organic substance would be too costly. The concentration of the sulphuric acid and the current density must not be too high, for otherwise so much hydrogen is separated at the cathode that it is not completely removed by the secondary processes, and the deposited copper becomes spongy and pulverulent. Small changes can be brought about in the deposited metal by altering the current density, and these have a great influence on the hardness and electrical conductivity of the copper; use is made of this fact in practice.
The smaller the number of ciipric ions the lower is the concentration of cuprous ions ; according to the above equation the concentration of the latter is proportional to the square root of that of the cupric ions. The concentra- tion of the cupric ions is greatly reduced by the addition of the acid, and to a still greater extent by the addition of salts which are able to form copper double salts. It may easily be conceived that similar relationships hold good for other metals. In the deposition of silver, organic substances are frequently added to the bath " to increase the polish of the metal."
Precipitation of Metal from a Solution containing Two Metal Salts. — If a solution contains two metals of different solution pressures, say silver and copper as nitrates, two cases may occur on electrolysis. The electromotive force used is either so great (over 1*14 volts) that it exceeds the solution pressures of both silver and copper, or it is sufficient (between 07 and 1*14 volts) just to overcome the solution pressure of one of the metals. This leads to a method, suggested and applied by Freudenberg (9), for the separa- tion of one metal from another analytically. The method is not good when the solution pressures of the two metals lie close together. In technical work, too, great use has been made of this principle, for instance in the separation of gold from the platinum metals (the gold being much more readily deposited from hydrochloric acid solution), or of silver from copper and other metals (from nitric acid solution). The
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