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

Abstract

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.