Ion-selective electrodes for thiocyanate based on the dinuclear Zinc(II) complex of a Bis-N,O-bidentate Schiff base

An ion selective electrode based on the dinuclear complex formed by two zinc(II) ions and two molecules of the bis-N,O-bidentate Schiff base 2,2'-[methylenebis(4,1-phenylenenitrilomethylidyne)]bisphenol exhibits thiocyanate selectivity with a good discrimination of nitrite, nitrate, and azide. The selectivities of electrode membranes with various compositions indicate that this potentiometric selectivity is based on the formation of a 1:1 complex between the thiocyanate anion and the dinuclear ionophore. The 2:1 ratio of thiocyanate ions and the dinuclear ionophore that results from higher ratios of cationic sites and ionophore worsens the selectivity, suggesting that binding of a thiocyanate to both zinc(II) centers of the dinuclear ionophore is not favorable. Interestingly, the selectivity patterns of these electrodes differ radically from that of a highly sulfate selective electrode based on a compound reported previously to be the analogous mononuclear 1:1 complex of zinc(II) and the same Schiff base. It is suggested that the previously reported 1:1 complex with zinc(II) may indeed have been a polymer of the same elemental composition.