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Copper in health
 Electrolysis 
The blister copper is already virtually pure (in excess of 99% copper). But for today's market, this is not really pure enough! It is purified further using electrolysis. This is known as electrolytic refining.

The blister copper is cast into large slabs which will be used as the anodes in the electrolysis apparatus (see page 4). The electrolytic refining of copper produces the high quality, high purity copper required by industry.

Photo of copper cathodes
Picture 9. The giant copper cathodes being lifted out of the solution. Notice all the electrolytic cells underneath.
Electrolysis
You may well have carried out this process in your school laboratory.

In industry this is carried out on a massive scale. Even the best chemical method cannot remove all the impurities from the copper, but with electrolytic refining it is possible to produce 99.99% pure copper.

The blister copper anodes are immersed in an electrolyte containing copper sulphate and sulphuric acid. Pure copper cathodes are arranged between the blister copper anodes and a current of over 200A passes through the solution.

What happens in electrolysis?
Under these conditions, copper atoms dissolve from the impure anode to form copper ions. These migrate towards the cathodes where they are deposited back as pure copper atoms.

At the anode: Cu(s) → Cu2+(aq) + 2e-

At the cathode: Cu2+(aq) + 2e- → Cu(s)

This is the same reaction as the one you may have tried in the school laboratory.

Interactive graphic of electrolysis
Interactive graphic of electrolysis
Picture 10. Laboratory electrolysis apparatus.

Laboratory electrolysis
When we close the switch, copper ions on the anode will start to move through the solution to the cathode. The copper atoms have already given up two electrons to become ions and their electrons are free to move in the wires. Closing the switch pushes electrons clockwise and forces some copper ions into the solution.

The cell pushes the ions from the anode to the cathode. At the same time, it pushes the free electrons around the wires (these electrons are already spread through the wires). Electrons in the cathode recombine with copper ions from the solution, forming a new layer of copper atoms.

Gradually, the anode is eroded and the cathode grows. Insoluble impurities in the anode fall to the bottom in a pile. This valuable biproduct is removed.

What happens to the impurities?
Gold, silver, platinum and tin are insoluble in this electrolyte and so do not deposit on the cathode. They form a valuable 'sludge' that collects under the anodes.

Soluble impurities of iron and nickel dissolve in the electrolyte, which has to be continually purified to prevent excessive deposition onto the cathodes, which would reduce the purity of the copper.

Recently, stainless steel cathodes have replaced copper cathodes. Identical chemical reactions take place. Periodically, the cathodes are removed and pure copper is scraped off.

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Question 4
In the school laboratory, a student set up an electrolysis of copper sulphate using copper electrodes. She passed a current of 2A through the solution for 10 minutes.

Calculate:

a) the number of faradays that will deposit 1 mole of copper faradays
b) in 10 minutes,
i. the charge, in coulombs, that flows through the apparatus coulombs
ii. the number of faradays that flows through the apparatus faradays
iii. the number of moles of copper deposited on the cathode in 10 minutes moles
iv. the mass of copper deposited on the cathode in 10 minutes grammes

The molar mass of copper is: Ar(Cu) = 64.