The salt solution cell

Metals immersed in water tend to ionise and pass into solution, leaving their electrons on the surface of the metal itself. A difference of potential therefore occurs between the solution, positively charged, and the metal, negatively charged.

Metals tend to pass into aqueous solution as ions; in the case of copper and zinc we have the following reactions: Cu ↔ Cu2+ + 2e –.

Because of the greater tendency to dissolve or, better, the higher stability of the oxidized state, more zinc atoms pass into the ionic state than copper ones.

When the ions pass into solution, the corresponding electrons remain on the metals, giving them a negative charge.

The process stops when the density of the electrons on the electrode is enough to make it too difficult for more ions to leave the electrode.

If the cell is shortcircuited (ie by joining the electrodes with a low resistance lead), the electrons would pass from zinc to copper and the sheet of zinc would dissolve entirely (this process is similar to corrosion).

It is worth mentioning that the positive ions do not spread throughout the water, but remain very close to the electrodes because of the presence of electrons on them (exerting an attractive force on the positive ions). This positive layer is named the Helmholtz layer and the electrode is then said to be polarized.

• Digital multimeter — 2275.10
• 1 × Connecting wire, red — 2522.03-04
• 1 × Connecting wire, black — 2522.04-08
• 2 × Crocodile clips — 2531.52
• 1 × Copper electrode — 4716.21
• 1 × Zinc electrode — 4716.16
• Test cell covers — 4718.10
• Beaker 50 ml — 1118.15

• Potassium nitrate solution, 1 M — R:8
• Sodium chloride solution 1 M

• Record the voltage and describe any change in the measurement.
• Comment on your results. Are there any differences between the two measurements?