Electrolysis of Brine

One Brine, Three Products

Strong brine and direct current go in; three things come out: chlorine gas on the anode side, hydrogen and a solution of sodium hydroxide, caustic soda, on the cathode side. Overall, 2NaCl + 2H₂O → Cl₂ + H₂ + 2NaOH.

The three come in fixed proportions. By the formula, every tonne of chlorine comes with 1.13 t of sodium hydroxide and about 28 kg of hydrogen, and uses 1.65 t of salt and 0.51 t of water. Plants get 1.07⁠–⁠1.13 t of caustic per tonne of chlorine; the shortfall goes to side reactions.

On the stand, a model of the principle of a bipolar electrolyser: seven cells between two end heads, the right four cut open towards you. Real cells are much thinner; the typical electrolyser of the EU reference document has 181.

2NaCl + 2H₂O → Cl₂ + H₂ + 2NaOH · per tonne of chlorine: about 1.13 t of caustic, about 28 kg of hydrogen (computed)

At the Anode, Chloride Gives Up Electrons

Brine flows into the anode compartment. The anode is expanded titanium mesh, coated with oxides of ruthenium, iridium and titanium. Here chloride ions give up electrons: 2Cl⁻ → Cl₂ + 2e⁻. Two chlorine atoms make a molecule, and the gas rises as bubbles.

The electrons leave through the anode, the titanium anode pan and the wiring outside, back to the supply. Water can also be oxidised to oxygen at the anode, so the anode side is kept acid, at a pH of about 2⁠–⁠4, which favours chlorine.

Before coated anodes, anodes were graphite, which wore away. H. B. Beer saw that titanium could carry the coating: he filed the first oxide-coating patent in 1965, and his 1967 recipe (30 % ruthenium oxide, 70 % titanium oxide) became the industrial standard.

2Cl⁻ → Cl₂ + 2e⁻ · titanium mesh coated with ruthenium, iridium and titanium oxides · anode side at pH 2⁠–⁠4

At the Cathode, Water Takes Them

The cathode compartment holds sodium hydroxide solution; the cathode is nickel mesh. Electrons from the supply flow into it, and water molecules take them: 2H₂O + 2e⁻ → H₂ + 2OH⁻. Hydrogen comes off and hydroxide is left behind.

Sodium ions come through the membrane from the anode side; with the hydroxide they make caustic soda. The cell holds it at about 32 % by mass; outside, steam concentrates it to the commercial 50 %.

Under standard conditions the two half-reactions’ potentials differ by about 2.19 V (computed here): the least voltage that can split brine.

2H₂O + 2e⁻ → H₂ + 2OH⁻ · nickel cathode · caustic at about 32 % in the cell, concentrated to 50 %

A Membrane in Two Layers

Between the compartments is a perfluorinated ion-exchange membrane, usually in two layers. The layer facing the anode carries sulphonate groups: strong, and a good conductor. The layer facing the cathode carries carboxylate groups: thin, it lets through little but sodium and holds back hydroxide and chloride. PTFE fibres woven in keep it from tearing.

The acid groups are fixed to the polymer and negatively charged: positive ions move between them, negative ions are pushed away. Each sodium ion takes 3.5⁠–⁠4.5 water molecules through with it.

At the front, a specimen with its two layers drawn apart: the thicker sulphonate layer behind, the thinner carboxylate layer in front. Membranes range from 0.2 to 5 m² a sheet and usually last three to five years.

Sulphonate layer facing the anode · carboxylate layer facing the cathode · PTFE reinforcement · 3.5⁠–⁠4.5 water molecules per Na⁺

Keeping Chlorine from Caustic

Inside the membrane, sodium ions carry most of the current. Hydroxide is negative and the field pulls it towards the anode; any that gets through makes oxygen, hypochlorite and chlorate in the anode compartment and wastes current. The carboxylate layer stops most of it, not all: by the EU’s reference document, caustic loses 3⁠–⁠7 % of its current efficiency this way.

Without a separator the chlorine would dissolve in the caustic as hypochlorite and chlorate, and no chlorine gas would come off. The membrane also keeps chloride on the anode side, so the caustic carries only about 50 mg of sodium chloride a litre.

Hydrogen and chlorine together explode: at room temperature, about 4 % hydrogen by volume in chlorine is enough. The chlorine from a membrane cell carries 0.03⁠–⁠0.3 %.

OH⁻ leaking through: 3⁠–⁠7 % of current efficiency lost · about 50 mg/L NaCl in the caustic · hydrogen in the chlorine 0.03⁠–⁠0.3 %, explosive from about 4 %

From 2.19 Volts to 3.2

Splitting brine takes at least the difference of the two half-reactions’ potentials: about 2.19 V under standard conditions. The EU’s reference document gives membrane cells a zero-current voltage of about 2.35 V, which holds the electrodes’ potential difference and a little overpotential to get the reactions going.

More current means more voltage: the overpotentials at the electrodes, the resistance of the solutions, the bubbles and the membrane, and of the conductors all grow with the current density. The document writes it as U₀ + K·j, with K of 0.1⁠–⁠0.3 V per kA/m² for membrane cells.

Its typical bipolar plant runs at 6 kA/m², 3.2 V a cell. The cells work at about 90 °C.

Standard potential difference about 2.19 V (computed) · zero-current voltage about 2.35 V · about 3.2 V at 6 kA/m² (typical bipolar plant)

756 Kiloampere-Hours a Tonne

Each chlorine molecule takes two electrons away. By Faraday’s law a tonne of chlorine takes 2.72 × 10⁹ C of charge, about 756 kAh. That number is set by the formula alone, not by how the cell is built.

Energy is charge times voltage. At 2.19 V, with all the current making chlorine, a tonne takes about 1,650 kWh; the document’s typical bipolar plant uses about 2,600 kWh of direct current a tonne.

The electrons leave at the anode end, run through the busbars and the rectifier, and come back in at the cathode end. In a bipolar electrolyser the cells are in series: behind each cell’s cathode pan is the next cell’s anode pan, and the supply is connected only at the two ends.

1 t of chlorine = 756 kAh (computed) · at least about 1,650 kWh (computed) · typical bipolar plant about 2,600 kWh DC

A Row of Cells, Pressed Together

A plant’s electrolyser is built like a filter press: the cells hang on two rails between two end heads and are pressed together by tie rods. Between the cells are gaskets and membranes; each cell has hoses up to the outlet headers and down to the inlet headers.

In the EU document’s typical bipolar plant, each electrolyser has 181 cells of 2.70 m² of membrane each; at 6 kA/m² each carries 16.2 kA, and five of them can make 158,000 t of chlorine a year.

A lower current density saves electricity, but the same output then needs more or bigger electrolysers, which cost more to build.

Typical bipolar electrolyser: 181 cells · 2.70 m² each · 16.2 kA · 181 × 3.2 V ≈ 580 V (computed)

How Clean the Brine Must Be

The saturated brine fed to the cells holds 310⁠–⁠315 g of sodium chloride a litre. Sodium carbonate and sodium hydroxide first precipitate its calcium and magnesium; for membranes that is not enough, and a chelating resin brings calcium and magnesium together below 20 µg a litre.

Calcium precipitates near the cathode side of the membrane and inside it, and the current efficiency drops; magnesium forms a fine precipitate near the anode side, and the voltage rises.

The depleted brine leaves the anode compartment at about 200 g a litre. It is dechlorinated completely (leftover chlorine would damage the resin), resaturated with salt, purified, and fed back.

After precipitation: Ca < 2 mg/L, Mg < 1 mg/L · after the resin: Ca + Mg < 20 µg/L · depleted brine about 200 g/L

Mercury, Asbestos, Membrane

Brine was split two ways before. The diaphragm cell (Griesheim, 1885) parted the electrodes with a porous diaphragm, long made of asbestos, and gave a weak caustic with salt in it. The mercury cell (Castner–Kellner, 1892) dissolved the sodium in a mercury cathode and reacted it with water elsewhere: strong, pure caustic, but mercury escaped into air and water.

Membranes arrived in the 1970s; the first industrial membrane plant opened in Japan in 1975. In Europe their share of capacity rose from 11 % to 59 % between 1997 and 2012. The EU banned chlor-alkali production with mercury electrodes from 11 December 2017; in 2025 membranes made up 81.9 % of EU capacity.

The cost of mercury is on record: in the 1960s and 1970s a chlor-alkali plant at Dryden, Ontario, released about 10 t of mercury into the Wabigoon River, and its fish still carry eating advisories. Minamata disease in Japan was not caused by a chlor-alkali plant: it came from methylmercury released by acetaldehyde production.

Diaphragm cell 1885 · mercury cell 1892 · first industrial membrane plant 1975 (Japan) · EU ban on mercury electrodes from 11 December 2017

Where Chlorine and Caustic Go

Chlorine is hard to store and ship, so most of it is used where it is made: in 2012, about 94 % of Europe’s chlorine was used on the same or a neighbouring site. The biggest use is PVC, 33.3 %; then isocyanates and related chemicals (for polyurethanes), 30.3 %; inorganic uses such as disinfection, water treatment and pigments, 14.3 %.

Of Europe’s caustic soda that year, 30.1 % went to organic chemicals, 13.4 % to pulp and paper, 6.0 % to aluminium and other metals and 5.0 % to water treatment. The hydrogen is mostly burned on site for steam or used as a feedstock; in 2025 Europe used about 87.6 % of it.

It is also one of chemistry’s biggest users of electricity. In 2010 the chlor-alkali plants of the EU-⁠27 and EFTA countries used 35 TWh, about 1 % of the region’s final electricity consumption. In 2012 the world could make about 76.8 million tonnes of chlorine a year.

Europe 2012: PVC 33.3 %, isocyanates and related 30.3 %, inorganics 14.3 % · 35 TWh in 2010, about 1 % of Europe’s electricity

Taken Apart, Plate by Plate

One end of the electrolyser comes apart: the end head, the end element (a titanium anode pan with its anode), a gasket, the membrane, a gasket, and the next bipolar element (a nickel cathode pan with its cathode, backed by the next cell’s titanium anode pan); the other cells stay in the frame.

The part under the pointer lifts out alone and is named; left alone, the parts lift out in turn.

End head · end element · gasket · membrane · gasket · bipolar element · the other cells