The Cathodic Polarization Curves in Electrodeposition of Metals

The preconcentration of trace metals by electrodeposition is an integral part of anodic-stripping voltammetry. The method consists of the preelectrolysis of the stirred solution with a small mercury drop or solid electrode as the cathode (112-114). The metals, which are deposited and dissolve in the mercury, are then stripped from the amalgam after a suitable rest period by a reversal of the electrode potential. The resulting current-polarization curve is characteristic of the metal and its concentration. Concentrations as low as 10 M of metal ions require a preelectrolysis of about 60 min or longer. Other electrodes such as mercury films, platinum, gold, silver, and various forms of carbon have been used (77 ). 

The electrodeposition of an alloy requires, by definition, the codeposition of two or more metals. In other words, their ions must be present in an electrolyte that provides a cathode film, where the individual deposition potentials can be made to be close or even the same. Figure ll.l depicts typical polarization curves, that is, deposition... 

Application of complex salt electrolytes in metal electrodeposition processes was examined by comparison of silver electrodeposition processes from the simple (nitrate) and complex (ammonium) electrolytes [15]. Silver was deposited from 0.10 M AgNOs in 0.20 M HNO3 (the simple electrolyte) and 0.10 M AgNOs in 0.50 M (NH4)2S04 to which was added ammonium hydroxide to dissolve the precipitate of Ag sulfate (the complex electrolyte). The conductivities of both electrolytes were almost the same [3]. Silver was deposited onto a stationary vertical Pt cathode (1x1) cm placed in the middle of a cylindrical cell (diameter 6 cm and height 5 cm). The surface of the cell was covered by anode of a high purity Ag plate. Polarization curves were recorded at the Pt wire electrodes at which Ag from the ammonium complex electrolyte was previously electrodeposited. 

This is the general behavior that may be expected whenever there are two possible reactions in a given potential range. Most of the time in metal electrodeposition, these two reactions are the reduction of metal cations and the discharge of hydrogen ions, but other cathodic processes are also possible, e.g., the discharge of dissolved oxygen. A similar behavior is to be expected in the electrodeposition of binary alloys when the difference between the equilibrium potential of the two metal redox couples is remarkable. The typical shape of the polarization curve as briefly discussed... 

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