Organopalladium(II) Complexes as Ionophores for Thiocyanate Ion-Selective Electrodes

A thiocyanate poly(vinyl chloride) (PVC) membrane electrode based on [1,2-bis(diphenylphosphino)ethane]dihalopalladium( II), [(dppe)$PdX_2$, X = Cl ($L^1$), X = I ($L^2$)] as active sensor has been developed. The diiodopalladium complex, [(dppe)$PdI_2](L^2$) displays an anti-Hofmeister selectivity sequence: $SCN^-\;>\;I^-\;>\;{ClO_4}^-\;>\;Sal^-\;>\;Br^-\;>\;{NO_2}^-\;>\;{HPO_4}^-\;>\;AcO^-\;>\;{NO_3}^-\;>\;{H_2PO_4}^-\;>\;{CO_3}^{2-}$. The electrode exhibits a Nernstian response (-59.8 mV/decade) over a wide linear concentration range of thiocyanate ($(1.0\;{\times}\;10^{-1}\;to\;5.0\;{\times}\;10^{-6}$ M), low detection limit ($(1.1\;{\times}\;10^{-6}$ M), fast response $(t_{90%}$ = 24 s), and applicability over a wide pH range (3.5∼11). Addition of anionic sites, potassium tetrakis[p-chlorophenyl] borate (KTpClPB) is shown to improve potentiometric anion selectivity, suggesting that the palladium complex may operate as a partially charged carrier-type ionophore within the polymer membrane phase. The reaction mechanism is discussed with respect to UV-Vis and IR spectroscopy. Application of the electrode to the potentiometric titration of thiocyanate ion with silver nitrate is reported.