Downs Cell in Electrolytic Cells

The Downs Cell

• The Downs cell is used to produce sodium and chlorine in commercial quantities.
• The cell uses inert (unreactive) electrodes: a graphite anode and an iron cathode.
• At the cathode, Na+ ions are reduced to sodium metal: Na+(l) + e- → Na(l)
• At the anode, 2Cl- ions are oxidized to chlorine gas: 2Cl-(l) → Cl2(g) + 2e-
• The overall cell reaction is: 2Na+(l) + 2Cl- → 2Na(l) + Cl2(g)

Molten Electrolytes

• In the electrolysis of a molten electrolyte, anions are oxidized at the anode and cations are reduced at the cathode.
• Water is not present to interfere with the desired reactions.
• Calcium chloride is added to molten sodium chloride to lower its melting point, but it does not affect the overall cell reaction.

History of Sodium Isolation

• Sodium was first isolated by electrolysis in 1807 by Humphry Davy.
• Davy electrolysed molten sodium hydroxide to isolate sodium.
• He was the first person to isolate sodium, potassium, calcium, barium, magnesium, boron, and strontium using electrolysis.

Advantages and Disadvantages of Molten Electrolytes

• The advantage of using a molten electrolyte is that there is no water present to interfere with the desired reactions.
• The main disadvantage is that the process requires much more energy to maintain the electrolyte in a molten state.

Operating the Downs Cell

• The electrolyte is a mixture of sodium chloride and calcium chloride in a 1:2 ratio, which lowers the melting point of sodium chloride from 801 °C to about 600°C.
• An iron mesh screen is used to keep the products at the anode and cathode apart.
• The construction and operation of the Downs cell minimizes contact between the two products to prevent them from reacting to re-form sodium chloride.