Lab-grade crystals of aluminium sulfate
| IUPAC name
| Systematic IUPAC name
| Other names
Vitriol of argile
Vitriol of clay
|Molar mass|| 342.15 g/mol (anhydrous)|
666.42 g/mol (octadecahydrate)
|Appearance||White hygroscopic solid|
|Density|| 2.672 g/cm3 (anhydrous)|
1.62 g/cm3 (octadecahydrate)
|Melting point|| 770 °C (1,420 °F; 1,040 K) (anhydrous); 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|
Std enthalpy of
|Safety data sheet||ScienceLab|
Except where otherwise noted, data are given for materials in their standard state (at 25 °C [77 °F], 100 kPa).
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.
- Al2(SO4)3 + 3 NaOH → 2 Al(OH)3 + 3/2 Na2SO4
- 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.
- 2 Al(OH)3 + 3 H2SO4 → Al2(SO4)3 + 6 H2O
- 2 Al + 3 H2SO4 → Al2(SO4)3 + 3 H2
- Make alum
- Make aluminium nitrate
- Make aluminium hydroxide and oxide
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.