Surface-induced electrolytic dissociation of oxalic and phosphoric acid in mixed alcohol–water solvents

The electric conductance of solutions of oxalic, phosphoric and sulfuric acid (up to 0.025 M) in ethanol, methanol, and mixed alcohol–water solvents has been studied in the presence and absence of TiO2 (1–10% by mass). TiO2 enhanced the conductance of solutions of oxalic and phosphoric acid in anhydrous alcohol and in alcohol-rich mixed solvents. In water-rich mixed solvents “normal” behavior was observed, that is, TiO2 depressed the conductance of electrolyte solutions. The enhanced conductance is interpreted in terms of surface-induced electrolytic dissociation. In nonaqueous solvents, oxalic and phosphoric acid occur chiefly in molecular form, even in very dilute solutions. TiO2 binds the hydrogen oxalate or dihydrogen phosphate anions (which originate from neutral acid molecules) and the protons remain in solution in form of solvated cations. The lyonium cations have a substantially higher molar conductance than other ions due to the Grotthuss mechanism. This is why the enhancement of conductance is more substantial in solutions of weak acids than in solutions of other electrolytes.