Apparent molar volumes for selected nitrogen base complexes of Co³⁺, Ni²⁺ and Cu²⁺ at 298 K

Abstract

The apparent molar volumes of aqueous solutions of seven alkyl diamines and two aromatic diamines, and 15 of their complexes with Co³⁺, Cu²⁺ and Ni²⁺ have been measured at 298 K over a wide concentration range down to 10^-4 mol dm^-3. Intercomparison is made of the ion, ligand and related complex limiting apparent molar volumes which follow an additive rule. The following pairs of complex ions, with similar geometric ionic radius and structure but of opposite charge, [Co(en)₃]³⁺ and [Co(ox)₃]^3-, and [Co(ox)₂(en)]- and [Co(en)₂(ox)]⁺ (en = ethylenediamine, ox = oxalate), show similar apparent ionic volumes, and solvent electrostriction. Solutions of [Ni(en)₃] Cl₂ undergo considerable dissociation: the effective apparent molar volume is analysed in terms of known dissociation constants and additional volumetric data for the generated species. For the less dissociated [CO(NH₃)₆]Cl₃, logK₆ is estimated to be 5.5 from the volumetric data.