Concentration Oscillations During Diffusion With Slow First-Order Chemical Reaction in Aqueous Solutions of trans-Dichloro-bis(ethane-1,2-diamine)cobalt(III) Chloride, trans-[Co(en)₂Cl₂]Cl

Abstract

The rate equations for diffusion and chemical reaction have been solved for isothermal diffusion with the first-order reaction

A → B

in stirred diaphragm cells. Provided pseudo-steady-state conditions are established in the diaphragms, the concentration differences across the diaphragm for the reactant and product are predicted to show damped oscillations in time for certain values of the solute diffusivities. Diffusion coefficients have been determined for aqueous solutions of trans-dichlorobis (ethane-1,2-diamine)cobalt(III) chloride in which the 1 : 1 salt slowly converts into a 1:2 salt through the reaction [Co(en)₂Cl₂]⁺ + Cl ^- → [Co(en)₂Cl]²⁺ +2Cl^- with k = 3.6×10^-5 s^-1 at 25°C. Supplementary ternary diffusion coefficients have been determined for aqueous solutions of potassium acetate + potassium sulfate, a ternary mixed electrolyte with ionic mobilities similar to those of the aqueous cobalt salts.