Ionic Strength Function for Strong Electrolytes in Anhydrous Acetic Acid. II. Copper(II) Acetate Monomer-Dimer Equilibrium

Abstract

In the preceding study it was demonstrated that the classical DebyeHuckel theory can be applied to the influence of a conventional salt upon the solubility of a slightly soluble strong electrolyte in anhydrous acetic acid as the solvent (5). The treatment required the assumption that only the dissociated ions of the diverse salt contribute to the ionic strength in dilute solutions. As a consequence, the ionic strength function, 4t, was replaced by (C a) t} for a 1-1 electrolyte, and the degree of dissociation? a, was calculated from the dissociation constant (Kd) for the ion pair. In the present communication this approach is employed in identifying an ionic strength eSect upon the copper(II) acetate monomer-dimer equilibrium in glacial acetic acid. The recently published data of Kochi and Subramanian (4) are used for this purpose. A dimer-monomer dissociation of a metal acetate is a potentially useful equilibrium for testing the Debye-Huckel theory in glacial acetic acid because: (a) transition metal acetates are significantly less ionic than alkali metal acetates and perchlorates and are among the weakest electrolytes in this solvent; (b) reliable basicity constants for the acetates can be obtained independently of the dimer dissociation constant; and (c) the problem of a competing solvolysis is eliminated by the use of acetates. The solid state composition of copper(II) acetate corresponds to the empirical formula Cu(OAc) 2 * L, in which L is the solvent molecule (HO, HOAc, pyridine, etc.). Measured magnetic moments, molecular weights, and infrared spectra indicate a dimeric species, and a binuclear structure has been assigned to copper(II) carboxylate complexes in the crystaline phase (1,8). In the copper(II) complex [Cu2(RC00)4L2l, each of the copper atoms is bonded to one molecule of solvent and these are arranged trans to each other. Copper(II) acetate exists in a dimeric form in n3naqueous solvents as well (7); however, the extent of dissociation is highly dependent upon the type of solvent. The dissociation constant for the dimer in glacial acetic acid has been determined indirectly by Kochi and Subramanian (4), and a mean value of 5.8 X 10-4 was obtained at 57°C. Copper(II) acetate is a very weak base in acetic acid, having an ionization constant of 1.26 X 10-8 at