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 solution 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 the complexes in methanol have been determined from the potentiometry data and have been shown to below (K¹ ≃ 50 L mol^-1, K₂ ≃ 8 L mol^-1). The value for K₁ 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 constants for the ionophore with other metal ions must be even lower than that for the ionophore with silver. A chemical and voltammetric study of the interaction between the ionophore and certain metal ions in methanol has revealed that the ionophore structure breaks down in the presence of Ag⁺, Hg²⁺, and Pb²⁺. The Hg²⁺ ion is the most reactive of the three ions and rapidly leads to cleavage of the C(O)-S bonds in the ionophore, with the subsequent precipitation of a product containing the thiol fragment. This reaction explains the irreversible poisoning effect that the Hg²⁺ ion has on the silver response of the sensor. In contrast, the decomposition reaction in the presence of the Ag⁺ ion is extremely slow, which explains why the short-term silver response of the sensor is excellent. However, the reaction has severe implications with respect to the lifetime of the sensor. A mechanism is proposed for the ionophore decomposition reaction in the presence of metal ions.