The difference between Alum and Al₂(SO₄)₃ Structure and Application

Aluminum sulfate is a widely used industrial reagent, often confused with alum. Today we will introduce these two products separately, and find out the difference of Alum and Al₂(SO₄)₃ structure and application.

Al₂(SO₄)₃ Structure

Aluminum sulfate is an inorganic compound with the Al₂(SO₄)₃ structure, a molecular weight of 342.15, and a molar mass of 342g/mol. It is a white orthorhombic crystalline powder. It is soluble in water and insoluble in ethanol, and the solution is acidic. It loses some of its crystal water at 86.5℃ and all of it at 250℃. When heated, it expands violently and becomes a sponge-like substance. When burned to red heat, it decomposes into sulfur trioxide and aluminum oxide. Its melting point is 770℃ and its density is 2.71g/cm³. 

Al₂(SO₄)₃ Application

In the papermaking industry, it is used as a precipitant for rosin glue, wax emulsion and other sizing materials, as a flocculant in water treatment, and as an internal retention agent for foam fire extinguishers. It is also a raw material for making alum and aluminum white, a petroleum decolorizer, a deodorant, and a raw material for certain drugs. It can also be used to make artificial gems and high-grade ammonium alum.

The first major use of aluminum sulfate, which accounts for about 50% of the total output, is for papermaking. The second largest use is as a flocculant in drinking water, industrial water and industrial wastewater treatment, accounting for about 40% of the total output of aluminum sulfate. When aluminum sulfate is added to this type of water, colloidal aluminum hydroxide flakes can be generated that can adsorb and precipitate bacteria, colloids and other suspended matter. It can be used in drinking water treatment to control the color and taste of water. Aluminum sulfate is usually used as a flocculant for purifying drinking water and sewage treatment equipment, and is also used in the papermaking industry.

Alum Structure

The scientific name of alum is potassium aluminum sulfate dodecahydrate, also known as white alum, potassium alum, potassium aluminum alum, potassium alum, and potassium alum. It is a complex salt of potassium sulfate and aluminum sulfate containing crystal water. It is a colorless cubic crystal, which often appears as an octahedron, or forms a polyhedron with a cube or rhombic dodecahedron. Sometimes it is shaped like a hexagonal plate. It belongs to the α-type alum complex salt and has a glassy luster. The molar mass of alum is 474g/mol, the density is 1.757g/cm³, and the melting point is 92.5℃. It loses 9 molecules of crystal water at 92.5℃ and 12 molecules of crystal water at 200℃. It is soluble in water and insoluble in ethanol.

Alum Application

Alum is sour, astringent, cold, and toxic. Therefore, it has antibacterial and astringent effects, and can be used as a traditional Chinese medicine. Alum can also be used to prepare aluminum salts, baking powder, paint, tanning materials, clarifiers, mordants, papermaking, waterproofing agents, etc. It can also be used as a food additive. In our lives, it is often used for water purification and as an edible expansion agent.

In summary, we can roughly understand the difference of Alum and Al₂(SO₄)₃ structure and application.