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

Abstract

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 LiClO 4 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 *β pq for the reaction qM z+ + pH 2O = M q(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, Δ G t pq = −RT[ln{β pq (mix)/β pq (aq)}] fot the reaction: qM z+ + pOH − = M q(OH) p qz−p+ (β pq = [M q(OH) p qz−p+]/[M +] q[OH −] p = *β pq/K p i; K i denotes the autoprotolysis constant of the solvent) were strongly dependent on the composition and charges of the complexes. However, the values (1/ p)Δ G t pq 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)Δ G t pq = ( /p)( q)Δg pq − Δg M) − Δg OH (Δg i stands for the partial molar free energy change of transfer of species i) and the contribution of Δg OH to (1/ p)Δ G t pq is the same in all the cases, the results obtained indicated that the values, (1/ q)Δg pq − Δg M′ 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, Δ H t pq and Δ S t pq , respectively, were evaluated. The value, (1/ p)Δ H t pq = ( q/p)((1/ p)Δ h pq − Δ h M) − Δ h OH, strongly depended on metals, where Δ h i denotes the partial molar enthalpy change of transfer of species i. For a given metal ion, (1/ p) t pq 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)Δ h pq − Δ h m depends on both p/q and Δ h M. For a strongly solvated metal ion ( i.e., (1/ p)Δ H t pq 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)Δ S t pq may become less positive. Therefore, the effect due to (1/ p)Δ H t pq on ((1/ p)Δ G t pq may be compensated by the effect due to (1/ p)Δ S t pq and thus the ((1/ p)Δ G t pq value becomes practically independent of metal ions.