How is potassium hydroxide formed through electrolysis

Potassium hydroxide (potassium hydroxide)


Author: Hans Lohninger

CAS no.1310-58-3
formulaKOH
molar mass56.11 g / mol
Melting point410 ° C
boiling point1327 ° C
density2,044 g / cm3
solubility in water
[0 ° C]
[100 ° C]

970 g / L
1780 g / L

Potassium hydroxide ("caustic potash"), KOH, is a colorless, strongly alkaline substance that, along with sodium hydroxide, is a prototype of a base. Potassium hydroxide is very soluble in water, the aqueous solution is called potassium hydroxide. Like NaOH, KOH is hygroscopic and dissolves in air. KOH also dissolves well in ethanol and glycerin.

Potassium hydroxide is commercially available in the form of cookies or sticks and, due to its hygroscopic properties, must be stored in airtight containers or bags.

Melting diagram of the KOH-water system.

If KOH is dissolved in water, there is a very strong depression of the freezing point. The eutectic of KOH and water is 31% by weight KOH and has a freezing point of -65 ° C.

Manufacturing

Around 800,000 tons of potassium hydroxide are produced worldwide every year. Today it is largely produced by electrolysis of an aqueous solution of potassium chloride (see chlor-alkali electrolysis):

2 H2O + 2 KCl H2 + 2 KOH + Cl2

In the past, KOH was represented by "causticizing" potassium carbonate, with calcium carbonate precipitating out as a solid precipitate and thereby shifting the equilibrium to the right:

K2CO3 + Ca (OH)2 2 KOH + CaCO3

application

Potassium hydroxide has a multitude of uses, the most important of which are briefly listed below:

  • Electrolyte in batteries: KOH is used as an electrolyte in nickel-cadium batteries and in mercury button cells (zinc / mercury oxide or cadium / mercury oxide). Especially with Cd / HgO cells, potassium hydroxide has the advantage that the button cell still works well even at very low temperatures, since 31% potassium hydroxide freezes only at around -60 ° C and cadmium is practically insoluble in potassium hydroxide.
  • Production of soft soaps: If you replace caustic soda with potassium hydroxide in soap production, soft soap (potassium stearate) is produced instead of curd soap (sodium stearate).
  • Starting basis for organic and inorganic potassium compounds (e.g. potassium phosphate fertilizer, potassium permanganate, potassium cyanide)
  • Water electrolysis: By electrolysis of a 30% potassium hydroxide solution on nickel electrodes, pure hydrogen and pure oxygen are obtained (separated at the electrodes),
  • Absorption of carbon dioxide in breathing apparatus: Potassium hydroxide forms with CO2Potassium carbonate and can thus remove the excess carbon dioxide from the air you breathe.
  • Potassium hydroxide is approved as a food additive (E525) without maximum quantity restrictions and is used as an acidity regulator in jam or instant teas, among other things.