Rhodium(II) Phthalocyanine as a Selective Carrier in Thiocyanate‐Selective Membrane Electrode

Abstract

An anion‐selective electrode based on rhodium(II) phthalocyanine (RhPc) as a neutral carrier is reported. The prepared electrode displays an anti‐Hofmeister behavior with a high selectivity for thiocyanate. The potentiometric response characteristics and selectivities of the membrane electrodes with various ratios of cationic additive to ion‐carrier indicate that the observed potentiometric response is based on the formation of a 1:1 complex between RhPc and thiocyanate as an axial ligand. The membrane electrodes containing ∼60 mol percent of cationic additive (relative to ionophore) exhibit a near‐Nernstian response in a concentration range of thiocyanate between 1×10−6 M to 1×10−1 M, a detection limit of 7.9×10−7 M, and a slope of −56.3 mV/decade in pH 6.0 using 0.1 M phosphate buffer solution. In comparison to the previously reported CoPc‐based membrane electrode, the prepared electrode showed better selectivity for thiocyanate over the relatively more hard anions, such as SO4 2−, H2PO4 −, HCO3 −, NO3 −, F−, Cl−, C2O4 2− and OAc−. This preference in response is assisted by the softness property of Rh2+ in the center of RhPc. The prepared electrode revealed a fast response time, sub‐micromolar detection limit, and a long‐time stability of more than 2 months (with less than 3% decrease in the slope of the potentiometric response). Application of the electrode with the optimum composition for determination of thiocyanate in pharmaceutical preparations (erythromycin thiocyanate) and human urine samples is reported. The electrode is also applied as an indicator electrode in potentiometric titration of milimolar solutions of thiocyanate and silver nitrate.