Determination of Mercury(II) by Solvent Extraction-Polarographic Method

Abstract

The application of polarogphic method to the CHCl₃-extract of the ternary complex composed of Hg(II), Cl- and pyridine(py) is studied. Mercury(II) reacts with chloride ion in the presence of pyridine and forms the ternary complex HgCl₂py₂. The complex is extractable into chloroform in the pH range from 5.5 to 6.5. The ternary complex can be reduced at the dropping mercury electrode and the reduction wave is applied to the determination of mercury(II). At first, mercury(II) is extracted into chlorofrom as the ternary complex HgCl₂py₂. The 6 N HNO₃, the most suitable supporting electrolyte solution, and methylcellosolve, the mixing solvent, are added to the chloroform extract. This ternary mixture is employed as the polaro- graphic electrolyte solvent. In the AC polarographic method, it is unnecessary to purge dissolved oxygen. The reduction of the ternary complex HgCl₂py₂ may be considered to proceed in one step involving the addition of two electrons, as shown by HgCl₂py₂ + 2e^- Hg↓ + 2Cl^- + 2 py The half-wave potential of this wave is about +0.18 V vs. SCE. The plot of log{i²/(i_a-i)} against potential E is linear and has a slope of 30 mV at 20°C. The limiting current i_d is proportional to the root of the height of the mercury column. Consequently, the electrode reaction is reversible and diffusion-controlled. The relative temperature coefficient of i_d is 1.44% deg^-1 at 20°C. The available range of the present method covers from 0.2 mg (5 x 10^-5 mol/l) to 1 mg (3 X 10^-4 mol/l) of mercury (II) in 20 ml of sample water and the time required for a single deter-mination is about 30 min. Cu²⁺, small amounts of Fe²⁺ and Fe³⁺ do not interfere, but I^-, Br^- and SCN^- interfere.