276 ELECTRO-ANALYSIS. chap.
is wholly used up in reducing the nitric acid, but after a time it begins to be evolved at the cathode. When evolution of hydrogen has taken place for a short time (ten minutes if a 2 per cent, nitric acid solution be used), the reduction may be assumed to be complete. |
Copper Refining. — The different behaviours of solutions of various metals on electrolysis have led to methods for separating the metals from each other. Some metals, like aluminium and uranium, are not deposited by the current, some of low solution pressure are deposited by an electro- motive force between the pole plates which is insufficient to separate those of high solution pressure. As an example of tliis type of separation we may take the technically important deposition in the refining of copper which has recently been fully studied by Neumann (6),
In the refining of this metal thin plates of copper are used as catliode, and the anode is a piece of crude cast copper. These are suspended in a wide vessel containing copper sulphate solution and sulphuric acid. When the current is passed, copper deposits in a coherent form on the cathode, and the crude copper is dissolved from the anode. The electromotive force may be from 0*25 to 07 volt, and as a rule the tension between the electrodes is 0*35 volt ; the current density is generally between 02 and 09 ampere per square decimetre. The optimum temperature for the process is about 40°. The impurities in the crude copper, metals of higher solution pressure (iron, zinc, nickel, and cobalt), gradually dissolve, so that the solution in the bath becomes richer in the sulphates of these metals, and poorer in copper sulphate. The other impurities, such as gold, silver, bismuth, antimony, and lead, remain undissolved, or form insoluble compounds (principally basic salts), and falling from the anode, collect in the so-called anode slime. Arsenic, and also antimony and bismuth partially, pass into solution, and must occasionally be removed from the bath by the addition of copper oxide. (Tin also may pass into solution, but is without influence on the nature of the copper deposit.)
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