A Hg2+ ion selective membrane electrode based on poly-o-toluidine Zr(IV) tungstate composite cation exchange material was fabricated using solution casting method. The effect of membrane composition and plasticizer on the potential response of the membrane electrodes were investigated by using varying amounts of electroactive material and plasticizer. The electrode with 100 mg of electroactive material and 12 drops of plasticizer exhibited better performance. The optimized membrane composition was used for the fabrication of ion selective membrane electrode which exhibited typical Nernstian response towards Hg2+ ions in the concentration range 1 x 10(-1)-1 x 10(-7) M with a Nernstian slope of 28.107 mV per decade change in Hg2+ ion concentration. The response time of the electrode under study for various concentrations of Hg2+ ions was found to be 15 s and the electrode can be used for 90 days without any considerable divergence in response potential. It can also be successfully used at various concentrations of Hg2+ ions in the pH range from 1.5 to 3.5. It was found selective for Hg2+ ions in the presence of various monovalent, divalent and trivalent interfering ions. It was also employed as an indicator electrode in the potentiometric titration of Hg2+ ions using ethylenediaminetetraacetic acid, disodium salt (EDTA) as titrant. The sharp end point corresponds to the stoichiometry of Hg(II)-EDTA complex. C) 2013 Elsevier B.V. All rights reserved.