A Thermodynamic Study on Hydrolytic Reactions of Lead(II) Ion in an Aqueous Solution and Dioxane–Water Mixtures. I. A Potentiometric Study

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Abstract
The hydrolytic reactions of lead(II) ions were studied at 25 °C by means of potentiometric titrations by using a glass electrode in water and dioxane–water mixture (dioxane contents: 0.1 and 0.2 mol fractions) containing 3 mol dm−3 LiClO4 as a constant ionic medium. Four complexes, Pb3(OH)33+, Pb4(OH)44+, Pb3(OH)42+, and Pb6(OH)84+ were found in the solvent systems examined at pH\lesssim8, and the formation constant *βpq for the reaction, qPb2++pH2O=Pbq(OH)p(2q−p)++pH+, was determined. The autoprotolysis constants Ki in the same solvent systems were also obtained by use of a hydrogen electrode. The formation constant βpq(=*βpq/Kip) for the reaction, qPb2++pOH−=Pbq(OH)p(2q−p)+ was evaluated by combining the formation constants *βpq and the autoprotolysis constants obtained. The free energy change of transfer from water to a dioxane-water mixture for the reaction could be estimates as ΔGpqt=−RTln{βpq(mix)⁄βpq(aq)}. The values (1/p)ΔGpqt of hydroxo complexes of various metal ions such as lead(II), copper(II), nickel(II), beryllium(II), and cadmium(II) ions were approximately independent of the complexes at a given concentration of dioxane. This result suggested that the formal charge per metal ion of the complex, z′ (=z−p/q; z: charge of a metal ion), rather than the total charge of the complex, zq–p, is essential for the interactions between the hydroxo complexes and solvent molecules.

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