Conductometric study of cobalt(II), nickel(II), copper(II), and zinc(II) perchlorates in methanol-ethylene glycol mixtures.

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The ionic association constants of M(ClO4)2 (M=Co, Ni, Cu, Zn) were determined by conductometric measurements in methanol–ethylene glycol mixtures of 100, 80, 60, 40, 20, and 0 wt% methanol compositions at 5, 15, 25, 35, and 45 °C. The conductometric data were analyzed using a modified Fuoss and Edelson method. The association constant of an ion pair M2+ClO4−, K1A, was studied and thermodynamic functions, ΔH1A\ominus and ΔS1A\ominus, for that association were determined at 25 °C. Free-energy changes, ΔG1A\ominus (=–RTlnK1A), at 25 °C which slowly decreased with an increase in the ethylene glycol content in a solvent, are in good agreement with estimates from Fuoss' theory regarding ionic association. Values of both ΔH1A\ominus and ΔS1A\ominus were positive in many systems: i.e., unfavorable enthalpy and favorable entropy changes, and those were obtained near to estimates from the Fuoss' theory in pure methanol, 100 wt%. However, there have been a few systems with a negative ΔH1A\ominus (Co, Ni, and Zn at 40 and 20 wt%, and Co at 0 wt%), contrary to theoretical expectations. These results are discussed from the standpoint of a changing solvation between M2+ and MClO4+. In all the solvents, moreover, the ΔH1A\ominus and ΔS1A\ominus values have been in a sequence according to Co<Ni<Cu>Zn. The results of limiting molar conductances, Λ0, are discussed regarding Walden products.
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