Downs cell anodeA block of graphite, a form of carbon. Chlorine gas forms on it and bubbles up.
Downs cell cathodeA ring round the anode, cut through here. Sodium metal forms on it and floats up.
Iron screenIt hangs between the two electrodes. It keeps the sodium and the chlorine apart.
Molten sodium chlorideThe electrolyte: salt kept liquid by heat. Calcium chloride is mixed in to lower its melting point.
Downs cell chlorine outletThe chlorine gas collects under the hood. It leaves the cell by this pipe.
Sodium outletMolten sodium gathers in a collecting ring over the cathode. It is led out through this pipe.
Diaphragm cell 7
Diaphragm cell anodeThis plate is the positive electrode. Chloride ions give up electrons here and chlorine gas forms.
Diaphragm cell cathodeThis plate is the negative electrode. Hydrogen gas and hydroxide ions form here from water.
DiaphragmA porous wall that the brine seeps through. It keeps the chlorine from the sodium hydroxide.
Brine inletBrine is fed in here, on the anode side. Brine is a strong solution of salt in water.
Diaphragm cell chlorine outletChlorine gas made at the anode leaves the cell here.
Hydrogen outletHydrogen gas made at the cathode leaves the cell here.
Sodium hydroxide outletSodium hydroxide solution drains from the cathode side. It still holds some salt.
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: Downs cell, https://en.wikipedia.org/wiki/Downs_cell (a carbon anode and an iron cathode in molten sodium chloride; calcium chloride is added so the mixture stays liquid near 600 degrees Celsius, where sodium chloride alone melts at 801; sodium and chlorine both float up, and perforated iron baffles lead them into separate chambers); Chloralkali process, https://en.wikipedia.org/wiki/Chloralkali_process , and Chlorine production, https://en.wikipedia.org/wiki/Chlorine_production (the diaphragm cell: two compartments separated by a permeable diaphragm; brine enters the anode compartment and flows through to the cathode compartment; chlorine at the anode, hydrogen and caustic soda at the cathode; a dilute caustic brine leaves the cell). (2) Purdue University, Bodner Research Web, Electrolytic Cells, https://chemed.chem.purdue.edu/genchem/topicreview/bp/ch20/faraday.php : in the Downs cell chlorine forms on a graphite anode set in the bottom of the cell and bubbles into a funnel at the top; sodium forms at the cathode and floats up into a collecting ring and is drained from it; the electrodes are separated by a screen of iron gauze; the feed is a mixture of calcium chloride and sodium chloride that melts at 580 degrees Celsius; electrolysis of the solution in water gives chlorine, hydrogen and sodium hydroxide, with a diaphragm between the chlorine and the sodium hydroxide, and the sodium hydroxide drained from the bottom. (3) 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 : the anode is separated from the cathode by a permeable diaphragm; chlorine is removed at the anode; the brine flows through the diaphragm to the cathode chamber; caustic brine forms at the cathode and hydrogen is removed in the cathode chamber. (4) Chemguide (Jim Clark), The diaphragm and membrane cells for the manufacture of chlorine, https://www.chemguide.co.uk/inorganic/group7/diaphragmcell.html : a porous diaphragm that the solution seeps through from the anode side to the cathode side; chlorine at the anode, hydrogen at the cathode, and a dilute sodium hydroxide solution that still holds sodium chloride. (5) J. W. Moore, C. L. Stanitski and P. C. Jurs, Chemistry: The Molecular Science (2008), the Downs cell figure and its caption, read by full-text search at https://archive.org/details/chemistrymolecul0000jurs : a circular iron cathode is separated from the graphite anode by an iron screen; the figure prints Iron screen and Na outlet. (6) A. C. Wilbraham and others, Chemistry (2017 printing), the Downs cell, read by full-text search at https://archive.org/details/chemistry0000anto : a perforated iron screen separates the circular cathode from the graphite anode. Iron screen is the name of sources 2 (its figure), 5 and 6; that the cathode is a ring round the anode stands in 5 and 6. 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 Downs cell makes sodium metal and chlorine gas by passing a current through molten sodium chloride. The second view is a diaphragm cell diagram: the same salt dissolved in water, which gives chlorine, hydrogen and sodium hydroxide.
Read the full guide
The Downs cell
Chlorine forms on the graphite anode in the center and rises under a hood to its outlet. Sodium forms on the cathode round it, floats up into a collecting ring and is led out by its own pipe. An iron screen hangs between the electrodes and keeps the two products apart.
The diaphragm cell
Brine enters on the anode side, where chlorine forms. It seeps through the porous diaphragm to the cathode side, where hydrogen and sodium hydroxide form. The diaphragm keeps the chlorine away from the sodium hydroxide.
Molten salt gives the metal. Salt in water gives hydrogen at the cathode instead, because water is easier to reduce than sodium ions.
Questions and answers
What is a Downs cell used for?
Making sodium metal. Molten sodium chloride is split by an electric current into liquid sodium at the cathode and chlorine gas at the anode.
Why is calcium chloride added in the Downs cell?
To lower the melting point. Sodium chloride alone melts at about 800 degrees Celsius; mixed with calcium chloride it stays liquid at about 600.
What does the diaphragm do in a diaphragm cell?
It is a porous wall between the anode and the cathode. Brine seeps through it, and it keeps the chlorine made at the anode away from the sodium hydroxide made at the cathode.
What are the products of a diaphragm cell?
Chlorine gas at the anode, hydrogen gas at the cathode, and a solution of sodium hydroxide that drains from the cathode side.