Kinetics of reaction between bromophenol blue and hydroxide ions in aqueous salt solutions at 298.15 K. Application of Pitzer's equations for ionic activity coefficients to kinetic salt effects

Abstract

Second-order rate constants are reported for the reaction between bromophenol blue and hydroxide ions in concentrated aqueous salt solutions at 298.2 K. Pitzer's equations for trace ionic activity coefficients in aqueous salt solutions are used to express the dependence of rate constants on ionic strength. The derived equation contains terms describing the role of charge–charge interactions together with further terms characterising interactions involving added anions and added cations. The experimental data are satisfactorily accounted for and the derived parameters pinpoint specific ionic effects, illustrating the importance of ion size and the hydrophilic/hydrophobic character of ions forming the added salts. The latter include alkali-metal, tetramethylammonium and tetraethylammonium cations. Kinetic data for reactions in aqueous solutions containing halide salts of these alkali-metal and tetramethylammonium cations are fitted in a single step to an equation for kinetic salt effects, thus leading to a common set of parameters for the complete data set. A separate fitting exercise was required for data describing the effects of added tetraethylammonium salts. The effects of these salts on the rate constant for reaction require consideration of ion–ion interaction parameters which are more complicated than simple pairwise additive, indicating a sensitivity of reacting and transition-state anions to ion–ion interactions involving added tetraethylammonium cations.