ELECTROLYTE-SOLVENT INTERACTION .20. PICRIC ACID IN MIXTURES OF ACETONITRILE AND BASIC SOLVENTS

The conductance of picric acid has been measured in the following mixed solvents: water-ethanol (WE), acetonitrile-ethanol (AE), acetonitrile-methanol (AM), and acetonitrile-water (WA). Association constants KΛ were calculated from the data. In solvents WA and WE, KΛ increases with decreasing dielectric constant D, but the log KΛ-D-1 curves are sharply concave upward. In solvents AE and AM, association decreases with decreasing dielectric constant. Both characteristics conflict completely with theory based on electrostatic control of ion association. The assumption that four molecules of base ROH (R = H, CH3, or C2H5) react with one molecule of picric acid, one to give the cation ROH2+ and three to solvate the three nitro groups of the picrate ion, leads to a conductance equation to which the data conform. The constant Ks = [ROH2+] [X-]/[HPi][ROH]4, where X- denotes the solvated picrate ion, is 2.5 × 10-5 for picric acid in water-acetonitrile, 4.0 × 10-7 in acetonitrile-methanol or acetonitrile-ethanol, and 0.4 in water-ethanol.