Solvent Extraction of Cobalt(ll) and Nickel(ll) by a Quaternary Ammonium Thiocyanate

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Abstract
The solvent extraction of cobalt(II) and nickel(II) into xylene solutions of a quaternary ammonium thiocyanate (R4N.NCS) has been investigated and shown to depend markedly upon the identity of the counteranion (X−) present in the aqueous phase. The extent of extraction of both metals increases with increasing stability of the corresponding ion-pair, R4N.X. The dependence of metal distribution upon the concentrations of R4N.NCS, R4N.X, and X− is consistent with the extraction of cobalt from sulfate (X− = 1/2SO4 2), chloride (X− = Cl−), bromide (X− = Br−), and nitrate (X− = NO3 −) media according to the stoichiometry The importance of certain deviations from ideal behavior, namely association of the extractant in the organic phase and CoSO4 ion-pair formation in the aqueous phase, has been demonstrated. Examination of the electronic spectra of the organic extracts reveals that cobalt is extracted as the tetrahedral anion Co(NCS)4 2- while nickel forms an octahedral complex, probably Ni(NCS)4(H2O)2 2. Addition of n-octylamine (R.NH2) to the thiocyanate extractant enhances the extraction of nickel, and spectroscopic data are consistent with the existence of the equilibrium