Catalysis of solvated electron reactions by ion-pairing

The effect of Coulomb interactions on the rate of reduction of nitrate ions by solvated electrons in water-ethanol mixtures has been studied by pulse radiolysis. When the dielectric constant of the medium decreases, the rate constant decreases almost exponentially with 1/epsilon owing to the increase of the Coulomb repulsion between the two ions. In pure ethanol the rate constant is comparatively low, k(e- + NO3-) = 3.107M-1s-1 at zero ionic strength. Addition of the inert divalent cation Ca++ increases the rate of reaction. This is due to the formation of ion-pairs (NO3Ca)+ which, having an overall positive charge, react much faster with solvated electrons. The rate constant of the reaction in ethanol has been determined k(e- + (NO3Ca)+) = 1,2.1010M-1s-1 at zero ionic strength