Aluminium sulphate, molecular formula Al2(SO4)3, is a chemical compound of aluminium from the group of sulphates. It forms a colourless powder with a density of 2.71 g/cm3.
Octadecahydrate, which is more commonly used in the laboratory, forms white crystals that dissolve very well in water. They are insoluble in ethanol. The aqueous solution reacts acidically. Due to this reaction, an acid is formed during eye contact, which can damage the cornea. When heated, the octadecahydrate melts already at 86 °C and splits off the water of crystallization above 340 °C. This produces aluminium sulphate, a loose, white powder. During strong annealing at about 800 °C, aluminium sulphate decomposes into sulphur trioxide and aluminium oxide:
Al2(SO4)3 → Al2O3 + 3 SO3
In nature, aluminium sulphate is found in the mineral alunogen, for example. It can also be extracted from naturally occurring alums.
Industrial production is carried out by dissolving aluminium hydroxide in concentrated warm sulphuric acid. Another possibility is the addition of 70% sulphuric acid to bauxite or kaolin under heat and pressure. However, this produces a product that is contaminated with iron ions. By adding potassium sulphate to the product, potassium aluminium sulphate crystallises out. This is how alum is produced today.
Aluminium sulphate is required in paper production during the sizing process to adjust the pH value. In drinking water treatment it is used as a flocculant. Like alum, it is suitable as a mordant in dyeing. It is also an important intermediate product for the production of other aluminium compounds. Aluminium sulphate is approved as food additive E 520. It serves as a firming agent and stabiliser. Further applications:
- Seed dressing
- Component of foam extinguishing agents
- Flame retardant
- Insulating salt (on fresh cement or lime surfaces, against water and nicotine stains) in renovation
- Ingredient of plant strengthening and protecting agents and for the blue colouring of hydrangeas
Aluminium sulphate was included by the EU in 2013 in the Community’s rolling action plan (CoRAP) under Regulation (EC) No 1907/2006 (REACH) as part of the substance evaluation. This involves reassessing the effects of the substance on human health and the environment and, if necessary, taking follow-up action. The reasons for the inclusion of aluminium sulphate were the concerns about consumer use, high (aggregated) tonnage, high risk characterisation ratio (RCR) and widespread use, as well as the risks arising from possible classification in the group of CMR substances and the potential risk from sensitising properties. The re-evaluation has been ongoing since 2015 and is being carried out by France. In order to be able to arrive at a final evaluation, further information was requested.