2_5 Downs Cell: Making Sodium by Electrolysis of Molten Sodium Chloride

Sodium is too reactive to be made by heating its ores with carbon, the method used for less reactive metals such as iron. Its ion holds onto electrons only weakly, but the metal is such a strong reducing agent that no cheap chemical can push electrons back onto

\mathrm{Na^+}

. The answer is electrolysis, which forces electrons onto the ions with an electric current. Almost all sodium metal is made this way, in the Downs cell, from molten sodium chloride.

Why the Salt Must Be Molten

Solid sodium chloride does not conduct, because its ions are locked in the lattice. Dissolving it in water frees the ions, but then water is reduced at the cathode instead of sodium ions, giving hydrogen gas and hydroxide. Any sodium that did form would react with the water at once anyway. So the salt must be fused (melted): the liquid contains only

\mathrm{Na^+}

and

\mathrm{Cl^-}

ions, and sodium ions are the only species that can be reduced.

Pure NaCl melts at 801 °C, which is expensive to maintain and close to sodium's boiling point of 883 °C. Adding calcium chloride (often with some barium chloride) lowers the melting point of the mixture to about 600 °C. At that temperature sodium is a liquid that can be drawn off, it does not vaporize into a metallic fog, and the hot melt and products attack the cell materials much less.

Construction of the Cell

• Anode: a large cylindrical block of graphite at the centre of the cell. Graphite conducts and is not attacked by chlorine.
• Cathode: a circular iron or copper ring surrounding the anode. Sodium does not react with it.
• Separator: a cylindrical iron gauze screen between the electrodes. Ions pass through it freely, but it keeps the sodium and chlorine apart, since they would otherwise react straight back to sodium chloride.
• Chlorine dome: a hood over the anode collects the chlorine gas and leads it away.
• Sodium collector: liquid sodium is less dense than the molten salt, so it rises around the cathode into a collecting ring and is drawn off from time to time.

Electrode Reactions

In the melt the salt is fully broken into ions. When the current flows, sodium ions move to the negative cathode and gain electrons (reduction), while chloride ions move to the positive anode and lose electrons (oxidation):

\begin{aligned} \text{Cathode: } & \mathrm{Na^+ + e^- \rightarrow Na(l)} \\ \text{Anode: } & \mathrm{2Cl^- \rightarrow Cl_2(g) + 2e^-} \end{aligned}

[Na+].[Na+].[Cl-].[Cl-]>>[Na].[Na].ClCl

The cell gives sodium of about 99.9% purity and dry chlorine, which is itself a valuable product. A little calcium is also reduced at the cathode. It is denser than sodium and largely sinks back into the melt, so it does not contaminate the sodium.

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How Much Electricity Is Needed

Each sodium ion needs one electron, so one mole of sodium (23 g) needs one mole of electrons, which is one faraday of charge, about 96 500 C. The mass of metal produced by a charge

Q

follows from Faraday's law:

m = \frac{Q \times M}{n \times F} \qquad \text{e.g. } \frac{(1.0\times10^{5}\,\text{A})(3600\,\text{s})(23\,\text{g/mol})}{(1)(96\,500\,\text{C/mol})} \approx 86\,\text{kg per hour}

Here

M

is the molar mass and

n

the number of electrons per ion. Industrial cells run at very high currents, so a single cell produces tens of kilograms of sodium per hour, and electricity is the main cost of the process. The sodium is used to make sodium compounds such as sodium peroxide and sodium hydride, as a reducing agent to extract metals such as titanium, and as a coolant in some nuclear reactors.

Tips for remembering

• Molten, not dissolved: in water, H₂ forms at the cathode instead of sodium, so the salt has to be melted, not dissolved.
• Salt plus salt to melt lower: adding CaCl₂ brings the melting point down from 801 °C to about 600 °C, saving energy and keeping sodium from boiling off.
• Sodium floats up, chlorine bubbles up: a gauze screen keeps them apart, because the two would react straight back to NaCl.