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
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
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.
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
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
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