2_6 Diaphragm (Nelson) Cell: Making Sodium Hydroxide from Brine

Sodium hydroxide (caustic soda) is one of the most heavily produced chemicals, used for soap, paper, aluminium extraction and much more. It is made by electrolysing brine, a concentrated solution of sodium chloride in water. Unlike the molten salt in a Downs cell, brine contains water, and that changes what happens at the cathode. The same process also produces chlorine and hydrogen, so it is called the chlor-alkali process. The classic design is the diaphragm cell, also known as the Nelson cell.

What Happens at Each Electrode

At the positive anode, chloride ions are oxidized to chlorine gas. Water could in principle be oxidized to oxygen, but the high chloride concentration and the large extra voltage needed to form oxygen on the electrode (its overpotential) mean chlorine is released instead.

At the negative cathode, there is a choice between reducing

\mathrm{Na^+}

(standard potential −2.71 V) and reducing water (−0.83 V). Water is much easier to reduce, so hydrogen bubbles off and hydroxide ions build up around the cathode. Sodium ions are never discharged. They simply stay in solution as the partner of the new hydroxide ions.

\begin{aligned} \text{Anode: } & \mathrm{2Cl^-(aq) \rightarrow Cl_2(g) + 2e^-} \\ \text{Cathode: } & \mathrm{2H_2O(l) + 2e^- \rightarrow H_2(g) + 2OH^-(aq)} \end{aligned}

[Na+].[Na+].[Cl-].[Cl-].O.O>>[Na+].[Na+].[OH-].[OH-].[H][H].ClCl

The net effect is that the brine loses chloride ions and gains hydroxide ions, so the liquid near the cathode turns into a sodium hydroxide solution.

Construction of the Diaphragm Cell

• Cathode: a perforated steel vessel, U-shaped in cross-section, lined on the inside with a porous diaphragm. Historically the diaphragm was asbestos, but modern cells use polymer-based materials instead.
• Anode: graphite rods held inside the diaphragm and dipping into the brine. Modern cells use coated titanium, which lasts longer.
• Brine feed: a constant-level device keeps the anode compartment topped up with fresh brine.
• Gas outlets: chlorine rises into a dome above the anode, and hydrogen leaves the cathode side through a separate pipe.
• Product outlet: the alkaline liquid seeps through the diaphragm and perforations and drips into a collecting basin. Steam keeps the electrolyte warm and the holes clear.

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Two Problems and How the Design Solves Them

Problem 1: chlorine meets hydroxide. If the anode and cathode products mix, chlorine reacts with cold hydroxide solution to form hypochlorite (the active ingredient of bleach), and with hot solution to form chlorate. Both waste the products.

ClCl.[OH-].[OH-]>>[O-]Cl.[Cl-].O

The diaphragm physically separates the two compartments. It still lets sodium ions migrate towards the cathode, which keeps the current flowing, but it stops chlorine gas and the alkaline solution from mixing freely.

Problem 2: hydroxide ions reach the anode. Hydroxide ions are negative and are attracted to the anode. If they get there, they are oxidized to oxygen, which contaminates the chlorine. To stop this, the brine level on the anode side is kept slightly higher than on the cathode side. Liquid then flows steadily through the diaphragm towards the cathode, sweeping hydroxide ions away from the anode faster than they can drift back.

Purifying the Product

The liquid leaving the cell is roughly 11% NaOH and 16% unreacted NaCl. When it is evaporated, sodium chloride is less soluble than sodium hydroxide in the concentrating solution and crystallizes out first, so it can be filtered off and recycled as brine. The final solution is about 50% NaOH with roughly 1% NaCl, which is pure enough for most uses. Where very pure sodium hydroxide is needed, membrane cells are used. They have a polymer membrane that lets through only cations, and they are now the most common design.

Tips for remembering

• Water beats sodium at the cathode: in brine, water is easier to reduce than Na⁺, so the cathode gives H₂ and OH⁻, and that OH⁻ is the NaOH.
• Three products from salt water: NaOH, Cl₂ and H₂, which is why the industry is called chlor-alkali.
• Keep Cl₂ and OH⁻ apart: if they meet they make bleach (OCl⁻), so the diaphragm and a higher brine level on the anode side keep them separated.