Page:Text-book of Electrochemistry.djvu/292

 It might be supposed that it would be advantageous to separate all the copper at the cathode. According to Neumann, however, this is attended with poor results, for the deposit is then very spongy. In these investigations, Neumann used as anode a metal containing 50 per cent, copper and 50 per cent, nickel in one case, and in another 65 per cent, copper and 35 per cent. zinc. In the former case the electrolyte contained 46 grams of copper and grams of copper and 40 grams of sulphuric acid. The temperatures were 30° and 50° respectively, and the electro- motive force 0*5 volt. From these data we may conclude that only about 2 per cent, of the current passed through the copper sulphate, and therefore the greater part of the copper must have been deposited as the result of a secondary process. The current density, which at the beginning was 2*0 or 13, gi-adually diminished to 10 or 0*6 amp./sq. dm. respectively. Until the quantity of nickel or zinc in the bath became double that of the copper, the deposit was extremely good. If the proportion of copper is further decreased, the deposit becomes bud-like or warty, and later very spongy on account of the simultaneous separation of hydrogen ; at the same time the yield obtained from the current is greatly diminished. When this happens, a fresh quantity of electrolyte should be taken, and the copper in the old solution separated from the impurities by means of hydrogen sulphide. On account of secondary actions more metal is dissolved than is precipitated. Consequently the concentra- tion of the sulphuric acid diminishes (provided that the volume of the liquid does not decrease by evaporation of water). In the technical refining of copper insoluble sulphates are formed, and these sink to the bottom of the cell. The mode of action of the acid can be seen from the results obtained by Forster (7). It may first be mentioned that the acid greatly increases the conductivity of the electrolyte in the bath, and therefore prevents a good deal of loss of energy in the form of Joule heat. In the solution,

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