Abstract: Apparatus and methods are disclosed for the selective dissolution of a predetermined metal from, for example, a metal-metal combination. The metal or the metal-metal combination is made the working electrode of an electrolytic cell whose other electrode is electrolytically inert in the cell in use. The working electrode is held at a substantially constant electrical potential relative to a standard calomel electrode, and this potential and the pH of the electrolyte are selected such that the electrical energy passing through the cell during a selected time period is dependent on the amount of the predetermined metal electrolytically dissolved and substantially independent of any other metal which may be present.