Difference between revisions of "Aluminium sulfate"

Revision as of 13:15, 21 October 2016

Aluminium sulfate

Aluminium sulfate flocculant tablet on a watchglass.
Names
IUPAC name Aluminium sulfate
Systematic IUPAC name Aluminium sulfate
Other names Aluminium sulphate Alunogenite Cake alum Dialuminum sulfate Filter alum Papermaker's alum Vitriol of argile Vitriol of clay
Identifiers
Jmol-3D images	Image
SMILES [Al+3].[Al+3].[O-]S(=O)(=O)[O-].[O-]S([O-])(=O)=O.[O-]S([O-])(=O)=O
Properties
Chemical formula	Al2(SO4)3
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
Solubility in water	31.2 g/100 mL (0 °C) 36.4 g/100 mL (20 °C) 89.0 g/100 mL (100 °C)
Acidity (pKa)	3.3-3.6
Thermochemistry
Std enthalpy of formation (ΔfHo298)	-3440 kJ/mol
Hazards
Safety data sheet	ScienceLab
Flash point	Non-flammable
Related compounds
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.

Properties

Chemical

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.

Physical

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 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.

Availability

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 aluminium sulfate can be purchased from various chemical suppliers.

Preparation

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

Projects

• Make alum
• Make aluminium nitrate
• Make aluminium hydroxide

Handling

Safety

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 hydrolizes 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.

Storage

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

Disposal

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.

References

Relevant Sciencemadness threads

• Aluminium Sulphate woes
• aluminum sulfate - can I use it to make anything useful
• aluminium sulphate to h2so4