Aluminium potassium sulfate dodecahydrate
[CAS# 7784-24-9]

Identification

• Classification: API >> Special medicine >> Dermatology medication
• Name: Aluminium potassium sulfate dodecahydrate
• Synonyms: Potassium alum
• Molecular Formula: AlK.(SO₄)₂.12(H₂O)
• Molecular Weight: 474.37
• CAS Registry Number: 7784-24-9
• EC Number: 616-521-7
• SMILES: O.O.O.O.O.O.O.O.O.O.O.O.[O-]S(=O)(=O)[O-].[O-]S(=O)(=O)[O-].[Al+3].[K+]

Properties

• Density: 1.757 g/m (Expl.)
• Melting point: 92 °C (Expl.)
• Solubility: Soluble in water, glycerol. Insoluble in alcohol (Expl.)

Safety Data

• Hazard Symbols: GHS07 Warning
• Risk Statements: H302-H315-H319-H335
• Safety Statements: P261-P305+P351+P338

Discovery and Applications

Aluminium potassium sulfate dodecahydrate, commonly known as potash alum, is a double sulfate salt with the formula KAl(SO₄)₂·12H₂O. It belongs to a class of compounds historically referred to as alums, which are hydrated double sulfates of a monovalent cation and a trivalent metal ion. Natural alum minerals were known in antiquity and were obtained from alunite deposits and related sources. The substance was used long before its chemical composition was understood, particularly in dyeing and tanning processes. Systematic chemical characterization developed in the eighteenth and nineteenth centuries as analytical chemistry advanced and the composition of double salts became clearer.

The historical importance of potash alum is closely linked to the textile industry. It functions as a mordant, a substance that fixes dyes onto fibers by forming coordination complexes between the dye molecules and the textile material. In traditional wool and silk dyeing, aluminium potassium sulfate dodecahydrate was dissolved in water and applied to fibers prior to or together with natural dyes. The aluminium ion forms stable interactions that enhance color fastness and intensity. This application was documented in Europe, the Middle East, and Asia for many centuries and remained significant until the development of synthetic dyes and alternative mordants.

In addition to dyeing, potash alum has been widely used in leather tanning. The salt facilitates the treatment of hides by interacting with collagen fibers, contributing to preservation and improved handling characteristics. Alum-tanned leather, sometimes called tawed leather, differs from vegetable-tanned leather in both process and properties. The use of alum in tanning was established well before the rise of modern industrial chemistry and became part of established craft traditions.

Aluminium potassium sulfate dodecahydrate has also played a documented role in water treatment. When added to water, it undergoes hydrolysis to produce aluminium hydroxide species that form gelatinous flocs. These flocs adsorb suspended particles and facilitate their removal by sedimentation and filtration. The coagulation and flocculation properties of alum have been widely applied in municipal water purification and wastewater treatment. This application became especially important during the development of modern public water systems in the nineteenth and twentieth centuries.

Another established use of potash alum is in paper manufacturing. It has been employed in combination with rosin to size paper, improving resistance to ink penetration and enhancing writing quality. The alum-rosin sizing process was a dominant method in papermaking for many decades before the introduction of alternative sizing agents. Its role in controlling paper properties contributed to the expansion of large-scale printing and publishing industries.

In medicine and personal care, aluminium potassium sulfate dodecahydrate has been used for its astringent properties. Aqueous solutions have been applied topically to minor cuts to reduce bleeding by causing protein precipitation and local vasoconstriction. Solid alum blocks have also been used after shaving for similar effects. These applications are based on well-documented protein-precipitating behavior of aluminium salts.

The compound’s crystalline structure, characterized by twelve molecules of water of crystallization, has been studied as part of broader investigations into double salts and hydration in inorganic chemistry. Its well-defined octahedral coordination around the aluminium ion and the arrangement of sulfate groups have made it a representative example in discussions of crystal chemistry.

Aluminium potassium sulfate dodecahydrate thus occupies an important place in both historical practice and modern industry. From textile dyeing and leather processing to water purification and paper production, its verified applications demonstrate the enduring utility of this classical inorganic compound.

References

2025. Electrochemical activation of alum sludge for the adsorption of lead (Pb(II)) and arsenic (As): Mechanistic insights and machine learning (ML) analysis. Bioresource Technology.
DOI: 10.1016/j.biortech.2025.132563

2025. ARNAX, but not conventional adjuvants, alum and squalene, induces antigen-specific CD8+ T cell proliferation in vaccination with influenza split vaccine. International Immunopharmacology.
DOI: 10.1016/j.intimp.2025.114895