BrineA strong solution of salt in water. The current splits it into chlorine and sodium.
Graphite anodeChlorine gas forms on it. One hangs in each outer compartment, just above the mercury.
Chlorine gasIt is made at the anodes. It bubbles up and leaves through the lid.
Under the compartments 2
MercuryIt is the cathode of the outer compartments. Sodium dissolves in it and forms sodium amalgam.
Eccentric wheelIt turns and makes that end of the cell rise and fall. The rocking moves the mercury to and fro.
The middle compartment 5
Sodium hydroxide solutionSodium leaves the amalgam here. It forms sodium hydroxide in the water.
Iron cathodeWater is split at this electrode. Hydrogen gas and hydroxide ions form.
Hydrogen gasIt is made at the iron cathode from water.
Water inletWater runs into the middle compartment. It replaces the solution that is drawn off.
Sodium hydroxide outletThe sodium hydroxide solution is drawn off here. It is the product of the cell.
How this list was compiled: Every name and fact checked on 2026-10-01, a name kept only where two independent sources use it. (1) Wikipedia, text only: Castner-Kellner process, https://en.wikipedia.org/wiki/Castner%E2%80%93Kellner_process (invented in the 1890s; cells separated by walls that dip below the liquids and let the mercury pass beneath; the outer cells hold sodium chloride solution, a graphite anode and the mercury as cathode, and chlorine leaves at their top; the sodium dissolves in the mercury as an amalgam; the center cell holds sodium hydroxide solution and an iron cathode, where water gives hydrogen and hydroxide; a rocking mechanism with a rotating eccentric moves the mercury; sodium hydroxide solution is withdrawn and replaced with water); Chloralkali process, https://en.wikipedia.org/wiki/Chloralkali_process ; Chlorine production, https://en.wikipedia.org/wiki/Chlorine_production , and Sodium amalgam, https://en.wikipedia.org/wiki/Sodium_amalgam (later mercury cells: the amalgam flows to a separate reactor where water turns it into hydrogen, sodium hydroxide and mercury, which is recycled). (2) A. J. Hale, The Applications of Electrolysis in Chemical Industry (1918), pages 104 and 105, https://archive.org/details/applicationsofel00haleuoft : the rocking mercury cell of 1892; two partitions divide it into three compartments and stop just short of the floor, so a thin layer of mercury joins them; the two outer compartments hold brine and two graphite anodes, and the chlorine is drawn off by pipes at the top of each; water circulates through the middle compartment, which holds an iron grid cathode; an eccentric wheel gives the cell a rocking motion, so that end rises and falls and the mercury moves backwards and forwards; the sodium discharged on the mercury is carried to the middle compartment and turned by the water into caustic soda, while hydrogen is evolved. (3) B. Blount, Practical Electro-Chemistry (1906), https://archive.org/details/practicalelectr03blougoog : three compartments over a layer of mercury; salt solution in the outer two and water in the center; carbon anodes in the outer compartments and an iron grid as the cathode in the center; the cell rests on a knife edge at one end and an eccentric at the other; pipes lead chlorine from the anode compartments and hydrogen from the cathode compartment, and the caustic soda solution is drawn off; caustic soda and hydrogen appear at the iron cathode; the mercury is cathode toward the anodes and anode toward the cathode, and keeps the liquids of the compartments apart. (4) F. H. Thorp, Outlines of Industrial Chemistry (1898), https://archive.org/details/outlinesofindust00thorrich : the cell is made to rock slightly by a cam; water flows into the center compartment and a like amount of caustic solution overflows. (5) US Environmental Protection Agency, AP-42 section 8.11, Chlor-Alkali, https://www.epa.gov/sites/default/files/2020-09/documents/8.11_chlor-alkali.pdf : in the mercury cell the liquid mercury cathode and the brine flow through the cell together; chlorine forms at the anode; the sodium forms an amalgam with the mercury, which is reacted with water in a separate decomposer to give hydrogen and caustic soda solution. Licence: own work
The drawing: Drawn for this site on 2026-10-01 from plain geometry; nothing traced. Licence: own work
How it works
The Castner-Kellner cell makes sodium hydroxide and chlorine from brine, with a layer of mercury as its cathode. This diagram cuts the rocking cell along its length: two outer compartments, a middle one, and the mercury under all three.
Read the full guide
The outer compartments
Each holds brine and a graphite anode. Chlorine gas forms at the anode and leaves through the lid. Sodium is set free at the mercury below and dissolves in it, which makes sodium amalgam.
The middle compartment
An eccentric wheel rocks the trough, and the amalgam is carried under the walls into the middle. There the sodium leaves the mercury and forms sodium hydroxide, while hydrogen forms at the iron cathode. Water runs in and the solution is drawn off.
The mercury does two jobs: it is the cathode in the outer compartments and the anode in the middle one.
Questions and answers
What is the Castner-Kellner process?
A way of making sodium hydroxide by passing a current through brine over a mercury cathode. Chlorine gas and hydrogen gas are made as well. It dates from the 1890s.
Why is mercury used in the Castner-Kellner cell?
The sodium set free at the mercury dissolves in it as an amalgam, so it is carried away from the brine and the chlorine. The mercury also keeps the liquids of the compartments apart.
Why does the Castner-Kellner cell rock?
An eccentric wheel under one end turns, so that end rises and falls a little. The motion carries the mercury and its sodium amalgam from the outer compartments into the middle one.
How is a later mercury cell different?
The mercury flows along the floor with the brine, and the amalgam passes to a separate vessel, the decomposer. There water turns it into sodium hydroxide solution, hydrogen and mercury that is used again.