An organic–inorganic nano-composite poly-o-anisidine Sn(IV) tungstate was chemically synthesized by sol–gel mixing of the incorporation of organic polymer o-anisidine into the matrices of inorganic ppt of Sn(IV) tungstate in different mixing volume ratios. This composite material has been characterized using various analytical techniques like XRD (X-ray diffraction), FTIR (Fourier transform infrared), SEM (Scanning electron microscopy), TEM (Transmission electron microscopy) and simultaneous TGA (Thermogravimetric analysis) studies. On the basis of distribution studies, the material was found to be highly selective for Hg(II). Using this nano-composite cation exchanger as electro-active material, a new heterogeneous precipitate based on ion-sensitive membrane electrode was developed for the determination of Hg(II) ions in solutions. The membrane electrode was mechanically stable, with a quick response time, and can be operated within a wide pH range. The electrode was also found to be satisfactory in electrometric titrations.