Aluminium sulfate

Aluminium sulfate
Aluminium sulfate hexadecahydrate
IUPAC name
Aluminium sulfate
Other names
Cake alum
Filter alum
Papermaker's alum
aluminum salt (3:2)
3D model (JSmol)
ECHA InfoCard100.030.110
EC Number233-135-0
E numberE520 (acidity regulators, ...)
RTECS numberBD1700000
Molar mass342.15 g/mol (anhydrous)
666.44 g/mol (octadecahydrate)
Appearancewhite crystalline solid
Density2.672 g/cm3 (anhydrous)
1.62 g/cm3 (octadecahydrate)
Melting point 770 °C (1,420 °F; 1,040 K) (decomposes, anhydrous)
86.5 °C (octadecahydrate)
31.2 g/100 mL (0 °C)
36.4 g/100 mL (20 °C)
89.0 g/100 mL (100 °C)
Solubilityslightly soluble in alcohol, dilute mineral acids
Acidity (pKa)3.3-3.6
-93.0·10−6 cm3/mol
monoclinic (hydrate)
-3440 kJ/mol
Safety data sheetSee: data page
NFPA 704
US health exposure limits (NIOSH):
PEL (Permissible)
REL (Recommended)
2 mg/m3[2]
IDLH (Immediate danger)
Related compounds
Other cations
Gallium sulfate
Magnesium sulfate
Related compounds
See Alum
Supplementary data page
Refractive index (n),
Dielectric constantr), etc.
Phase behaviour
Except where otherwise noted, data are given for materials in their standard state (at 25 °C [77 °F], 100 kPa).
N verify (what is YesYN ?)
Infobox references

Aluminium sulfate is a chemical compound with the formula Al2(SO4)3. 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[3][4] and waste water treatment plants, and also in paper manufacturing.

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)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.

Aluminium sulfate is sometimes called alum or papermaker's alum in certain industries. However, the name "alum" is more commonly and properly used for any double sulfate salt with the generic formula XAl(SO
, where X is a monovalent cation such as potassium or ammonium.[5]


In the laboratory

Aluminium sulphate may be made by adding aluminium hydroxide, Al(OH)3, to sulfuric acid, H2SO4:

2 Al(OH)3 + 3 H2SO4 → Al2(SO4)3+6H2O

or by heating aluminum metal in a sulfuric acid solution:

2 Al(s) + 3 H2SO4 → Al2(SO4)3 + 3 H2 (g)

From alum schists

The alum schists employed in the manufacture of aluminium sulfate are mixtures of iron pyrite, aluminium silicate and various bituminous substances, and are found in upper Bavaria, Bohemia, Belgium, and Scotland. These are either roasted or exposed to the weathering action of the air. In the roasting process, sulfuric acid is formed and acts on the clay to form aluminium sulfate, a similar condition of affairs being produced during weathering. The mass is now systematically extracted with water, and a solution of aluminium sulfate of specific gravity 1.16 is prepared. This solution is allowed to stand for some time (in order that any calcium sulfate and basic ferric sulfate may separate), and is then evaporated until ferrous sulfate crystallizes on cooling; it is then drawn off and evaporated until it attains a specific gravity of 1.40. It is now allowed to stand for some time, and decanted from any sediment.[6]

From clays or bauxite

In the preparation of aluminum sulfate from clays or from bauxite, the material is gently calcined, then mixed with sulfuric acid and heated gradually to boiling; it is allowed to stand for some time, the clear solution drawn off.

From cryolite

When cryolite is used as the ore, it is mixed with calcium carbonate and heated. By this means, sodium aluminate is formed; it is then extracted with water and precipitated either by sodium bicarbonate or by passing a current of carbon dioxide through the solution. The precipitate is then dissolved in sulfuric acid.[6]

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