Asymmetrical Schiff bases as carriers in PVC membrane electrodes for cadmium (II) ions

5-[((4-Methyl phenyl) azo)-N-(6-amino-2-pyridin) salicylaldimine] (S-1), and 5-[((4-methyl phenyl) azo)-N-(2-diamino-2-cyano-1-ethyl cyanide) salicylaldehyde] (S-2) with N and O donor atoms are effective ionophores to make Cd2+-selective membrane electrodes. The electrodes based on S-1 and S-2 exhibits a Nernstian or near-Nernstian response for cadmium ion over a wide concentration range 1.5 x 10(-1)-7.5 x 10(-7) with a slope of 28 and 2.0 x 10(-1)-4.0 x 10(-7) with a slope of 22, respectively. They have quick response and can be used for three or four months without any divergence in potential. The proposed sensors show fairly good selectivity over some alkali, alkaline earth, transition and heavy metal ions. The electrodes based on S-1 and S-2 can be used in the pH range 3.5-9. These electrodes were used as an indicator electrode in potentiometric titration of cadmium ion with EDTA and in the direct determination of cadmium ion in aqueous solutions.