Relationship between volts and amps for electrolytes

Demonstration

Solutions may show ohmic or non-ohmic characteristics.

Apparatus and Materials

• Power supply, low voltage, DC
• Ammeter, 1 A
• Voltmeter, 10 V
• Gas voltameter (see technical note)
• Copper voltameter (see technical note)

Health & Safety and Technical Notes

Saturated copper sulfate solution is harmful. Wear eye protection and keep it off the skin. Eye protection is also needed for the electrolysis of water, since the bursting gas bubbles may spray into eyes.

Read our standard health & safety guidance

A typical copper 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. Copper sulfate solution is used as the electrolyte (saturated, with 5% IM sulfuric acid added).

The gas voltameter referred to is equipment for the electrolysis of acidified water. At its simplest it need only be a beaker of water with a little sulfuric acid in it and carbon rod electrodes. A little acid is added to provide plenty of hydrogen ions, but it is the water which is used up and produces the gases; the original acid remains. (NB Oxygen attacks the carbon electrode, giving CO2 and loose carbon granules.)

Procedure

1. Connect the copper voltameter into a simple series circuit as shown. Alter the power supply to give voltages across the voltameter of 2 V, 4 V.... up to 10 V, and record pairs of readings, p.d. and current. Plot a graph of potential difference (y axis) against current (x axis).

2. Repeat this procedure using the gas voltameter.

Teaching Notes

• The graph of potential difference against current for the electrolysis of copper sulfate solution with copper electrodes is a straight line almost through the origin. This shows that it is an ohmic material with constant resistance.
• In blue copper sulfate solution, the current is carried by positively charged copper ions and negatively charged sulfate ions. The copper ions give the solution its blue colour. The copper ions drift slowly across to the negative electrode, and the positive ions drift slowly towards the positive electrode. Although the ions drift slowly, the current starts as soon as the battery is connected. This is because the whole liquid is full of a dense population of ions which start drifting immediately.

• The graph of potential difference against current for water electrolysis (part b) will show a straight line, but it does not go through the origin. It needs a small potential difference before it will begin to conduct. This suggests the existence of a back e.m.f.
• When the potential difference applied to the cell is small, some of the hydrogen bubbles produced at the cathode by electrolysis adhere to and soon coat this electrode. Likewise, oxygen bubbles adhere to and soon coat the anode. This process is known as polarization. The electrodes then behave as if they are made of hydrogen and oxygen, reversing the current in the cell and so giving a back e.m.f.
• When a larger external potential difference applied to the cell, it supplies or removes electrons from the electrodes at a sufficient rate to overcome this effect.

This experiment was safety-tested in January 2007