Abstract
Thermodynamic dissociation constants (SK) of p-nitroanilinium ion (BH+) have been determined at 25 °C in aqueous solutions containing 20, 40, 60, and 80 wt% acetonitrile (ACN) by spectrophotometric measurements. Standard Gibbs energies of transfer, [Formula: see text] (B) of p-nitroaniline (B) from water to other solvents have been evaluated from the measurement of solubilities at 25 °C. Solvent effect on the dissociation of the acid (BH+), δ(ΔG0) = 2.303RT[p(sK) − p(wK], results in a characteristic minimum with a change in the solvent composition. The result in this protic – dipolar aprotic mixture (H2O + ACN) has been compared with that in aqueous mixture of protic solvent (H2O + ethylene glycol) available from literature. The solvent effect has been discussed in terms of the standard Gibbs energies of transfer [Formula: see text], from water to aqueous mixtures of organic co-solvent of the uncharged base (B), the hydrochloride of the base (BHCl), and hydrochloric acid (HCl) and also in terms of the individual species involved in the dissociation process. The results indicate that solvent effect on dissociation is an involved process, guided by the combined effects of various types of solute–solvent interactions of the Brønsted acid and its conjugate base in addition to the relative solvent basicities.
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