2 78 ELECTRO-ANALYSIS. chap.
however, the principal part is played by the cuprous ions. When the current density is very small (less than 0*01) cuprous sulphate is formed at the anode (at the ordinary temperature). As the current density increases, greater quantities of cupric sulphate are produced. The relative proportion of cuprous to cupric sulphate formed increases with rising temperature, so that at 100° and with current densities up to 0'3 amp./sq. dm. cuprous salt is almost exclusively formed. This cuprous salt is highly detrimental to the electrolysis ; for it decomposes partially according to the equation —
CU2SO4 + H2O ^ CU2O + HaSO* (or 2du + H2O ^ CuaO -f 2H).
The more acid {i.e. hydrogen ion) is present, the higher may the cuprous ion concentmtion be without this deoom- lK)sition occurring.
On the other hand, the cuprous ions are in equilibrium with the cupric ions —
2Cu = Cu + Cu
(caprouB ion =: copper + cupric ion).
At a certain acid (hydrogen ion) concentration, which increases with rising temperature, no cuprous oxide is precipitated. When it does deposit partially at the cathode and forms badly conducting spots, it gives rise to the warty appearance of the deposited copper. The copper is therefore less coherent, and the separation of the cuprous oxide should consequently be prevented by addition of sulphuric acid. The formation of cuprous oxide may also be hindered by the addition of certain organic substances (e.(/. alcohol). Probably the organic substance simply acts in a reducing capacity. Oettel (<9), who studied the accuracy of the copper voltameter, found that an addition of 5 per cent, of alcohol is suflBcient to prevent the disturbing effect of cuprous oxide at the cathode when the current density is small. In practice sulphuric acid is used, for the
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