Copper purification and the electrolysis core practical

At the positive copper anode, copper atoms lose electrons and enter the solution as copper ions. The anode loses mass rather than producing oxygen gas.

These processes happen together: copper ions removed from the solution at the cathode are replaced by copper ions formed at the anode. Ideally, the cathode's mass gain equals the anode's mass loss, and the copper-ion concentration remains approximately constant. This is a transfer of copper, not the creation of extra copper.

Using this transfer to purify copper

To purify copper, use impure copper as the anode, a thin sheet of pure copper as the cathode, and copper sulfate solution as the electrolyte.

Copper from the impure anode enters the solution as ions. Copper ions then form pure copper on the cathode, which becomes thicker as the anode becomes thinner. Impurities are not deposited along with the copper: insoluble impurities collect beneath the anode as sludge. This separates copper from those impurities. The sludge can contain valuable metals such as silver.

Measuring mass changes with copper electrodes

Use two pieces of copper foil, a 100 cm³ beaker, copper sulfate solution, a low-voltage d.c. supply, connecting leads and crocodile clips. An ammeter measures the current, and a variable resistor allows it to be adjusted. You also need emery paper, a balance, a stop clock, distilled water and access to propanone in a fume cupboard.

1. Clean the copper foil with emery paper. Label the pieces anode and cathode, then measure and record each initial mass.
2. Half-fill the beaker with copper sulfate solution. Connect the anode to the positive terminal and the cathode to the negative terminal. Keep the electrodes apart: touching electrodes would allow current to bypass the solution.
3. Connect the ammeter and variable resistor in series with the electrolytic cell. Switch on and adjust the current to 0.2 A.
4. Run the electrolysis for 20 minutes, checking the ammeter and adjusting the variable resistor if needed to keep the current constant.
5. Switch off before removing the electrodes. Gently rinse them with distilled water, dip them into propanone in the fume cupboard, and allow the liquid to evaporate completely.
6. Weigh each dry electrode, keeping track of which is which. Repeat using 0.3 A, 0.4 A and 0.5 A, with the same electrolysis time.

Wear eye protection and avoid skin contact with copper sulfate solution. Wash any splashes from skin. Propanone is an irritant and highly flammable: there must be no naked flames in the laboratory while it is used.

Turning measurements into a conclusion

Record current in amperes and initial and final electrode masses in grams. Calculate each mass change using:

$$\text{mass change} = \text{final mass} - \text{initial mass}$$

The cathode should have a positive mass change and the anode a negative mass change. Alternatively, record mass gained and mass lost as positive quantities, but label them clearly.

Plot current on the horizontal axis and mass change on the vertical axis. For a fixed time, a larger current transfers more charge, so more copper is deposited at the cathode and more is removed from the anode. Ideally, the mass gained or lost is directly proportional to current: doubling the current doubles the amount of copper transferred.

Suppose the cathode gains 0.08 g at 0.2 A, 0.12 g at 0.3 A, 0.16 g at 0.4 A and 0.20 g at 0.5 A, all for the same electrolysis time. These example values demonstrate the proportional relationship.

Data for Investigating current and copper deposition

To estimate a value at 0.35 A, locate 0.35 on the current axis, move up to the line, then across to the mass axis. Here the estimated cathode gain is 0.14 g. Under ideal conditions, the corresponding anode loss would also be 0.14 g, or a signed mass change of −0.14 g. Use the same interpolation method on your measured anode graph.

Real mass gains and losses may not match exactly. Copper can flake off the cathode during handling, or remaining liquid can affect a mass reading. Gently rinsing rather than wiping the electrodes, drying them fully and keeping current and time controlled improve the measurements. Repeat each current several times and calculate mean mass changes to check the reliability of the trend.

Comparing graphite electrodes

Replace the copper foil with two graphite rods and connect them to the same positive and negative terminals. Switch on and record what happens at each electrode.

Copper ions still gain electrons at the cathode, so copper is deposited there. At the inert anode, the electrode cannot supply copper ions. The water in the solution provides hydroxide ions, $\mathrm{OH^-}$. These lose electrons in preference to the sulfate ions, producing oxygen; sulfate ions remain in solution.

To identify the oxygen, collect the gas in a small inverted test tube initially filled with solution over the anode. Oxygen relights a glowing splint. Carry out the gas test separately from any use of propanone, with the flammable solvent removed. Switch off the power before dismantling the apparatus.

Higher tier: expressing the reactions as half equations

At the cathode, with either electrode material, copper ions gain electrons. This is reduction:

$$\mathrm{Cu^{2+} + 2e^- \rightarrow Cu}$$

At a copper anode, copper atoms lose electrons. This is oxidation:

$$\mathrm{Cu \rightarrow Cu^{2+} + 2e^-}$$

At a graphite anode, hydroxide ions lose electrons and oxygen forms. This is oxidation:

$$\mathrm{4OH^- \rightarrow O_2 + 2H_2O + 4e^-}$$

The electron positions show the distinction: reduction uses electrons, whereas oxidation releases them.

Oxygen relights a glowing splint. Ideally, copper electrodes give equal mass gain and loss, with approximately constant copper-ion concentration.

Copper purification

• Anode: impure copper.
• Cathode: thin sheet of pure copper.
• Electrolyte: copper sulfate solution.
• Copper transfers to the cathode; insoluble impurities collect as anode sludge.

Core practical recall

Clean, label and weigh copper electrodes → electrolyse for 20 minutes at constant current → switch off → rinse, use propanone safely, dry and reweigh.

Investigate 0.2, 0.3, 0.4 and 0.5 A, keeping time constant. Plot mass change against current; interpolate to estimate at 0.35 A. Repeat each current and calculate means.

$\text{mass change} = \text{final mass} - \text{initial mass}$