The application of stripping processes in voltammetry

Abstract

The use of the hanging mercury drop electrode in conjunction with preelectrolysis and chronovoltammetric stripping has found wide application for the determination of trace amounts of certain ions which form amalgams or insoluble salts with mercury. When the concentrations of the trace elements to be determined are of the order of 10-6 M, the influence of the double-layer capacity on the stripping curve is negligible. However, the double-layer capacity is important when the concentrations are of the order 10-8 to 10 M, and at such concentrations, the presence of impurities or surface-active substances in the base electrolyte can be of vital importance. Some years ago, a two-electrode subtractive method of overcoming these effects was described6. An improved chronovoltammetric procedure will be discussed; this involves a variable time of pre-eleotrolysis and the addition of variable amounts of standard solutions. The new method gives more precise and reproducible results without the preparation of a complete calibration curve. Numerous examples are given to illustrate the proposed method. INTRODUCTION A predominant feature of modern analytical chemistry is the great importance of the determination of trace amounts of inorganic and organic substances. Electrochemical techniques of various types have been widely applied for such analysis but the polarographic method, proposed nearly fifty years ago by J. Heyrovsky, is undoubtedly the most important and has found extensive applications in trace analysis for inorganic and organic species present in solutions. Polarography in its conventional form has allowed many very important achievements, yet it has a considerable limitation in that concentrations below about lOmol/l cannot be determined. The diffusion current, d' thatflows in a polarographic circuit when a sufficient concentration of background electrolyte is present in the test solution, is composed of two components: the electrolysis current, i, and the capacity (condenser) current, i. Thus = + The concentration of the test substance is determined by measuring the height of the polarographic wave which depends on both these currents, bi only the electrolysis current component is proportional to the concentration of the 449