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Potassium Hydroxide

What is potassium hydroxide?

Potassium hydroxide (formula KOH), also called caustic potash or lye is a chemical compound which uses mainly for making soft soap. Like sodium hydroxide, potassium hydroxide is also a strong base that attacks aluminum and zinc. It is made by the electrolysis of potassium chloride solution.

Potassium hydroxide or caustic potash chemical formula and uses of KOH solution in food and making soap

Potassium hydroxide is a strong alkali which available in liquid and crystalline solid forms. The solid form is readily dissolved in water to form an aqueous KOH solution. Potassium hydroxide is marketed in several forms including pellets, flakes, and powders.

Potassium hydroxide and its solution are highly reactive and corrosive which irritates our skin, eyes, and other tissue.

Properties

At room temperature, caustic potash is a white crystalline solid that absorbs moisture from the air and attacks aluminum and zinc. It forms a clear solution with water. It is also dissolved in low molecular alcohols like methanol, ethanol, and propanol.

Some common properties of KOH are given below the table,

Potassium hydroxide
Chemical formula KOH
Other names caustic potash
Potash lye
Potassia
Potassium hydrate
CAS number 1310-58-3
Molar mass 56.11 g mol−1
Appearance White crystalline solid
Odor odorless
Density 2.044 g cm−3
Melting point 360 °C, 680 °F, 633 K
Boiling point 1327 °C, 2421 °F, 1600 K
Solubility Soluble in water and alcohols (methanol, isopropanol), glycerol but insoluble in ether, liquid ammonia
Acidity (pKa) 14.7
Crystal structure rhombohedral
Heat capacity 65.87 J mol−1 K−1
Standard molar
entropy
79.32 J mol−1 K−1
Standard enthalpy of
formation
−425.8 kJ mol−1
Gibbs free energy −380.2 kJ mol−1

Chemical properties

Potassium hydroxide reacts violently with acid to form corresponding salts.

  • It reacts with methanol to form potassium methoxide.
    KOH + CH3OH → CH3OK + H2O
  • It is a highly nucleophilic anion that attacks polar bonds of inorganic and organic compounds. For example, aqueous KOH saponifies esters.
    KOH + RCOOR’ → RCOOK + R’OH
  • KOH attracts SiO2 to give soluble potassium silicates.
  • It reacts with carbon dioxide to produce potassium bicarbonate
    KOH + CO2 → KHCO3
  • Potassium hydroxide solutions react with the aluminum to form potassium tetra hydroxy aluminate (III) and hydrogen gas.
    2 Al + 2 KOH + 6 H2O → 2 KAl(OH)4 + 3 H2

Potassium hydroxide preparation

Previously, KOH was produced by adding potassium carbonate to a strong solution of calcium hydroxide.
Ca(OH)2 + K2CO3 → CaCO3 + 2 KOH

Calcium carbonate was removed from the solution as a precipitate. After this, the solution boils to give potassium hydroxide or caustic potash.

Presently, the process is replaced by a new method where electrolysis of aqueous potassium chloride gives KOH.
2 KCl + 2 H2O → 2 KOH + Cl2 + H2

Hydrogen and chlorine gas are produced as a by-product during the electrolysis of KCl.

Uses of potassium hydroxide

Potassium hydroxide has a wide variety of applications in various industries such as the cleaning industry, chemicals industry, agriculture industry, food processing industry, manufacturing industry, cosmetics, etc.

Potassium hydroxide for soap making

Soap is a salt product that is obtained by combining alkali with fats or fatty acids. The alkali used for making soap is NaOH or KOH.

KOH or caustic potash or lye is a chemical which use specifically for making soft soap. It induces saponification of the fats and oils to form liquid soap. It is softer than the more common sodium hydroxide soaps.

Uses in the chemical industry

  • It is used for making many potassium salts and potassium chemicals such as carbonate, cyanide, permanganate, phosphate, bromate, bromide, potassium aluminate, laurate, formate, gluconate, etc.
  • Aqueous potassium hydroxide solution is used as the electrolyte for the production of alkaline batteries such as nickel-cadmium, nickel-hydrogen, and manganese dioxidezinc batteries.
  • It is used for the production of various household cleaning products.
  • In the petroleum and natural gas industry, potassium hydroxide helps remove organic acids and sulfur compounds.
  • It is used as a catalyst in the production of biodiesel from fats and oils.
  • In analytical chemistry, KOH is a useful agent which adjusts the pH of chemical solutions.
  • It is used for semiconductor chip fabrication.

Uses in agriculture

  • In agriculture, KOH is used to adjust and regulate the pH level of the soil.
  • It is used for the production of fertilizers that supply potassium to crops.
  • KOH is also used for making herbicides and fungicides. For example, phosphorous acid and KOH solutions may be sprayed for pest and disease prevention.

KOH in food

In the food industry, potassium hydroxide acts as a food thickener, pH controller, and food stabilizer agent. KOH is a common food additive to make soft drinks, chocolate, and cocoa. It acts as a thickener and stabilizer in ice cream.

It is also used for washing and the chemical peeling of fruit and vegetables. According to FDA consideration, it is a safe food ingredient when used in accordance with Good Manufacturing Practices.

KOH in cosmetics

In cosmetics, KOH is used for making manicure products and shaving products. Potassium hydroxide is a basic material which use in the production of lotions, shampoos, hairspray, etc

Potassium hydroxide solution

Aqueous potassium hydroxide solution is a colourless inorganic liquid that is used as a strong base. The pH values in an aqueous solution depending on the concentration of KOH.

KOH in water dissociates completely to form K+ and OH ions. Due to complete dissociation, the concentration of KOH is equal to the concentration of OH.

Problem: If the concentration of an aqueous potassium hydroxide solution is 0.1 mol dm−3. How to calculate the pH value of such KOH solution?

Solution: The concentration of OH of said solution = 0.1 mol dm−3 = 10−1.

∴ pOH = −log[10−1]
or, pOH = 1

From the equation, pH + pOH = 14
pH + 1 = 14
∴ pH = 13

Problem: How to calculate the pH value of 0.0001 mol dm−3 KOH solution?

Solution: The concentration of OH of said solution = 0.0001 mol dm−3 = 10−3 mol dm−3.

∴ pOH = −log[10−3]
or, pOH = 3

From the equation, pH + pOH = 14
pH + 3 = 14
∴ pH = 10

From the above two problems, if the concentration of KOH lies between 0.0001 M to 0.1 M, the pH range of such solutions will be 10 to 13. When the concentration of potassium hydroxide increases, the pH value of the solution also increases.