.ALPHA.-Hydroxycarboxylate Selectivity by Spontaneous Formation of Dimeric Dialkyltin(IV) Complexes in Membrane of Anion-Selective Electrode

Abstract

The liquid-membrane ion-selective electrode based on bis(pentafluorobenzyl)tin(IV) dibromide showed higher selectivities to α-hydroxycarboxylates than to other carboxylates without any hydroxyl group and those with a β-hydroxy group. The calibration curve for citrate was linear over a range of 10−4 to 10−2 mol dm−3 with a potential slope of −60 mV/decade. The equilibrium study on the complexation between dimethyltin(IV) and a series of carboxylates indicates the formation of stable, neutral and dimeric complexes only with α-hydroxycarboxylates, [(Me2Sn)2(H−1hc)2] {(H−1hc)−2 : species formed by deprotonation of α-hydroxycarboxylates (hc−) at α-OH groups}. The corresponding species may be spontaneously formed as electroactive species in the liquid membrane containing bis(pentafluorobenzyl)tin(IV) dibromide and acquire the selectivity to α-hydroxycarboxylates.