Thermodynamics of Lithium-Isotope-Exchange Reactions. III. Electrochemical Studies of Exchange between Isotopic Metals and Aqueous Ions

This paper reports our results for the direct experimental determination of the equilibrium constant for the lithium-isotope-exchange reactionLi(s)7+6LiCl(aq)=6Li(s)+7LiCl(aq).This reaction was studied in an electrochemical quadruple cell without liquid junction of the type Li(s)7|7LiBr(PC)| 7Li(Hg)|7LiCl(aq)| Hg2Cl2 (s)| Hg(l)| Hg2Cl2 (s)|6LiCl(aq)|6Li(Hg)|6LiBr(PC)|6Li(s),where PC denotes the solvent propylene carbonate. The experimental value of the equilibrium constant at 296.6°K for this reaction is K = 1.046±0.013. The experimental value of K is compared with values for K calculated from statistical thermodynamic theory for various model reactions. The results of these calculations are consistent with a tetrahedrally coordinated structure for the aqueous lithium ions.