Electrolysis of copper sulfate solution

Demonstration

To some students the feeling for coulombs of charge comes most easily from electrolysis.

Apparatus and Materials

• Copper voltameters (exactly similar), 3
• Ammeters (0-1 A), DC, 3
• Rheostats (10-15 ohms), 3, rated to carry at least 1 A
• Batteries, 12 V, 3
• Top-pan chemical balance with a sensitivity of no less than 0.01 g
• Electrolyte: saturated solution of copper sulfate to which is added 5 per cent IM sulfuric acid

Health & Safety and Technical Notes

Saturated copper sulfate solution is harmful. Wear eye protection and keep off the skin.

Read our standard health & safety guidance

The copper voltameter should be designed so that it is easy to remove and replace the cathode. A typical voltameter consists of two clean copper electrodes held to the sides of a rectangular glass jar by bulldog clips and each fitted with a soldered terminal. This arrangement facilitates easy replacement in the same place.

Procedure

1. Before the lesson, set up three exactly similar circuits as shown and adjust the rheostats to give suitable currents.

2. In the case of electrodes with an immersed area of 8 x 5 cm, currents of 1 A in one case and 0.5 A in the other two cases would be very suitable: this corresponds to current densities of about 0.025 A per cm2 and 0.012 A per cm2.
3. Switch off the circuits and remove the cathodes. Wash, dry, and then lightly clean them with emery cloth or paper.
4. When the demonstration is to be performed, weigh the cleaned dry cathodes and replace them in the three circuits. Then switch on the 3 circuits together. After ten minutes, switch off the currents in the circuit carrying 1 A and one of the circuits carrying 0.5 A. After twenty minutes, switch off the third current. Remove the cathodes. Wash, dry and reweigh them, taking care to record which is which. You should find that the copper carried across is proportional to current x time.

Teaching Notes

• Like all metal ions, and hydrogen, the blue copper ions carry positive charges and the sulfate ions carry negative charges. So the plate on which the copper collects is connected to the negative of the battery.
• From these experiments you can extract the idea that the mass of copper carried across the cell is proportional to the current and to the time and hence to current x time.
• Each coulomb passing through copper sulfate solution deposits...
0.000 000 329 kg of copper
• The experiment which determined that number, had to have a standard ammeter and a stop watch. But once that is settled, you can reverse the argument and use that number in testing your own ammeters. That number can be sent on a post-card to a new laboratory elsewhere. This is much easier than sending a delicate standard ammeter by parcel post!

This experiment was safety-tested in October 2006