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| title | chunk | source | category | tags | date_saved | instance |
|---|---|---|---|---|---|---|
| Chloralkali process | 2/2 | https://en.wikipedia.org/wiki/Chloralkali_process | reference | science, encyclopedia | 2026-05-05T10:46:40.961258+00:00 | kb-cron |
In the mercury-cell process, also known as the Castner–Kellner process, the "outer" electrolytic cells each contain an anode immersed in brine, which floats on a layer of mercury. The "inner" cells each contain a cathode in a sodium hydroxide solution, floating on the same mercury layer. The walls dividing the cells have gaps below the surface of the mercury layer. This allows mercury to flow between cells, while preventing the aqueous solutions from doing so. In the "outer" cell, chloride ions are oxidized at the anode, producing chlorine gas which bubbles out of the cell. The mercury layer acts as the cathode, here sodium ions in the brine are reduced and form an amalgam with the mercury. Once in the amalgam, sodium atoms are free to move to the "inner" cell. In the "inner" cell, the mercury layer now acts as the anode. Sodium atoms in the amalgam are oxidized and enter aqueous solution. Meanwhile at the cathode, water is split into hydrogen gas and hydroxide ions. Mercury cells are being phased out due to concerns about the high toxicity of mercury and mercury poisoning from mercury cell pollution such as occurred in Canada (see Ontario Minamata disease) and Japan (see Minamata disease).
== Unpartitioned cell ==
The initial overall reaction produces hydroxide and also hydrogen and chlorine gases:
2 NaCl + 2 H2O → 2 NaOH + H2 + Cl2 Without a membrane, the OH− ions produced at the cathode are free to diffuse throughout the electrolyte. As the electrolyte becomes more basic due to the production of OH−, less Cl2 emerges from the solution as it begins to disproportionate to form chloride and hypochlorite ions at the anode:
Cl2 + 2 NaOH → NaCl + NaClO + H2O The more opportunity the Cl2 has to interact with NaOH in the solution, the less Cl2 emerges at the surface of the solution and the faster the production of hypochlorite progresses. This depends on factors such as solution temperature, the amount of time the Cl2 molecule is in contact with the solution, and concentration of NaOH. Likewise, as hypochlorite increases in concentration, chlorates are produced from them:
3 NaClO → NaClO3 + 2 NaCl This reaction is accelerated at temperatures above about 60 °C. Other reactions occur, such as the self-ionization of water and the decomposition of hypochlorite at the cathode, the rate of the latter depends on factors such as diffusion and the surface area of the cathode in contact with the electrolyte. If current is interrupted while the cathode is submerged, cathodes that are attacked by hypochlorites, such as those made from stainless steel, will dissolve in unpartitioned cells. If producing hydrogen and oxygen gases is not a priority, the addition of 0.18% sodium or potassium chromate to the electrolyte will improve the efficiency of producing the other products.
== Electrodes ==
Due to the corrosive nature of chlorine production, the anode (where the chlorine is formed) must be non-reactive and has been made from materials such as platinum metal, graphite (called plumbago in Faraday's time), or platinized titanium. A mixed metal oxide clad titanium anode (also called a dimensionally stable anode) is the industrial standard today. Historically, platinum, magnetite, lead dioxide, manganese dioxide, and ferrosilicon (13–15% silicon) have also been used as anodes. Platinum alloyed with iridium is more resistant to corrosion from chlorine than pure platinum. Unclad titanium cannot be used as an anode because it anodizes, forming a non-conductive oxide and passivates. Graphite will slowly disintegrate due to internal electrolytic gas production from the porous nature of the material and carbon dioxide forming due to carbon oxidation, causing fine particles of graphite to be suspended in the electrolyte that can be removed by filtration. The cathode (where hydroxide forms) can be made from unalloyed titanium, graphite, or a more easily oxidized metal such as stainless steel or nickel.
== Manufacturer associations == The interests of chloralkali product manufacturers are represented at regional, national and international levels by associations such as Euro Chlor and The World Chlorine Council.
== See also == Electrochemical engineering Gas diffusion electrode Solvay process, a similar industrial method of making sodium carbonate from calcium carbonate and sodium chloride
== References ==
== Further reading == Bommaraju, Tilak V.; Orosz, Paul J.; Sokol, Elizabeth A.(2007). "Brine Electrolysis." Electrochemistry Encyclopedia. Cleveland: Case Western Reserve University.
== External links == Media related to Chloralkali process at Wikimedia Commons Animation showing the membrane cell process Animation showing the diaphragm cell process