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What Is Alum?

Alum is an inorganic chemical compound that is generally made up of water molecules, aluminium or other metals and sulphates. Alum is basically a hydrated double sulphate salt of aluminium.  The general chemical formula for alum is XAl(SO4)2·12H2O. Alums can occur as minerals. For example, alunite and leucite.

Alum crystals are also available in both offline and online markets, such as grocery shops and medical stores or e-commerce websites. These chemical compounds are generally sold as white fitkari, red fitkari or white alum stones and red alum stones.

Alum Production

While there are many types of alums, the most important ones include potassium, sodium, and ammonium. These are also produced industrially, usually by combining aluminium sulphate and the sulphate monovalent cation. It is produced by the process of precipitation from an aqueous solution. For instance, to produce potassium alum, aluminium sulphate and potassium sulphate are dissolved in water. During evaporation, the alum crystallises out of the solution. Aluminium sulphate is also commonly procured by treating minerals like bauxite, cryolite or alum schist with sulfuric acid. It can also be obtained by treating a mixture of aluminium sulphate and sulfuric acid with ammonia.

Properties of Alum

Most alums tend to have an astringent and acidic taste. They are odourless, colourless and often found in the form of a white crystalline powder.

  • Alums are highly soluble in water (especially in hot water).
  • They generally crystallise in the regular octahedral form when precipitated from aqueous solutions.
  • The alum crystals can be liquified if heat is applied.
  • Alums generally exist in the form of a white and transparent crystalline form.
  • Its boiling point is around 200 degrees Celsius.
  • Its melting point is 92.5 degrees Celsius.
  • Its density is 1.725 g/cm3.
  • They react with acid to litmus.

Types of Alum

Alum exists in different forms: potash alum, soda alum, ammonium alum, and chrome alum. We will discuss them briefly below.

1. Potash Alum – Alum in its single form is also commonly known as potassium alum or potash alum. The chemical formula for potash alum is KAl(SO₄)₂·12H₂O, and its chemical name is potassium aluminium sulphate. The common alum is a potash alum or potassium alum. The molar mass of the potash alum is 258.192 g/mol. It can occur in a white crystal form and smells like metallic water. Potash alum is also sometimes called white alum.

2. Soda Alum – It is also called sodium alum or just SAS, is a common item found in baking powder and as an acidulant in food. Its chemical name is sodium aluminium sulphate, and has a molar mass of 458.28 g/mol. It exists in a white crystal form and has a smell that of metallic water. The chemical formula for sodium alum is NaAl(SO₄)₂·12H₂O.

3. Ammonium Alum – This type is referred to as ammonium sulphate alum. Its more common name is AAS. The chemical formula for potash alum is NH3Al(SO4)2·12H2O. Ammonium alum has a molar mass of 132.14 g/mol. It also exists in a white crystal form with a metallic water smell.

4. Chrome Alum – The common abbreviation for chromium alum is CAS. It is also known as chromium alum and has a chemical formula KCr(SO4)2·12H2O. It is also known as chromium aluminium sulphate and has a molar mass of 283.22 g/mol. Like other alums, it has a metallic water smell but mostly exists in a purple crystal form.

5. Selenate Alum – Here, selenium takes the place of sulphur. In this form of alum, the anion present is selenate instead of sulphate. It has the property of being a strong oxidising agent. The molecular formula for selenate alum is Al2O12Se3, and its molecular mass is 482.9 g/mol.

Alum Water Treatment

Alum water treatment is carried out to basically treat polluted waters. In this use case scenario, alum acts as a coagulant and is used in the coagulation-flocculation process of polluted water. Alum treatment is carried out prior to the sedimentation and filtration process. This is done mainly to enhance the ability of a treatment process in the removal of unwanted particles.

During the coagulation process, destabilisation of the charges on the particles occurs. Coagulants that have opposite charges to those of the suspended solids are added to the water. This is done to neutralise the negative charges on dispersed non-settable solids, which include clay and some organic substances.

Al2(SO4)3.18H2O + 6HCO3- → 2Al(OH)3 + 6CO2 +18H2 + 3SO4-2

After the neutralisation of the charge, the small-suspended particles stick together.

Uses of Alum

Some of the common uses of alum are given below:

  • Alum is used in the baking and pickling process.
  • Used in the tanning process of leather.
  • As stated above, used widely in the coagulation and flocculation process of water treatment.
  • Acts as an acidulating agent in cooking.
  • Used as a drying agent in a textile company.
  • It is used as an antiseptic agent and adjuvant in vaccines.
  • Used as an acidic component of baking powder.
  • Alums possess the property of anti-inflammatory agents and are commonly used in the gargling process to reduce teeth gum inflammation and pain. It is found as an ingredient in some toothpaste.

Alum Concept Video


Frequently Asked Questions on Alum


What is alum?

Alum is a hydrated double sulfate salt of aluminium.

What is the general formula of alum?

The general formula of alum is XAl(SO4)2.12H2O

Is alum water soluble?

Yes, alums are highly soluble in water.

In which form do alums exist?

Alums exist in crystalline form. They are transparent and white.

What are the different types of alum?

The different types of alum are soda alum, potash alum, chrome alum and ammonium alum.


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