Aluminium Sulfate – a chemical compound

Aluminium Sulfate – a chemical compound

Aluminium sulfate is a salt with the formula Al2(SO4)3. It also known as alum, is a chemical compound.  It is soluble in water and is mainly used as a coagulating agent (promoting particle collision by neutralizing charge) in the purification of drinking water and wastewater treatment plants, and also in paper manufacturing. It is a white crystalline solid, commonly used as a coagulating agent in the purification of drinking water and wastewater treatment plants.

The anhydrous form occurs naturally as a rare mineral millosevichite, found for example in volcanic environments and on burning coal-mining waste dumps. Aluminium sulfate is rarely, if ever, encountered as the anhydrous salt. It forms a number of different hydrates, of which the hexadecahydrate Al2(SO4)3·16H2O and octadecahydrate Al2(SO4)3·18H2O are the most common. The heptadecahydrate, whose formula can be written as [Al(H2O)6]2(SO4)3·5H2O, occurs naturally as the mineral alunogen.

Here are some key points about aluminum sulfate:

  • Chemical Formula: The chemical formula for aluminum sulfate is Al2(SO4)3, indicating that each molecule consists of two aluminum (Al) atoms bonded to three sulfate (SO4) groups.
  • Appearance: Aluminum sulfate typically appears as white crystals or a white powder. It is soluble in water, and its aqueous solution has a slightly acidic nature.
  • Solubility: It is soluble in water. When dissolved in water, it dissociates into aluminum ions (Al3+) and sulfate ions (SO4^2-).
  • pH: The addition of aluminium sulfate to water results in the release of acidic hydrogen ions, which can lower the pH of the solution. This property is often utilized in water treatment processes to adjust pH levels.
  • Hygroscopic: It has a tendency to absorb moisture from the air, making it hygroscopic.


  • Water Treatment: Aluminum sulfate is widely used in water treatment processes to clarify water by causing impurities and particles to coagulate and settle. This helps in the removal of suspended solids and reduces turbidity.
  • Paper Industry: It is used as a sizing agent in the paper industry to improve the paper’s resistance to water.
  • Textile Industry: Aluminum sulfate is employed in the textile industry as a mordant, helping to fix dyes to fabrics.
  • Soil Amendment: In gardening and agriculture, it may be used to lower soil pH in certain conditions.

Health Hazards

While aluminum sulfate is generally considered safe when used in water treatment within recommended concentrations, excessive exposure to aluminum compounds has been associated with certain health concerns. It’s important to use aluminum sulfate in accordance with regulations and guidelines to prevent environmental and health issues.