INTERACTION BETWEEN SILVER AND OTHER HEAVY-METAL IONS AND THE IONOPHORE S,S-DIPROPYL PYRIDINE-2,6-DICARBOTHIOATE

A detailed study has been made of the interaction of the neutral metal ion carrier S,S-dipropyl pyridine-2,6-dicarbothioate in methanol solu tion with silver perchlorate. Evidence has been found by potentiometry and confirmed by electrospray mass spectrometry for the existence of both 1:1 and 1:2 metal/ligand complexes, The formation constants of th e complexes in methanol have been determined from the potentiometry da ta and have been shown to be low (K-1 approximate to 50 L mol(-1), K-2 approximate to 8 L mol(-1)). The value for K-1 has been confirmed by NMR and voltammetric measurements. The high Ag+ selectivity found for the ionophore in a PVC-based sensor implies that the formation constan ts for the ionophore with other metal ions must be even lower than tha t for the ionophore with silver. A chemical and voltammetric study of the interaction between the ionophore and certain metal ions in methan ol has revealed that the ionophore structure breaks down in the presen ce of Ag+, Hg2+, and Pb2+, The Hg2+ ion is the most reactive of the th ree ions and rapidly leads to cleavage of the C(O)-S bonds in the iono phore, with the subsequent precipitation of a product containing the t hiol fragment. This reaction explains the irreversible poisoning effec t that the Hg2+ ion has on the sih er response of the sensor. In contr ast, the decomposition reaction in the presence of the Ag+ ion is extr emely slow, which explains why the short-term silver response of the s ensor is excellent. However, the reaction has severe implications with respect to the lifetime of the sensor, A mechanism is proposed for th e ionophore decomposition reaction in the presence of metal ions.