:: Aluminium Sulphate


Aluminium sulfate
, alternatively spelled either aluminum or sulphate, is a chemical compound with the formula Al2(SO4)3.

It is soluble in water and is mainly used as a flocculating agent in the purification of drinking water and waste water treatment plants, and also in paper manufacturing.

Aluminium sulfate is sometimes referred to as a type of alum. Alums are double sulfate salts, with the formula
AM(SO4)212H2O, where A is a monovalent cation such as potassium or ammonium and M is a trivalent metal ion such as aluminium.

The anhydrous form occurs naturally as a rare mineral millosevichite, found e.g. 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)316H2O and octadecahydrate Al2(SO4)318H2O are the most common.

The heptadecahydrate, whose formula can be written as [Al(H2O)6]2(SO4)35H2O, occurs naturally as the mineral alunogen.


Aluminium sulfate is used in water purification and as a mordant in dyeing and printing textiles. In water purification, it causes impurities to coagulate which are removed as the particulate settles to the bottom of the container or more easily filtered. This process is called coagulation or flocculation.

When dissolved in a large amount of neutral or slightly alkaline water, aluminium sulfate produces a gelatinous precipitate of aluminium hydroxide, Al(OH)3. In dyeing and printing cloth, the gelatinous precipitate helps the dye adhere to the clothing fibers by rendering the pigment insoluble.

Aluminium sulfate is sometimes used to reduce the pH of garden soil, as it hydrolyzes to form the aluminium hydroxide precipitate and a dilute sulfuric acid solution. An example of what changing the pH level of soil can do to plants is visible when looking at the Hydrangea macrophylla. The gardener can add aluminium sulfate to the soil to reduce the pH level which in turn will result in the flowers of the Hydrangea turning a different color.

In construction industry it is used as waterproofing agent and accelerator in concrete. Another use is a foaming agent in fire fighting foam.

It is also used in styptic pencils, and pain relief from stings and bites.

It can also be very effective as a molluscicide, killing spanish slugs.






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