A thermodynamic study on hydrolytic reactions of divalent metal ions in aquaeous and dioxane-water mixed solvents

We have studied hydrolytic reactions of various divalent metal ions such as beryllium, copper, nickel cadmium and lead in aqueous and dioxane—water mixed solvents containing 3 mol dm−3 LiClO4 as a constant ionic medium at 25 °C [1–4]. It has generally been found that the composition of the hydrolytic species and the formation constant *βpq for the reaction qMz+ + pH2O = Mq(OH)p(qz−p)+ + pH+ were little affected by the solvent composition up to 0.5 mole fraction (ca. 88% w/w) of dioxane in the medium. Free energy changes of transfer, ΔGtpq = −RT[ln{βpq(mix)/βpq(aq)}] fot the reaction: qMz+ + pOH− = Mq(OH)pqz−p+ (βpq = [Mq(OH)pqz−p+]/[M+]q[OH−]p = *βpq/Kpi; Ki denotes the autoprotolysis constant of the solvent) were strongly dependent on the composition and charges of the complexes. However, the values (1/p)ΔGtpq were approximately independent of the complexes examined at a given concentration of dioxane. Since the free energy change of transfer can be expressed as (1/p)ΔGtpq = (/p)(q)Δgpq − ΔgM) − ΔgOH (Δgi stands for the partial molar free energy change of transfer of species i) and the contribution of ΔgOH to (1/p)ΔGtpq is the same in all the cases, the results obtained indicated that the values, (1/q)Δgpq − ΔgM′ depend only on p/q (= z − z′ where z′ represents the formal charge per metal ion of the complex). Enthalpy changes for the hydrolytic reactions of some divalent metal ions and the autoprotolysis reaction of the solvents were determined by use of a fully automatic on-line-controlled system developed in our laboratory [5] and the enthalpy and entropy changes of transfer of the reaction, ΔHtpq and ΔStpq, respectively, were evaluated. The value, (1/p)ΔHtpq = (q/p)((1/p)Δhpq − ΔhM) − ΔhOH, strongly depended on metals, where Δhi denotes the partial molar enthalpy change of transfer of species i. For a given metal ion, (1/p)tpq became more negative (or less positive) with an increase in z′ the value was practically independent of the composition of the complexes. The results obtained indicated that the value of (1/q)Δhpq − Δhm depends on both p/q and ΔhM. For a strongly solvated metal ion (i.e., (1/p)ΔHtpq may be largely negative for such a ion), the ion may have a large ordering effect for the solvent molecules even in the secondary solvation shell of the ion, and thus, (1/p)ΔStpq may become less positive. Therefore, the effect due to (1/p)ΔHtpq on ((1/p)ΔGtpq may be compensated by the effect due to (1/p)ΔStpq and thus the ((1/p)ΔGtpq value becomes practically independent of metal ions.