# Chloralkali process The **chloralkali process** is the industrial [[Electrolysis|electrolysis]] of brine, a concentrated solution of [[Sodium_chloride|sodium chloride]] in [[Water|water]], to make three products at once: [[Chlorine|chlorine]] gas at the anode, [[Hydrogen|hydrogen]] gas at the cathode, and [[Sodium_hydroxide|sodium hydroxide]] (caustic soda) in the solution around the cathode.[^openstax-ch16][^ball-ch14] The overall reaction is `2NaCl + 2H₂O → Cl₂ + H₂ + 2NaOH`, and because it runs on [[Electric_current|electric current]] rather than heat, the quantity of each product is fixed by [[Faraday's_laws_of_electrolysis|Faraday's laws of electrolysis]]: one mole of electrons passed through the cell makes half a mole of chlorine, half a mole of hydrogen and one mole of sodium hydroxide, no more and no less.[^haverkort-faraday] Chlorine and caustic soda are among the highest-tonnage products of the [[Chemical_industry|chemical industry]], feeding the manufacture of [[Plastic|plastics]], paper, water treatment, [[Aluminium|aluminium]] refining and soap. In the microsim below the reader sets the cell current in kiloamperes and the run time and reads off the tonnes of Cl₂, H₂ and NaOH that `m = (I·t/F)·(M/z)` delivers, together with the electrical [[Energy|energy]] consumed at the chosen cell [[Voltage|voltage]] in [[Kilowatt-hour|kilowatt-hours]] per tonne; the cell is drawn in 3D as a membrane cell, with sodium ions crossing the membrane from the brine side to the caustic side. On Wikitube's [[PORTAL_Chemistry|Chemistry]] flagship this is the main article for the Part XIII — Practice section *Chlor-alkali: Faraday's laws* (row K69), the chlorine spine sim; the [[Hall–Héroult_process|Hall–Héroult]] cell, in which aluminium is won from its oxide by three electrons per atom, is the same sim's variant on the aluminium spine. ## History Chlorine was first prepared in 1774 by Carl Wilhelm Scheele, who heated the mineral pyrolusite with hydrochloric acid, and it was recognised as an element by Humphry Davy in 1810, who named it for its colour.[^rsc-chlorine] The quantitative link between charge and chemical change came in 1834, when [[Michael_Faraday|Michael Faraday]] reported that the amount of a substance decomposed by a current is proportional to the quantity of electricity passed and, for the same charge, proportional to the substance's chemical equivalent weight; those two statements are the laws the microsim computes.[^faraday1834] Through most of the nineteenth century, however, chlorine was still made chemically, by oxidising hydrochloric acid with manganese dioxide as Scheele had, and alkali came from the soda processes rather than from electrolysis. Electrolytic production became practical only when cheap [[Electric_power|electric power]] from [[Electric_generator|dynamos]] arrived in the 1880s and 1890s. Two cell designs appeared almost together: cells that kept the anode and cathode products apart with a porous diaphragm, and the mercury-cathode cell of Hamilton Castner and Carl Kellner, in which sodium is deposited into flowing mercury as an amalgam and decomposed with water elsewhere. Both families were in industrial use by the turn of the twentieth century and dominated the industry for seventy years.[citation needed] The third design, the membrane cell, followed the invention of perfluorinated cation-exchange membranes in the 1960s and came into commercial use in the 1970s; it has since displaced the older cells wherever plants have been rebuilt.[citation needed] The 2013 Minamata Convention on Mercury lists chlor-alkali production with mercury cells among the manufacturing processes to be phased out, with a deadline of 2025, and the last mercury cells in the European Union had already closed by the end of 2017.[^minamata][^eurochlor] World chlorine production in the 2020s is of the order of 90 million tonnes a year, matched by a slightly larger mass of caustic soda.[citation needed] ## Process systems All three cell types share the same chemistry and differ only in how they keep the products apart. At the anode chloride is [[Redox|oxidised]], `2Cl⁻ → Cl₂ + 2e⁻`; at the cathode water is reduced, `2H₂O + 2e⁻ → H₂ + 2OH⁻`, which leaves hydroxide ions paired with the sodium ions of the brine as sodium hydroxide.[^openstax-ch16] The cathode reaction is the same hydrogen-evolution reaction as in an [[Alkaline_water_electrolysis|alkaline water electrolyser]], and Haverkort's text writes its rate in Faraday's form, `j = n·F·N`: the current density j equals the number of electrons per molecule n, the [[Faraday's_laws_of_electrolysis|Faraday constant]] F ≈ 96,485 C per mole of electrons, and the molar flux N of the species produced.[^haverkort-faraday] Integrated over a whole cell and a whole run, that is the microsim's equation. With a current I flowing for a time t the charge is I·t, the moles of electrons are I·t/F, and the mass of a product of molar mass M that needs z electrons per molecule is `m = (I·t/F)·(M/z)`, with z = 2 for Cl₂ (M = 70.90 g/mol), z = 2 for H₂ (M = 2.016 g/mol) and z = 1 for NaOH (M = 40.00 g/mol).[^openstax-ch16] The worked number behind the sim: one kiloampere for one hour is 3.6 × 10⁶ C, or 37.3 mol of electrons, and makes 1.32 kg of chlorine, 37.6 g of hydrogen and 1.49 kg of sodium hydroxide (computed here). Scaling up, a cell at 100 kA running for 24 h makes 3.17 t of chlorine, 90 kg of hydrogen and 3.58 t of caustic, and one tonne of chlorine always costs 756 kA·h of charge, whatever the cell. The sim's readouts are these Faraday-law quantities, which are a ceiling: in a real cell some of the current is lost to side reactions such as oxygen evolution at the anode and to chlorine that reaches the cathode side, and the ratio of actual to theoretical product is the cell's current efficiency. The [[Stoichiometry|stoichiometry]] is fixed by the [[Mole_(unit)|mole]] and the [[Electric_charge|charge]] on the electron, which is why chlor-alkali plants are rated in kiloamperes and why the price of chlorine follows the price of electricity. ### Membrane cell The membrane cell is the design the microsim draws and the one that has replaced the others. A cation-exchange membrane, a perfluorinated [[Polymer|polymer]] carrying sulfonate and carboxylate groups, divides the cell into an anode compartment fed with purified brine and a cathode compartment containing sodium hydroxide solution. The membrane lets sodium [[Ion|ions]], with their water of hydration, cross from the brine side to the caustic side to balance the charge carried by the electrons in the external circuit, while blocking chloride ions in one direction and hydroxide ions in the other. The result is caustic soda of high purity, almost free of chloride, delivered from the cell at roughly a third by mass, which needs only modest evaporation to reach the 50 % grade of commerce.[citation needed] Because the membrane is thin and the compartments narrow, the ohmic drop across the [[Electrolyte|electrolyte]] is small, and the membrane cell has the lowest electrical energy consumption of the three designs. Its demands are on brine purity: calcium, magnesium and other multivalent ions [[Precipitation_(chemistry)|precipitate]] inside the membrane as hydroxides and raise its resistance, so the brine is treated with ion-exchange resins before it enters the cell. In the 3D scene the reader watches sodium ions cross the membrane at the rate Faraday's law dictates for the chosen current, one ion per electron, while chlorine bubbles rise on the brine side and hydrogen on the caustic side. ### Diaphragm cell The diaphragm cell keeps the products apart with a porous separator, historically a mat of asbestos deposited on the cathode screen and now a polymer-modified or asbestos-free diaphragm. Brine flows from the anode compartment through the diaphragm into the cathode compartment, and it is this flow, kept up by a difference in liquid level, that stops hydroxide from diffusing back toward the anode where it would react with chlorine. The cathode liquor is therefore a mixture: dilute sodium hydroxide together with most of the unconverted [[Salt_(chemistry)|salt]], which must be concentrated by [[Evaporation|evaporation]] until the salt crystallises out and can be returned to the brine circuit. The evaporation is the diaphragm cell's largest cost, and the product still carries traces of chloride and chlorate that some customers cannot accept. Its advantages are tolerance of less pure brine and a lower cell voltage than the mercury cell, which kept it in wide use until membrane cells matured. ### Mercury cell The mercury cell uses a flowing film of [[Mercury_(element)|mercury]] as its cathode. Because hydrogen evolution has a very large [[Overpotential|overpotential]] on mercury, water is not reduced there; sodium ions are reduced instead and dissolve in the mercury as a dilute sodium amalgam, `Na⁺ + e⁻ + Hg → Na(Hg)`. The amalgam flows to a separate vessel, the decomposer, where it meets water over a [[Graphite|graphite]] packing and reacts to give hydrogen and a concentrated, very pure sodium hydroxide solution, `2Na(Hg) + 2H₂O → 2NaOH + H₂ + 2Hg`, and the stripped mercury is pumped back to the cell.[citation needed] The chemistry is elegant: chlorine and caustic never share a compartment, so no separator is needed and the caustic needs no evaporation. The price is the mercury itself, lost to the products, the air and the waste streams of the plant, and the cell's higher voltage, since the amalgam route costs more energy per tonne than direct water reduction. The Minamata Convention's phase-out of the process rests on the first of these, and the mercury cell is now an end-of-life technology.[^minamata] ## Unpartitioned cell If the anode and cathode share one solution with no diaphragm or membrane, the products meet. Chlorine dissolves in the alkaline catholyte and disproportionates, `Cl₂ + 2OH⁻ → ClO⁻ + Cl⁻ + H₂O`, so that an unpartitioned cell run cold makes sodium hypochlorite solution, the active ingredient of household bleach, instead of chlorine and caustic.[^openstax-ch18] Run hot, the hypochlorite disproportionates again, `3ClO⁻ → ClO₃⁻ + 2Cl⁻`, and the product is sodium chlorate, the feedstock for the chlorine dioxide used in pulp bleaching.[^openstax-ch18] Both are made deliberately in cells designed without a separator, and hypochlorite generators of the same kind are installed on site at swimming pools and water works so that no chlorine gas has to be shipped or stored. For a chlor-alkali plant proper, mixing is exactly what the separator exists to prevent: every mole of chlorine that reaches hydroxide is a mole of product lost and a mole of current wasted, which is why the Faraday-law yield of the microsim is a ceiling that the three partitioned designs approach from below. ## Electrodes The electrode reactions set the voltage, and the voltage sets the energy in the sim's second readout. From the standard potentials tabulated by OpenStax, Cl₂/Cl⁻ at +1.358 V and 2H₂O/H₂, OH⁻ at −0.828 V, the minimum cell voltage for `2Cl⁻ + 2H₂O → Cl₂ + H₂ + 2OH⁻` is 1.358 − (−0.828) = 2.19 V (computed here); the [[Standard_electrode_potential|standard electrode potential]] for oxidising water to oxygen is lower, +1.229 V, so oxygen rather than chlorine would be expected at the anode on thermodynamic grounds alone.[^openstax-appL] Chlorine wins in practice because oxygen evolution has a large overpotential on the anode materials used and chloride is present at high concentration, a textbook case of [[Electrochemical_kinetics|kinetics]] overriding [[Thermodynamics|thermodynamics]] that the OpenStax chapter uses to introduce overpotential.[^openstax-ch16] A working cell runs well above 2.19 V. Haverkort's accounting of the losses in an electrolyser, `V_cell = V_eq + η_c − η_a − Δφ − ΔV`, adds the cathode and anode overpotentials and the ohmic drops in the electrolyte and the electrodes to the equilibrium voltage; the activation terms dominate at low current density and the ohmic terms at high, and each tenfold rise in current density costs roughly 0.12 V of [[Tafel_equation|Tafel]] overpotential at each electrode.[^haverkort-losses] With the sim's default cell voltage of 3.0 V, the 756 kA·h that a tonne of chlorine requires becomes 2,270 kWh, and a tonne of caustic soda, needing 670 kA·h, costs 2,010 kWh (computed here); every 0.1 V saved on the cell saves about 76 kWh per tonne of chlorine. The anodes were graphite for the first sixty years of the industry, consumed slowly by the oxygen side reaction and shedding carbon into the chlorine. They were replaced from the late 1960s by dimensionally stable anodes: [[Titanium|titanium]] plates coated with a mixed oxide of ruthenium and titanium, which catalyse chlorine evolution at low overpotential and do not [[Corrosion|corrode]], a change Trasatti describes as one of the most consequential in industrial electrochemistry.[^trasatti2000] Cathodes in diaphragm and membrane cells are steel or [[Nickel|nickel]], often coated with a high-surface-area catalyst to cut the hydrogen overpotential, and the mercury cell's cathode is the metal itself. The electrodes are the parts of the cell where [[Electrochemistry|electrochemistry]] and [[Materials_science|materials science]] meet, and every improvement in them shows up directly in the kilowatt-hours per tonne of the sim's second readout. ## Manufacturer associations The producers organise themselves regionally. Euro Chlor, founded in 1953 and based in Brussels as a sector group of the European chemical industry council, represents the European chlor-alkali producers, publishes the industry's production and energy statistics and coordinated the sector's exit from mercury technology.[^eurochlor] The Chlorine Institute, founded in 1924 in the United States, concentrates on the safe production, handling and transport of chlorine and issues the technical pamphlets that North American plants and railroads work to.[^chlorine-institute] The World Chlorine Council, established in 1993, is the umbrella body that links these and the other regional associations, including the Japanese and Chinese producers' groups, and speaks for the industry in international forums such as the Minamata Convention negotiations.[^wcc] For a reader of this article the associations matter as sources: their statistics are where the tonnages and the kilowatt-hours per tonne that anchor the microsim's readouts are reported. ## See also - [[Faraday's_laws_of_electrolysis]] - [[Hall–Héroult_process]] - [[Electrolysis]] - [[Sodium_hydroxide]] - [[Chlorine]] - [[Electrolysis_of_water]] - [[Alkaline_water_electrolysis]] - [[Overpotential]] - [[Electrochemical_cell]] ## References [^openstax-ch16]: Flowers, Paul; Neth, Edward; Robinson, William, et al. (2019). *Chemistry: Atoms First 2e*. OpenStax. Chapter 16, Electrochemistry, §16.7 Electrolysis, pp. 753–790 (electrolysis of aqueous sodium chloride: the half-reactions, why chloride rather than water is oxidised, and the quantitative stoichiometry of electrolysis) (page to pin). https://openstax.org/details/books/chemistry-atoms-first-2e [^openstax-appL]: Flowers, Paul; Neth, Edward; Robinson, William, et al. (2019). *Chemistry: Atoms First 2e*. OpenStax. Appendix L, Standard Electrode (Half-Cell) Potentials, pp. 1125–1130. https://openstax.org/details/books/chemistry-atoms-first-2e [^openstax-ch18]: Flowers, Paul; Neth, Edward; Robinson, William, et al. (2019). *Chemistry: Atoms First 2e*. OpenStax. Chapter 18, Representative Metals, Metalloids, and Nonmetals, pp. 851–928 (the halogens: disproportionation of chlorine in cold and hot base to hypochlorite and chlorate) (page to pin). https://openstax.org/details/books/chemistry-atoms-first-2e [^haverkort-faraday]: Haverkort, Willem (2024). *Electrolysers, Fuel Cells and Batteries: Analytical Modelling*. Chapter 1, Electrochemistry, p. 25 (F ≈ 96,485 C per mole of electrons) and p. 28 (Faraday's law j = n·F·N; standard potential −0.83 V for 2H₂O/H₂, OH⁻). https://open.umn.edu/opentextbooks/textbooks/electrolysers-fuel-cells-and-batteries-analytical-modelling [^haverkort-losses]: Haverkort, Willem (2024). *Electrolysers, Fuel Cells and Batteries: Analytical Modelling*. Chapter 1, p. 33 (one decade of current costs about 120 mV of Tafel overpotential) and pp. 36–38 (the cell-voltage loss budget, Eq. 1.36; activation losses dominate at low current density and ohmic losses at high). [^ball-ch14]: Ball, David W. (2011). *Introductory Chemistry*. Chapter 14, Oxidation and Reduction, pp. 673–719 (electrolysis as the industrial route to chlorine and sodium hydroxide from brine) (page to pin). https://open.umn.edu/opentextbooks/textbooks/introductory-chemistry [^rsc-chlorine]: Royal Society of Chemistry. "Chlorine." *Periodic Table*. https://www.rsc.org/periodic-table/element/17/chlorine [^faraday1834]: Faraday, Michael (1834). "Experimental Researches in Electricity. Seventh Series." *Philosophical Transactions of the Royal Society of London* 124: 77–122. [^minamata]: United Nations Environment Programme (2013). *Minamata Convention on Mercury*, Annex B, Part I (manufacturing processes in which mercury or mercury compounds are used: chlor-alkali production, phase-out date 2025). [^trasatti2000]: Trasatti, Sergio (2000). "Electrocatalysis: understanding the success of DSA®." *Electrochimica Acta* 45 (15–16): 2377–2385. [^eurochlor]: Euro Chlor. "About Euro Chlor." https://www.eurochlor.org/ [^chlorine-institute]: The Chlorine Institute. "About the Institute." https://www.chlorineinstitute.org/ [^wcc]: World Chlorine Council. "About WCC." https://worldchlorine.org/ ## Further reading - Haverkort, Willem (2024). *Electrolysers, Fuel Cells and Batteries: Analytical Modelling* — Chapter 1 (Electrochemistry) and Chapter 6 (Electrolysers); book 053 on the Portal Books shelf. - Flowers, Paul; Neth, Edward; Robinson, William, et al. (2019). *Chemistry: Atoms First 2e*, OpenStax — Chapter 16 (Electrochemistry) and Appendix L; book 051. - Ball, David W. (2011). *Introductory Chemistry* — Chapter 14 (Oxidation and Reduction); book 056. ## External links - [Electrolysers, Fuel Cells and Batteries: Analytical Modelling](https://open.umn.edu/opentextbooks/textbooks/electrolysers-fuel-cells-and-batteries-analytical-modelling), Open Textbook Library record - [Euro Chlor](https://www.eurochlor.org/), [The Chlorine Institute](https://www.chlorineinstitute.org/) and the [World Chlorine Council](https://worldchlorine.org/) — the producers' associations - The Wikipedia pair's *External links* section lists further reference sites <!-- MATTERSIM:BEGIN g24 — Matter & Energy Cluster microsim (framework build, specs/sims/Chloralkali_process.json); do not hand-edit inside --> **Microsim — three.js (Wikitube framework), pending deploy:** *Chloralkali process* will play here once `https://wikitube-3d-microsims.netlify.app/matter/Chloralkali_process.html` is live. <!-- pending: <div class="wt-sim" data-src="https://wikitube-3d-microsims.netlify.app/matter/Chloralkali_process.html" data-title="Chloralkali process"></div> --> <!-- MATTERSIM:END --> ## Wikipedia : Wikitube **Strict pair:** [Wikipedia](https://en.wikipedia.org/wiki/Chloralkali_process) : [Wikitube](https://en.wikitube.io/wiki/Chloralkali_process) · pinned revision [1370512649](https://en.wikipedia.org/w/index.php?oldid=1370512649) · 2026-09-11 ## Previous hub tags Hubs: `Life_Physics`. Portals: [[PORTAL_Chemistry]]. --- *Matter & Energy Cluster child articles, wave 1 · 2026-09-11 · drafted · Chemistry row K69 · sim pending (matter/Chloralkali_process).*