Two-phase reactions of tin(IV)–tetraphenylporphine complexes as carrier of anion selective electrodes

Two-phase reactions of [Sn(OH)2(tpp)] with lipophilic anions and inorganic acids were studied and compared with those of [Zr(OH)2(tpp)] having the same oxidation state of the central metal ion but different structural characteristics. The reaction with tetrakis[3,5-bis(trifluoromethyl)phenyl]borate formed a 2:1 cationic dimer and 1:1 and 1:2 ion-pairs, while the reaction with dodecyl sulfate formed 1:1 and 1:2 ion-pairs. The 1:1 and 1:2 species formed concomitantly in the reaction with HCl and CH3COOH having higher coordination abilities, while forming in stepwise manner in the reaction with HClO4 and HNO3 having lower coordination abilities. The 2:1 cationic dimer of Sn(IV) had significantly lower stability than that of Zr(IV). The affinity of Sn(IV) for Cl− relative to OH− was much higher, compared with Zr(IV). Except in extremely acidic media, [SnCl2(tpp)] working as a carrier in PVC membranes hydrolyzed to give [Sn(OH)Cl(tpp)] or [Sn(OH)2(tpp)], which showed stronger but less-selective potential responses at lower pH and weaker but more-selective responses to salicylate at higher pH. The affinities and responses of Sn(IV) complexes to oxygen-containing anions were weaker than those of Zr(IV) complexes.