A Cadmium-Selective Electrode Based on Ionophores with Nitrogen-, Sulfur-, and Oxygen-Containing Functional Groups

We assessed a possibility of using octadecylamine (ODA) as an ionophore component of the membrane of a cadmium-selective electrode (Cd-SE). The equilibria in the membrane-solution system were studied depending on the acidity of the medium and the amount of the ionophore in order to reveal the mechanism of membrane functioning. At pH 1-2, octadecylamine is in the cationic form, and cadmium ions in a 0.1 M KI solution are in the form of [CdI3](-). The [ODA](+)[CdI3](-) ion pair formed under these conditions is most stable at pH 1 and pI 1, and the membrane responds only to triiodocadmate ions. The optimized composition of the Cd-SE membrane (wt %) includes polyvinyl chloride 32.8, octadecylamine 1.7 (ODA concentration 100 mM), and o-nitrophenyl octyl ether 65.5 with an internal electrolyte of CdCl2 (0.01 M) + HCl (0.01 M) + KI (0.1 M). The following electrochemical characteristics of the Cd-SE were found: linearity range 1 x 10(-4)-1 x 10(-2) M, slope of the electrode function -53 mV/decade, and limit of detection for cadmium 1 x 10(-6) M. The potentiometric selectivity coefficients of the cadmium-selective electrode were determined by the method of bi-ionic potentials. The proposed Cd-SE was used to determine cadmium in a POSK 50-18 alloy and milk, using stationary and flow-through modes, respectively. We found the optimal operation conditions of the flow-injection system, ensuring the stability of the baseline and the maximum sensitivity and productivity of analyzes. The accuracy of the results of the determination of cadmium was confirmed by atomic absorption spectrometry and using the standard addition method.