Ligand Function of Ion-Selective Electrodes Reversible to Zinc and Cobalt Thiocyanate Complexes: Causes of Formation, Mathematical Description, and Analytical Applications

Resumo

The manifestation of the ligand function of \({\text{Zn}}\left( {{\text{NCS}}} \right)_{4}^{{2 - }}{\text{-}}\) and \({\text{Co}}\left( {{\text{NCS}}} \right)_{4}^{{2 - }}{\text{-}}\)selective electrodes based on higher quaternary ammonium salts is explained by the exchange displacement of zinc and cobalt thiocyanates by SCN^– ions from the membrane into the near-electrode layer of the solution. Its action is limited by the dissociation of the quaternary ammonium salt as an associate with \({\text{Zn}}\left( {{\text{NCS}}} \right)_{4}^{{2 - }}\) and \({\text{Co}}\left( {{\text{NCS}}} \right)_{4}^{{2 - }}.\) ions. The effect of the background concentration of CoCl₂ or ZnCl₂ on the selectivity of \({\text{Zn}}\left( {{\text{NCS}}} \right)_{4}^{{2 - }}{\text{-}}\) and \({\text{Co}}\left( {{\text{NCS}}} \right)_{4}^{{2 - }}{\text{-}}\)selective electrodes based on higher quaternary ammonium salts to SCN^– ions is studied. The introduction of CoCl₂ or ZnCl₂ into the solution binds cobalt or zinc ions released from the membrane into thiocyanate complexes. The high selectivity of the \({\text{Zn}}\left( {{\text{NCS}}} \right)_{4}^{{2 - }}{\text{-}}\) and \({\text{Co}}\left( {{\text{NCS}}} \right)_{4}^{{2 - }}{\text{-}}\)selective electrodes to SCN^– ions is due to the higher extraction ability of the cobalt and zinc complexes with SCN^– ions compared to acid complexes with the competing anions. It is demonstrated that \({\text{Zn}}\left( {{\text{NCS}}} \right)_{4}^{{2 - }}{\text{-}}\) and \({\text{Co}}\left( {{\text{NCS}}} \right)_{4}^{{2 - }}{\text{-}}\)selective electrodes can be used for the determination of SCN^– ions in process solutions in the production of polyacrylonitrile fibers.