Spectrophotometric study of dissociation of p-nitroanilinium ion in urea-water solvents at 25°C—effect of solvent

Abstract

The dissociation constants ( K s ) of p-nitroanilinium ion have been determined at 25°C spectrophotometrically in urea-water mixtures containing 11.52, 20.31, 29.64 and 36.83 wt.% urea. Standard free enrgies of transfer, Δ G° t (B) of the base p-nitroaniline from water to the mixed solvents have been evaluated at 25°C from the measurement of solubilities. Strength of the acid, p-nitroanilinium ion increases with the increase in proportion of urea in the solvents, the rate of increase being sharp at the begining. The solvent effect on the dissociation of the acid: δ(Δ G°) = 2.303 RT [p( K s ) − p( K w )] is unusually large in urea—water solvent system compared to those in aqueous mixtures containing methanol, ethylene glycol, acetone and acetic acid. Solvent effect has been discussed in terms of the standard free energies of transfer (Δ G° t ) from water to aqueous urea solvents of the uncharged base (B), the hydrochloride of the base (BHCl) and the hydrochloric acid (HCl) and also of the individual species involved in the dissociation process. The corresponding data in ethylene glycol—water solvent system obtained from literature are also compared. The difference in solvent effects on the dissociation in the two solvent systems has been shown to result from the contrasting behaviour of Δ G° t (H +)−Δ G° t (BH +) arising from the difference in the extent of the solute—solvent interactions.