Interactions between metal-complex ions and water. Part 2.—Osmotic and mean activity coefficients of trivalent metal-complex chlorides in aqueous solutions at their freezing points

Abstract

The method of freezing-point depression has been used to determine the osmotic and activity coefficients of [ML₆]Cl₃, [M = Co^III or Cr^III; L = NH₃, H₂O, urea, ethylenediamine/2(= en/2), 1,3-diaminopropane/2(= tn/2), or 2,2′-bipyridine/2 (=bpy/2)] in aqueous solutions. Measurements have been made in the molality range 0.01–0.65. The osmotic coefficients have been fitted to the Debye–Hückel (DH) equation and the mean activity coefficients have been calculated. At a given molality these values increase in the following order: [Co(tn)₃]Cl₃ < [Co(en)₃]Cl₃ < [Co(NH₃)₆]Cl₃ < [Co(bpy)₃]Cl₃ < [Cr(urea)₆]Cl₃ < [Cr(H₂O)₆]Cl₃. This order does not agree with that of the radii of the metal-complex ions. The value of the expression ar^–2_c–27.27σ_c,p changes with σ_{c, p}, where a, r_c and σ_{c, p} are the ion size parameter of the DH equation, the ionic radius and the polarization surface charge density of a given metal-complex ion, respectively, and 27.27 is a constant derived from the critical distance of ion association. A similar relation was observed for alkali metal and tetra-alkylammonium chlorides using cation surface charge densities. These relationships suggest that the interactions between cations and the chloride ion are affected by the water structure.