Electrolytic processes for obtaining pure metals such as copper and aluminum on the industrial scale are very important and are often discussed in high school chemistry classes and introductory chemistry and physical chemistry seminars in undergraduate school. The electrolytic purification step of copper consists of the anodic dissolution of impure copper and simultaneous deposition of pure copper at the cathode. The electrolyte is a concentrated aqueous copper(II) sulfate−sulfuric acid solution. During electrolysis, impurities such as Fe and Zn become oxidized to ions in solution. Other impurities such as Ag, Au, Pt, and Pd are not oxidized and fall to the bottom as a sludge. Isolation of copper from 5-cent coin1 and the electrolytic recovery of copper from brass2 are
laboratory simulations of industrial electrolytic purification. The microscopic process is described with electrolysis of
copper sulfate solution using copper electrodes.3 Transfer of copper metal from the anode to the cathode occurs during electrolysis. At the anode, each copper atom loses two electrons to form one Cu2+ ion, which dissolves in the solution, as shown below.
Cu(s) → Cu 2 +(aq) + 2e−
The positively charged copper ions move toward the cathode where they gain two electrons and become copper atoms, which are deposited on the cathode, as shown below.
Cu 2 +(aq) + 2e − → Cu(s)
However, when electrolyzing aqueous CuSO4, dissolution of Cu2+ ions from the anode cannot be observed because the aqueous CuSO4 is already blue. Even the deposition of copper on the cathode is somewhat difficult to observe during the