Ionic equilibrium in mixtures of polar protophobic and protophilic non-hydrogen bond donor solvents: Acids, salts, and indicators in acetonitrile with 4 mass % dimethylsulfoxide

Abstract

Acetonitrile (AN) and dimethylsulfoxide (DMSO) belong to the so-called polar non-hydrogen bond donor solvents, in accordance with Bordwell's classification. At that, according to Kolthoff, AN is a protophobic, and DMSO is an expressed protophilic solvent. This article presents a study of the acid-base and related equilibria in a binary AN – DMSO solvent system with mass ratio 96: 4, which corresponds to the molar fraction xDMSO = 0.021. Owing to the aforesaid property of DMSO, the utilization of this binary mixture makes it possible to neglect the interfering influence of water traces, which is difficult to overcome in pure AN. As result, in the solvent under study, the proton is solvated first of all by DMSO despite predominance of AN. The solvent composition was chosen basing on the pKa values of the picric acid, which were determined via UV–visible spectroscopy at different AN – DMSO ratios. The interfering influence of water traces was checked. An operational paH+∗ scale was developed using a quinhydrone electrode in a concentration cell with liquid junction, basing on a picrate buffer solution with paH+∗ = 3.27. The cation-anion association constants of a set of salts were determined using the conductance method, the pKa values of the buffer acids (benzoic, salicylic, 2,4- and 2,6-dinitrophenols) were calculated, and the constants KAHA−f of homoassociation (A− + HA ⇄ AHA−) were estimated. The response of the glass electrode to the acidity of the solutions was proved within the range of 2.5 ≤ paH+∗ ≤ 15.5 (slope = 59.69), the paH+∗ values of 16 picrates of organic bases were determined, and the pKas of 7 indicators, nitrophenols and sulfonephthaleins, were obtained spectroscopically in the solvent under study. The dissociation constants of salts, acids, and indicators in AN–DMSO (96:4) mixed solvents were compared with those in other non-hydrogen bond donor solvents. Basing on some reliable assumptions, the values of the Gibbs energy of the H+ transfer from the binary solvent to AN and DMSO were estimated as ΔGtro = +44 ± 3 and −22.6 ± 3 kJ mol−1, respectively.