How The Castner-Kellner Electrolytic Process Works

Castner-Kellner electrolytic process

The Castner-Kellner electrolytic process, invented in the 1890s by American Hamilton Castner and Austrian Carl Kellner, is a method of electrolysis on an aqueous alkali chloride solution (usually sodium chloride solution) to produce the corresponding alkali hydroxide.


Charles Watt received the first patent for electrolyzing brine in England in 1851. However, his method was not economically viable for producing sodium hydroxide because it could not prevent the chlorine formed in the brine solution from reacting with its other constituents. Hamilton Castner, an American chemist and engineer, invented the mercury cell, which solved the mixing problem and was granted a U.S. patent in 1894. Carl Kellner, an Austrian chemist, arrived at a similar solution around the same time. To avoid a legal battle, they formed a partnership in 1895 and established the Castner-Kellner electrolytic process Alkali Company, which built plants using the process throughout Europe.The mercury cell process is still in use today. Current mercury cell plant operations are being chastised for mercury pollution in the environment, which has resulted in severe mercury poisoning in some cases (as occurred in Japan). Mercury cell plants are being phased out as a result of these concerns, and an ongoing effort is being made to reduce mercury emissions from existing plants.

Procedure of the Castner-Kellner Process 

The apparatus depicted is divided into two types of cells, which are separated by slate walls. The first type, shown on the right and left of the diagram, employs a sodium chloride solution electrolyte, a graphite anode (A), and a mercury cathode (B) (M). The other type of cell, depicted in the center of the diagram, employs a sodium hydroxide electrolyte and a mercury anode (M), as well as an iron cathode (D). The mercury electrode is shared by both cells. This is accomplished by lowering the walls separating the cells below the electrolyte level while still allowing the mercury to flow beneath them.

The reaction at anode (A) is as follows:

2 Cl− → Cl2 + 2 e−

As a byproduct of the process, the chlorine gas vents at the top of the outside cells and is collected. In the outer cells, the reaction at the mercury cathode is

Na+ + e− → Na (amalgam)

This reaction produces sodium metal, which dissolves in mercury to form an amalgam. The current is carried by the mercury from the outer cells to the center cell. Furthermore, a rocking mechanism (B, represented by the fulcrum on the left and the rotating eccentric on the right) agitates the mercury to transport the dissolved sodium metal from the outer cells to the center cell.

The anode reaction occurs at the interface between the mercury and the sodium hydroxide solution in the center cell.

2Na (amalgam) → 2Na+ + 2e−

Finally, the reaction occurs at the center cell’s iron cathode (D).

2H2O + 2e− → 2OH− + H2

The net result is a decrease in the concentration of sodium chloride in the outer cells and an increase in the concentration of sodium hydroxide in the center cell. As the process continues, some sodium hydroxide solution is withdrawn as an output product from the center cell and replaced with water. To replace what has been electrolyzed, sodium chloride is added to the outside cells.

Frequently Asked Questions and Answers

Which one is used in a Castner-Kellner cell’s outer chamber?

Answer: In this process, mercury cathode and graphite anode are used to produce caustic soda.

Is caustic soda made using the Castner-Kellner method?

Yes, it is.

Sodium hydroxide can be produced using a variety of methods, including the Castner-Kellner electrolytic process.

How does the Castner-Kellner electrolytic process produce work?

Answer: Sodium hydroxide is commercially produced by electrolysis of sodium chloride in a Castner-Kellner electrolytic process cell. Using a mercury cathode and a carbon anode, a brine solution is electrolyzed. Sodium amalgam is formed when sodium metal discharged at the cathode combines with mercury.

The Castner-Kellner electrolytic process is a method of producing alkali hydroxide using an electrolytic cell that contains a graphite anode, a mercury cathode and a diaphragm. It has high efficiency and selectivity, but its drawbacks are mercury pollution and energy consumption.

Hamilton Castner was a US chemist who dreamed aluminium dreams. He invented a process to make cheap sodium, making aluminium smelting commercially viable.

1. The Anode of the Castner-Kellner Electrolytic Process

The word “electrode” brings to mind the image of a mad scientist in his stygian laboratory creating life from a bolt of lightning. But the reality is much less sensational. Electrodes are an integral part of any process that requires electrical energy. The electric current produced by an electrode causes chemical reactions to take place. For example, the electrolysis of concentrated salt solutions produces chlorine gas and sodium hydroxide (NaOH). Sodium chloride is also formed at the cathode.

The Castner-Kellner electrolytic process is used for the production of lye (sodium hydroxide) by the electrolysis of brine solution. The anode in this type of cell is made of graphite and the cathode is made of mercury. A porous diaphragm separates the anode and cathode compartments. This prevents chloride ions from entering the anode compartment and hydroxide ions from entering the cathode compartment. This method of producing sodium hydroxide is still commonly used today although the use of mercury and graphite as electrodes has been replaced by a more environmentally friendly process using Nafion membrane.

Hamilton Castner, US chemist (1858aEUR”1899) and Austrian chemist Carl Kellner developed processes to make cheap sodium, which was essential for the production of aluminium. Their inventions solved the problem of converting molten sodium hydroxide to metal and reducing it at carbon electrodes. Their processes, however, were not economically viable because hydrogen gas would mix with the molten metal and react with its other constituents.

Kellner-Solvay Cell

In the Kellner-Solvay cell, a large rectangular trough is filled with a concentrated brine solution and provided with graphite anodes in the outer cells. The inner cell is filled with water and provided with a mercury cathode. The current from the battery passes through the mercury layer and dissolves the chlorine gas, leaving a sodium amalgam that moves into the collection cell (see diagram). Sodium ions are discharged at the mercury cathode and combine with it to form liquid sodium. Hydrogen gas is also discharged at the anode and it reacts with water to produce sodium hydroxide. The amalgam then flows into an iron tank where it is decomposed by a calculated amount of water giving the caustic soda, hydrogen and mercury.

2. The Cathode of the Castner-Kellner Electrolytic Process

The Castner-Kellner cell is the basic technology for the industrial production of sodium metal and chlorine gas. It was invented independently by Hamilton Castner and Karl Kellner in the late 19th century. It became the basis for the global production of these essential chemicals. Unfortunately it also became a significant source of mercury pollution and this led to a global treaty on the reduction of mercury emissions.

In the Castner-Kellner process molten salt (NaCl) is electrolyzed to produce sodium metal and chlorine gas. The positive electrode, called the anode, is made of graphite. The cathode is made of iron or nickel. The anode and cathode are separated by a diaphragm, which is a porous partition that prevents mixing of the sodium metal and chlorine gas.

It is crucial that the temperature of the anode and cathode is kept above 330 degrees Celsius. If the temperature is below that, the molten sodium hydroxide will solidify; above that, the metal will dissolve in the melt.

The key to the success of the Castner-Kellner cell was that Castner developed a more durable cathode. He used pyrolytic carbon, which is more stable than the charcoal he previously used as an anode. The resulting carbon electrodes are still in use today – they’re found in aircraft, sports cars and the forks of my bicycle. In addition, they are more durable and much cheaper than aluminium. They’re also less prone to corrosion. This is one of the reasons why scientists are constantly seeking to make better and more durable materials for electrodes.

3. The Diaphragm of the Castner-Kellner Electrolytic Process

Hamilton Castner was a restless, impatient student at Columbia University. He took the courses he felt he should, but skipped a lot of them and went to work on his aluminium dreams.

When he discovered that iron carbide could reduce molten caustic soda to elemental sodium, his dreams came true. Suddenly, aluminium smelting was a commercial possibility.

The Castner-Kellner electrolytic process is a process of electrolysis to produce alkali hydroxide from an aqueous sodium chloride solution. In the cell, graphite serves as an anode, and mercury is used as a cathode. A diaphragm separates the two cells to prevent the mixing of chlorine gas and sodium metal.

The chlorine ions migrate to the titanium plates and gain electrons, forming chlorine gas. Sodium ions (Na+) migrate to the cathode, where they are reduced to form sodium metal. The diaphragm, a porous partition made of asbestos or other suitable material, prevents the mix-up. The cell is operated at high current densities, ensuring a high production rate. Its disadvantages include mercury pollution and energy consumption, and it requires regular maintenance. Despite these drawbacks, the Castner-Kellner cell is still an important technology for large-scale production of sodium and chlorine. It is the basis of modern industrial processes such as the chloralkali industry, which produces chlorine and sodium hydroxide, which are essential for many manufacturing operations.

4. Electrolyte

Invented by Hamilton Castner and Karl Kellner in the late 19th century, this electrolytic process is now used globally to produce sodium metal and chlorine gas. Its a major industrial chemical process that powers the world’s chlorine and alkali plants and underpins many other industries such as aluminium production and sodium peroxide bleach. It also provides chlorine for the treatment of drinking water and the extraction of gold from ore. The process itself uses an electrolytic cell, which has a graphite anode and mercury cathode to generate chlorine and sodium metal.

The anode and cathode are separated by a diaphragm which prevents mixing of chlorine gas with the sodium metal. The diaphragm is made from asbestos, a fire-resistant material that is able to tolerate high temperatures and long-term operation of the electrolytic cell. The cell has several advantages including efficiency and selectivity, but it also poses problems such as mercury pollution, energy consumption, and maintenance. It is still a vital technology for the large-scale production of these essential chemicals. Caustic Soda, the chemical produced by this process is corrosive and is known to cause burns if it comes into contact with skin or eyes.