The hydrolysis of some acidic metal cations in acetonitrile containing traces of water

Abstract

Acetonitrile is a solvent in which the halogen acids are highly associated. The strongly polarizing Al³⁺ and Ti⁴⁺ ions, when present in acetonitrile with comparable concentrations of water, form very stable hydroxy complexes. These two factors bring about the formation of halogen acids when the metal ions are dissolved in solvent containing halide ions. The results show (at least for aluminium) that it is the dissociation of water rather than of solvent molecules which is involved. Values obtained by polarography for the degrees of hydrolysis of Al³⁺ and Ti⁴⁺ in 25mM H₂O and 0.1M tetrapropylammonium chloride were 0.82 and 0.18 respectively. In 0.1M perchlorate media the values for AlCl₃ and TiCl₄ were 0.45 and 0.40 respectively, indicating that the hydrolysis of Ti⁴⁺ is limited as a result of complexing by competing chloride ions. Potentiometric titrations, conductance measurements, and u.v. spectra (for TiCl₄) support the polarographic results. No significant hydrolysis could be detected for Mg²⁺, So³⁺, Ga³⁺, and Sn⁴⁺ in halide solutions of acetonitrile.