Ion–ion–solvent interactions in aqueous ionic cosolvent systems. IV. Transfer energetics of water, p-nitroaniline, and benzoic acid from emf/solubility measurements at different temperatures in aqueous sodium nitrate solvent system

Abstract

(K,) of aqueous mixtures of I , 2 , and 4 m sodium nitrate used as an ionic cosolvent system have been determined from emf measurements of the cell: Pt, H2 (g, 1 atm) / KOH (m I ) KC1 (tn2), solvent / AgC1-Ag at five equidistant temperatures ranging from 15535°C. The standard free energies (AGO) and entropies (AS') of autoionisation of the solvents were then evaluated from these data. Relative free energies (AGO) and entropies (AS') of autoionization of the solvents when coupled with the previously determined transfer free energies [AG:(H+)] and entropies [AS:(H++)] of H' yielded AGY(OH),,, [= AGY(0H-) -AG:(H,O)], As:(OH-), and ASF(H20). Values of AG:(Ht.) and A G Y ( 0 H ) obtained after correcting for AG;(H~o), as well as A@,,,,in,(H+) and AGfl,i,,(O~-) obtained after correcting the "cavity effect" and Born-type electrostatic effect suggests that while the "basicity" of the aqueous NaN03 solutions decreases, the "acidity" more or less increases with NaN03 concentration. The observed A$,,,.i,,(OH-)-and A~l r , i , , (~,~) -c o m p o s i t i o n profiles were also examined in the light of Kundu et al.'s four-step transfer process and the involved order-disorder phenomena, respectively, as proposed earlier.Standard free energies (A@) and entropies (AS:) of transfer of p-nitroaniline (pNA) and benzoic acid (HBz) for the solvent system have also been determined from solubility measurements at different temperatures. The observed AGY-and AGf,,,in,-composition profiles appear to reflect the salting-out effect of the salt and the AS,' -and A$,,,,in,-composition profiles confirm the applicability of either of these quantities rather than A$.,r,i,,(HX), as a better structural probe both for aquo-ionic and aquo-organic solvents. [Traduit par la revue] Introduction In our previous papers (1-3) we have reported the thermodynamics of HX, X = C1 (I), Br, and I (3), and also the transfer energetics of some electrolytes and the related "ion-ionsolvent" interactions (2) in aqueous solutions of 1, 2, and 4 rn NaN03 taking each composition as an individual solvent and taking the infinitely dilute solution of solutes in each solvent as the standard state as in aquo-organic solvent systems.Since autoionization constants of water in aquo-organic solvents reflect acidity and basicity of the solvents, it would be of particular interest to evaluate the same for this solvent system as well. Moreover, it is now increasingly recognised that thermodynamics of autoionization of water and the transfer energetics of various non-electrolytes including water, provide useful information regarding the three dimensional (3D) "structuredness" of the solvent (4-13). The basis of structural