Alum

Alum is a term used for a group of double salts that contain sulphate ions and are characterised by a similar crystalline structure. Chemically, an alum is a double sulphate of a trivalent metal (such as aluminium, chromium, or iron) and a monovalent metal (such as potassium, sodium, or ammonium). The most common and widely used form is potassium aluminium sulphate, represented by the chemical formula KAl(SO₄)₂·12H₂O. Alums are known for their crystalline nature, astringent taste, and wide range of industrial, medicinal, and domestic applications.

Chemical Composition and Structure

Alums share the general formula M⁺M³⁺(SO₄)₂·12H₂O, where M⁺ is a monovalent cation (like K⁺, Na⁺, NH₄⁺, Rb⁺, or Cs⁺) and M³⁺ is a trivalent cation (such as Al³⁺, Cr³⁺, or Fe³⁺).
For example:

• Potassium alum (common alum): KAl(SO₄)₂·12H₂O
• Ammonium alum: NH₄Al(SO₄)₂·12H₂O
• Sodium alum: NaAl(SO₄)₂·12H₂O
• Chrome alum: KCr(SO₄)₂·12H₂O
• Ferric alum: KFe(SO₄)₂·12H₂O

All alums are isomorphous, meaning they have the same crystal structure and can form mixed crystals with each other. They usually crystallise in the cubic system, producing transparent octahedral crystals. When heated, alums lose their water of crystallisation and become anhydrous.

Preparation of Alum

Alum can be prepared in laboratories and industrial settings through various processes. The most common method involves neutralising aluminium sulphate solution with a monovalent metal sulphate such as potassium sulphate. The reaction may be represented as:
Al2(SO4)3+K2SO4+24H2O→2KAl(SO4)2⋅12H2OAl_2(SO_4)_3 + K_2SO_4 + 24H_2O → 2KAl(SO_4)_2·12H_2OAl2​(SO4​)3​+K2​SO4​+24H2​O→2KAl(SO4​)2​⋅12H2​O
In industrial production, alum is often obtained from bauxite or alunite ores. The ore is roasted and treated with sulphuric acid to yield aluminium sulphate, which is then crystallised with potassium sulphate to produce potassium alum.

Physical and Chemical Properties

Alums are generally colourless, though those containing chromium or iron exhibit purple or violet hues. Their key properties include:

• Appearance: Transparent or white crystalline solid.
• Taste: Sweetish and astringent.
• Solubility: Soluble in hot water, moderately soluble in cold water, and insoluble in alcohol.
• Melting and Dehydration: On heating, they lose water of crystallisation and eventually decompose into aluminium oxide and other oxides.
• Reaction with Litmus: Acidic in aqueous solution due to hydrolysis of aluminium ions.
• Crystallisation: Easily crystallises upon cooling of hot saturated solutions.

Chemically, alums are mildly acidic and exhibit coagulating properties, making them useful in flocculation and water purification processes.

Types and Variants of Alum

1. Potassium Alum (KAl(SO₄)₂·12H₂O): The most common form, widely used in purification, dyeing, and medicinal applications.
2. Ammonium Alum (NH₄Al(SO₄)₂·12H₂O): Used in tanning and water purification.
3. Sodium Alum (NaAl(SO₄)₂·12H₂O): Employed in the paper industry and dyeing.
4. Chrome Alum (KCr(SO₄)₂·12H₂O): Deep violet crystals used in tanning leather and photographic processing.
5. Ferric Alum (KFe(SO₄)₂·12H₂O): Used as a mordant in dyeing and for coagulation in water treatment.

Industrial and Commercial Uses

Alum has extensive applications across various fields due to its coagulating, antiseptic, and stabilising properties.
1. Water Purification: Alum acts as a flocculating agent, helping to remove suspended particles and impurities from water. When added to water, it forms a gelatinous precipitate of aluminium hydroxide that traps fine particles, which can then be filtered or settled out. This makes alum essential in municipal water treatment plants.
2. Textile and Dyeing Industry: In dyeing and printing textiles, alum serves as a mordant, helping dyes adhere firmly to fabrics. It forms insoluble complexes with dyes and fibres, enhancing colour fastness.
3. Leather Tanning: Certain types of alum, especially chrome alum, are used in tanning processes to make leather soft, durable, and resistant to decay.
4. Paper Manufacturing: Alum is added to paper pulp to improve sizing and prevent ink from spreading. It also enhances the paper’s texture and durability.
5. Medicine and Health: Alum’s astringent and antiseptic properties make it valuable in medical and personal care applications:

• Used as a styptic to stop bleeding from minor cuts and shaving nicks.
• Employed in traditional medicine for its antimicrobial action.
• Incorporated in some dental powders and deodorants to reduce bacterial growth.

6. Food Preservation: In small quantities, alum has been used historically in pickling to maintain crispness of fruits and vegetables. However, its use in food is now regulated due to health considerations.
7. Fireproofing and Dye Fixation: In earlier times, alum was used in the fireproofing of fabrics and wood, and as a fixative for natural dyes in artwork and textiles.

Environmental and Safety Considerations

Although alum is generally considered safe in controlled quantities, excessive exposure or ingestion can be harmful. In high concentrations, aluminium compounds can contribute to soil and water acidity and have been linked to neurological health concerns when improperly used. Modern regulations restrict the quantity of alum allowed in food and water treatment to ensure safety.
Environmental impacts arise primarily from large-scale industrial discharge, where alum residues may alter pH levels and affect aquatic ecosystems. Sustainable alternatives and careful dosage control are therefore important in industrial applications.

Historical and Cultural Significance

The use of alum dates back to antiquity. Ancient Egyptians and Greeks employed it for dyeing fabrics, tanning hides, and purifying water. During the Middle Ages, alum was a valuable trade commodity, with deposits in regions such as Rocca, Italy, and later Egypt and India serving as major sources. European textile industries heavily relied on alum imports before discovering local sources in the sixteenth century.
In India, traditional households used fitkari (the Hindi term for alum) for purifying drinking water, healing wounds, and cosmetic uses long before modern chemical processing began.