Electrolysis of Copper(II) Sulfate and Purifying Copper

1Why electrical wire must be so pure

Copper conducts electricity better than any metal except silver, but only when it is very pure: even small traces of other elements raise its resistance, so wire made from it would waste energy as heat. Copper for wiring is therefore refined to better than 99.9% purity, and the refining is done by electrolysis. The same cell shows a second idea: what happens when the electrodes themselves take part.

2Copper(II) sulfate solution with inert electrodes

The solution contains $\mathrm{C}{\mathrm{u}}^{2+}\backslash mathrm\{Cu^\{2+\}\}$, $\mathrm{S}{\mathrm{O}}_4^{2-}\backslash mathrm\{SO\_4^\{2-\}\}$, ${\mathrm{H}}^{+}\backslash mathrm\{H^+\}$ and $\mathrm{O}{\mathrm{H}}^{-}\backslash mathrm\{OH^-\}$. With graphite or platinum electrodes:

• Cathode (−): copper is less reactive than hydrogen, so copper is deposited as a pink-brown coat: $\mathrm{C}{\mathrm{u}}^{2+}(\mathrm{a}\mathrm{q})+2{\mathrm{e}}^{-}\to \mathrm{C}\mathrm{u}(\mathrm{s})\backslash mathrm\{Cu^\{2+\}(aq)\; +\; 2e^-\; \backslash rightarrow\; Cu(s)\}$.
• Anode (+): sulfate is never discharged, so hydroxide ions are, and oxygen bubbles off: $4\mathrm{O}{\mathrm{H}}^{-}(\mathrm{a}\mathrm{q})\to 2{\mathrm{H}}_2\mathrm{O}(\mathrm{l})+{\mathrm{O}}_2(\mathrm{g})+4{\mathrm{e}}^{-}\backslash mathrm\{4OH^-(aq)\; \backslash rightarrow\; 2H\_2O(l)\; +\; O\_2(g)\; +\; 4e^-\}$.
• Solution: as copper ions are removed, the blue colour fades; the ${\mathrm{H}}^{+}\backslash mathrm\{H^+\}$ and $\mathrm{S}{\mathrm{O}}_4^{2-}\backslash mathrm\{SO\_4^\{2-\}\}$ left behind make it sulfuric acid.

$$2\mathrm{C}\mathrm{u}\mathrm{S}{\mathrm{O}}_4(\mathrm{a}\mathrm{q})+2{\mathrm{H}}_2\mathrm{O}(\mathrm{l})\to 2\mathrm{C}\mathrm{u}(\mathrm{s})+{\mathrm{O}}_2(\mathrm{g})+2{\mathrm{H}}_2\mathrm{S}{\mathrm{O}}_4(\mathrm{a}\mathrm{q})\backslash mathrm\{2CuSO\_4(aq)\; +\; 2H\_2O(l)\; \backslash rightarrow\; 2Cu(s)\; +\; O\_2(g)\; +\; 2H\_2SO\_4(aq)\}$$

3Copper(II) sulfate solution with copper electrodes

Now the anode is made of copper, which loses electrons more easily than hydroxide ions do. The anode itself dissolves:

$$\text{anode: }\mathrm{C}\mathrm{u}(\mathrm{s})\to \mathrm{C}{\mathrm{u}}^{2+}(\mathrm{a}\mathrm{q})+2{\mathrm{e}}^{-}\text{cathode: }\mathrm{C}{\mathrm{u}}^{2+}(\mathrm{a}\mathrm{q})+2{\mathrm{e}}^{-}\to \mathrm{C}\mathrm{u}(\mathrm{s})\backslash text\{anode:\; \}\; \backslash mathrm\{Cu(s)\; \backslash rightarrow\; Cu^\{2+\}(aq)\; +\; 2e^-\}\; \backslash qquad\; \backslash text\{cathode:\; \}\; \backslash mathrm\{Cu^\{2+\}(aq)\; +\; 2e^-\; \backslash rightarrow\; Cu(s)\}$$

Copper is simply carried from the anode to the cathode. The anode loses exactly the mass the cathode gains, no gas forms, and the blue colour stays the same because a copper ion enters the solution for every one that leaves.

Table 3.1: Copper(II) sulfate solution with two kinds of electrode

4Purifying copper

Copper from the smelter is about 99% pure. It is cast into thick slabs that are made the anodes, with thin sheets of pure copper as the cathodes, in acidified copper(II) sulfate solution (Figure 3.1). Copper dissolves from the anode and is deposited, pure, on the cathode. Impurities more reactive than copper, such as zinc and iron, dissolve and stay in solution; less reactive metals, such as silver and gold, do not dissolve and fall to the bottom as anode sludge, which is collected and sold.