Potasium Hydroxide

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Caustic Potash

Potassium hydroxide, also known as caustic potash or lye, is a strong inorganic base of potassium. The chemical formula of potassium hydroxide is KOH. It is one of the strongest bases which exist, and because of this property, it has multiple industrial as well as day-to-day uses. 

Potassium Hydroxide Chemical Formula

As explained earlier, the chemical formula of potassium hydroxide or caustic potash is KOH. It is a crystalline solid with a crystal structure which is similar to sodium chloride at higher temperatures.

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The chemical structure of KOH molecule is as follows:

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In the above structure, the distance between K+ and OH- ions varies anywhere between 2.69 to 3.15 angstroms. 

Although KOH can exist as a crystalline solid similar to sodium chloride, but due to its high affinity to water, it usually ends up forming crystalline hydrates. The most common hydrates of caustic potash are mono, di and tetra with either 1 water molecule, 2 water molecules and 4 water molecules, respectively, attached to one molecule of potassium hydroxide.

Properties of Caustic Potash

As we explained earlier, potassium hydroxide or caustic potash has multiple uses because of a wide array of chemical and physical properties that it possesses. We have made some general references to a few of such properties in previous sections, but here we look at those in a slightly more detailed manner.

At room temperature, potassium hydroxide exists as a colourless solid which is generally translucent. Since it has a high affinity to water, its crystals also contain water molecules in varying degrees. Commercially, it is sold as flakes, pellets or powder, depending on the usage. 

As the name suggests, it also has caustic nature, which basically means it has a burning sensation and is an irritant if exposed to naked skin. It is advised to be used and handled with caution. It also has a highly corrosive nature. It can corrode metals along with living cells or tissues.

Potassium hydroxide is a hydroxide of an alkali metal. Hence, it is a strong base. In fact, it is featured among some of the strongest stable bases. The pH value of a 1m mol/L solution of KOH at standard conditions is 10.98, which is very high. 

Caustic potash is highly soluble in water as well as organic alcohols. A lot of heat is released when it dissolves in water. In organic alcohols, potassium hydroxide forms an acid-base equilibrium as below:


Equation 1: Acid-base equilibrium of KOH with Methanol

It also reacts with inorganic and organic acids to form corresponding salts.

KOH + HCl 🡪 KCl + H2O

Equation 2: KOH reaction with an inorganic acid

Potassium hydroxide is used in a process called saponification to convert organic fats into soaps. A generic reaction looks as below:

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It is noncombustible in nature, so it doesn’t react with oxygen. But it can react with carbon dioxide to form potassium bicarbonate.


Equation 4: Caustic potash reacts with carbon dioxide

Potassium hydroxide is highly stable at high temperatures. It gets converted into liquid or gaseous form before disintegrating into its constituent elements at high temperatures. Because of high thermal stability, it can be melted and cast into any desirable shape which may be required for further uses.

Uses of Potassium Hydroxide

Potassium hydroxide is used in preparing soaps by a process called saponification. It is used as a base to convert oils/fatty acids and their esters into soapy salts. Soaps with potassium salts are more soluble than those with sodium salts, and hence are also referred to as soft soaps.

Although corrosive in nature, potassium hydroxide is used in food products as a thickening agent and stabilizer. It can only be used in a certain quantity so that the food items stay safe for human consumption.

It is used as a cleaning agent. One of the most common household cleaning uses is the drain cleaner chemical, which contains potassium hydroxide. Some of the drain cleaners may also contain sodium hydroxide because of its similar nature with potassium hydroxide.

Potassium hydroxide also has medical uses. It is used to prepare clinical specimens of fungal infections in hair, skin, nails etc. It has also been found to treat plane warts well.

Due to its highly conductive nature, it is also used as an electrolyte, especially in alkaline batteries.

Potassium hydroxide is a good etching agent and is used in anisotropic wet etching of semiconductor wafers during the manufacturing of electronic chips.

It is also used to prepare other potassium products and salts such as phosphate, permanganate etc. 

Preparation of Potassium Hydroxide

Potassium hydroxide is prepared at industrial scale by electrolysis of potassium chloride solution. The process can also be referred to as chloralkali process. The reaction takes place as below:

2KCl + 2H2O 🡪 2KOH + Cl2 + H2

Equation 5: Electrolysis of KCl

Chlorine and hydrogen are byproducts of this reaction.

Though KCl electrolysis is the current method of preparing KOH, historically, it was prepared using a salt metathesis reaction of calcium hydroxide and potassium carbonate. In a salt metathesis reaction, the two reactant compounds react to form 2 new compounds as products where their respective elements or constituents exchange places. During the preparation of potassium hydroxide, this reaction takes place as follows:

Ca(OH)2 + K2CO3 🡪 CaCO3 + 2KOH

Equation 6: Salt metathesis reaction to prepare potassium hydroxide

The reaction takes place in an aqueous solution. This method of preparation is not used currently and was in use till the 19th century. Invention and widespread use of electricity replaced the old method with the current electrolysis reaction method.

This ends our coverage on the topic “Caustic potash or Potassium hydroxide”. We hope you enjoyed learning and were able to grasp the concepts. We hope after reading this article you will be able to solve problems based on the topic. If you are looking for solutions of NCERT Textbook problems based on this topic, then log on to Vedantu website or download Vedantu Learning App. By doing so, you will be able to access free PDFs of NCERT Solutions as well as Revision notes, Mock Tests and much more.