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.