatic, G_t(non)^o component. It has been found that G_t(non)^o is negative.