The Downs cell

Sodium metal is made in industry, in large amounts, by the electrolysis of molten sodium chloride you have already studied.

Na⁺ (l) + e → Na (l)

2Cl^- (l) → Cl₂ (g) + 2e

2Na⁺ (l) + 2Cl^- (l) → 2Na (l) + Cl₂ (g)

(1) at the cathode; (2) at the anode; (1) × 2 + (2) overall

For this a special electrolytic cell is used, called the Downs cell (figure 12.2.9).

Figure to be added

Figure 12.2.9 — the Downs cell: a graphite anode (+) in the centre, a circular steel cathode (−) around it, a circular steel gauze diaphragm between them; chlorine gas leaves from above the anode and liquid sodium from above the cathode; the raw material is fed in at the top.

Lowering the melting point

The raw material is molten sodium chloride. Solid sodium chloride melts at a high temperature, about 840 °C. Adding about 40% calcium chloride to it lowers the melting point of the mixture to 600 °C.

Keeping the products apart

What would happen if the chlorine formed at the anode met the sodium formed at the cathode? They would react and form sodium chloride again. To prevent this, the anode and cathode are separated by a steel gauze diaphragm.

Chlorine gas is obtained as a by-product of this process, and it too can be used as a raw material for other products.

Why calcium chloride

Keeping a large cell at 840 °C costs a great deal of fuel, and sodium is volatile at that temperature. Lowering the melting point to 600 °C saves energy and makes the cell easier to run. Even with calcium chloride added, it is still the Na⁺ ions that are reduced at the cathode, so the product stays sodium.

The Downs cell

Molten NaCl with CaCl₂ at about 600 °C; graphite anode gives chlorine, steel cathode gives liquid sodium; a steel gauze keeps them apart.