Kinetics and Equilibria in the System Pyrophosphate‐Nickel (II). Evidence for Two Parallel Reaction Paths; Influence of Ionic Strength on the Ion Pair Constant

Abstract

AbstractThe kinetics of the reaction between Ni2+ and pyrophosphate have been investigated over a wider range of conditions than previously. At 25°, a variation of the ionic strength between 0.02 and 0.20 M (achieved by adding potassium nitrate at appropriate concentrations) showed that the salt effect, though acting on the formation constant of an ion pair rather than on the rate constant of the rate‐determining step, is essentially “classical”. By varying the pH between 4.5 and 7.0, at least two parallel reaction paths were observed, involving Ni2+ and the ions HP2O3−7 and H2P2O2−7. The latter path has a rate constant that is ∼28 times smaller than that of the former and makes an appreciable contribution to the observed rate only at the two lowest pH values. Possible reasons for this difference in reactivity are discussed. The rate constant for the path involving HP2O3−7 was 1.61 × 106M−1 s−1 at 25°, I = 0.1 M. The activation parameters for this path are ΔH≠ = 18.8 Kcal mole−1 and ΔS≠ = 33 e.u. All kinetic experiments were carried out by the T‐jump method. The equilibrium constants necessary for the evaluation of the results were determined by potentiometric titration.