Aluminium sulfate

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Aluminium sulfate
Aluminium sulfate crystals on watch glass.jpg
Lab-grade crystals of aluminium sulfate
IUPAC name
Aluminium sulfate
Systematic IUPAC name
Aluminium sulfate
Other names
Aluminium sulphate
Cake alum
Dialuminum sulfate
Filter alum
Papermaker's alum
Vitriol of argile
Vitriol of clay
Jmol-3D images Image
Molar mass 342.15 g/mol (anhydrous)
666.42 g/mol (octadecahydrate)
Appearance White hygroscopic solid
Odor Odorless
Density 2.672 g/cm3 (anhydrous)
1.62 g/cm3 (octadecahydrate)
Melting point 770 °C (1,420 °F; 1,040 K) (anhydrous); decomposes
86.5°C (octadecahydrate)
Boiling point Decomposes
31.2 g/100 mL (0 °C)
36.4 g/100 mL (20 °C)
89.0 g/100 mL (100 °C)
Solubility Insoluble in organic solvents
Acidity (pKa) 3.3-3.6
-3440 kJ/mol
Safety data sheet ScienceLab
Flash point Non-flammable
Related compounds
Related compounds
Aluminium sulfide
Except where otherwise noted, data are given for materials in their standard state (at 25 °C [77 °F], 100 kPa).
Infobox references

Aluminium sulfate is a chemical compound with the formula Al2(SO4)3, used mostly as a flocculating agent in the purification of drinking water or to reduce the pH of garden soil.



When aluminium sulfate reacts with an alkaline hydroxide, such as sodium hydroxide, aluminium hydroxide precipitates:

Al2(SO4)3 + 3 NaOH → 2 Al(OH)3 + 3/2 Na2SO4

Alumnium sulfate decomposes when heated to 770 °C, to release sulfur trioxide vapors and leave behind alumina:

Al2(SO4)3 → Al2O3 + SO3

Aluminium sulfate slowly hydrolyzes in water to release diluted sulfuric acid.


Aluminium sulfate is an odorless, white hygroscopic crystalline compound, moderately soluble in water and insoluble in organic solvents. It has an acidic taste.

Aluminium sulfate is rarely encountered in the anhydrous form. Aluminium sulfate forms a number of different hydrates, the most common ones being the hexadecahydrate Al2(SO4)3·16H2O and octadecahydrate Al2(SO4)3·18H2O. A rarer form, the heptadecahydrate, written as [Al(H2O)6]2(SO4)3·5H2O, occurs naturally as the mineral alunogen.


Aluminium sulfate can be bought from various swimming pool and various home-improvement and gardening retailer stores, as flocculant tablets. The tablet always consists of the hydrated form.

Anhydrous or hydrated aluminium sulfate can be purchased from various chemical suppliers.


Aluminium sulfate can be made by reacting sulfuric acid with aluminium oxide, hydroxide, halide or with hot aluminium metal.

2 Al(OH)3 + 3 H2SO4 → Al2(SO4)3 + 6 H2O
2 Al + 3 H2SO4 → Al2(SO4)3 + 3 H2




Aluminium sulfate has low toxicity compared to other compounds, but it may cause aluminium poisoning if ingested in large quantities. The salt is considered to be toxic to the reproductive system. The anhydrous form is irritant on skin and eye contact, and may irritate the lungs if inhaled. If released in the environment in large quantities, it will raise the level of aluminium in the soil as well as the soil acidity.

Aluminium sulfate slowly hydrolyzes in water to yield sulfuric acid which is highly corrosive. In one notorious case, a large amount of aluminium sulfate (20 tonnes) was accidentally dissolved in the Camelford water supply and the resulting sulfuric acid dissolved lead and copper from the city plumbing, causing short-term mass poisoning.


Anhydrous aluminium sulfate must be stored in airtight containers, while the hydrated forms can be stored in any container, just keep it away from moisture.


Aluminium sulfate can be safely poured down the drain, though it's best to heavily dilute it first, to prevent a build-up of sulfuric acid.



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