Alum

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What is Alum?

Alum is an inorganic compound composed of water molecules, aluminium, other metal than aluminium, and sulphates. Alum is a double salt in the hydrated form. Alum exists in different forms: potash alum, Soda alum, ammonium alum, and chrome alum. The general chemical formula for alum is XAl(SO4)2·12H2O.


Types of Alum

  1. Potash Alum - It is also known as potassium alum. The common name of potash alum is fitkari. The chemical formula for potash alum is KAl(SO₄)₂·12H₂O. Its chemical name is potassium aluminium sulphate. The common alum is potash alum or potassium alum.

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The molar mass of the potash alum is 258.192 g/mol. It exists in a white crystal form. It smells like metallic water. It is also commonly known as white alum.

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  1. Soda Alum - It is also known as sodium alum. Its common name is SAS. The chemical formula for sodium alum is NaAl(SO₄)₂·12H₂O. Its chemical name is sodium aluminium sulphate. The molar mass of the sodium alum is 458.28 g/mol. It exists in a white crystal form. It smells like metallic water.

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  1. Ammonium Alum - It is also known as ammonium sulphate alum. Its common name is AAS. The chemical formula for potash alum is NH3Al(SO4)2·12H2O. Its chemical name is ammonium aluminium sulphate. The molar mass of the ammonium alum is 132.14 g/mol. It exists in a white crystal form. It smells like metallic water.

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  1. Chrome Alum -  It is also known as chromium alum. Its common abbreviation for chromium alum is CAS. The chemical formula for potash alum is KCr(SO4)2·12H2O. Its chemical name is chromium aluminium sulphate. The molar mass of the chromium alum is 283.22 g/mol. It exists in a purple crystal form. It smells like metallic water.

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  1. Selenate Alum - In this type of the alum selenium takes the place of the sulphur. The anion present in this form of alum is selenate in place of sulphate. These alums possess the property of strong oxidizing agents. The molecular formula for selenate alum is Al2O12Se3. The molecular mass of the selenate alum is 482.9 g/mol.

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Alum is available both offline and online. Alum offline is generally available at grocery shops and medical stores. Alum Online is available on different e-commerce websites. Alums are generally sold by their common names like white fitkari, red fitkari, and white alum stones or red alum stones. Alums possess the property of anti-inflammatory agents. It is used in the gargling process to reduce teeth gum inflammation and pain.


Properties of Alum

  • Alums are highly soluble in water.

  • These compounds are sweet in taste.

  • They generally crystallize in the regular octahedral form.

  • The alum crystals get liquified when heated.

  • 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 92.5degrees celsius.

  • Its density is 1.725 g/cm3.


Alum Water Treatment 

Alum water treatment is generally carried to treat the polluted water. These compounds act as a coagulant. It is used in the coagulation-flocculation process of polluted water. It is a chemical water treatment technique typically applied prior to sedimentation and filtration to enhance the ability of a treatment process to remove particles.

Coagulation - it destabilizes the charges of the particles. Coagulants with charges opposite to those of the suspended solids are added to the water to neutralize the negative charges on dispersed non-settable solids such as clay and organic substances.

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

Once the charge is neutralized, the small-suspended particles are capable of sticking together. The slightly larger particles formed through this process are called microflocs and are still too small to be visible to the naked eye.


Uses of Alum

  • It is used in the pickling and baking process.

  • It is used in the tanning process of leather.

  • It is used in the coagulation and flocculation process of water treatment.

  • It is used as an acidulating agent in cooking.

  • It is used as a drying agent in a textile company.

  • It is used as an antiseptic agent.

FAQ (Frequently Asked Questions)

1. Is alum soluble in water?

Yes, alum is soluble in water. Alum is an ionic compound consist of water of hydration. These large number of water molecules contribute to the solubility of alum in water. It gets completely dissociated when dissolving in water.

2. What is an alum?

Alum is an inorganic compound composed of water molecules, aluminium, other metal than aluminium, and sulphates. Alum is a double salt present in the hydrated form. The general chemical formula for alum is XAl(SO4) 2·12H2O.

3. Write the properties of the alum.

The properties of the alum are given below:

  • Alums are highly soluble in water.

  • These compounds are sweet in taste.

  • They generally crystallize in the regular octahedral form.

  • The alum crystals get liquified when heated.

  • Alums generally exist in the form of a white and transparent crystalline form.