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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.

Aluminium sulfate
Aluminium sulfate hexadecahydrate
IUPAC name
Aluminium sulfate
Other names
Aluminum sulfate
Aluminium sulphate
Cake alum
Filter alum
Papermaker's alum
aluminum salt (3:2)
3D model (JSmol)
ECHA InfoCard 100.030.110
EC Number 233-135-0
E number E520 (acidity regulators, ...)
RTECS number BD1700000
Molar mass 342.15 g/mol (anhydrous)
666.44 g/mol (octadecahydrate)
Appearance white crystalline solid
Density 2.672 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)
Solubility slightly 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 sheet See: data page
NFPA 704
Flammability code 0: Will not burn. E.g., waterHealth code 1: Exposure would cause irritation but only minor residual injury. E.g., turpentineReactivity code 0: Normally stable, even under fire exposure conditions, and is not reactive with water. E.g., liquid nitrogenSpecial hazards (white): no codeNFPA 704 four-colored diamond
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 ☑Y☒N ?)
Infobox references

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 laboratoryEdit

Aluminium sulfate 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 + 3 H2SO4 → Al2(SO4)3 + 3 H2

From alum schistsEdit

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 bauxiteEdit

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 cryoliteEdit

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]


It is sometimes used in the human food industry as a firming agent for food starch, where it takes on E number E520, and in animal feed as a bactericide. Aluminum sulfate may be used as a deodorant, an astringent, or as a stiptic for superficial shaving wounds.[7]

It improves vaccine immunogenicity as a vaccine adjuvant "by facilitating the slow release of antigen from the vaccine depot formed at the site of inoculation."[7]

Aluminium sulfate is used in water purification and as a mordant in dyeing and printing textiles. In water purification, it causes suspended impurities to coagulate into larger particles and then settle to the bottom of the container (or be filtered out) more easily. This process is called coagulation or flocculation. Research suggests that in Australia, aluminium sulfate used this way in drinking water treatment is the primary source of hydrogen sulfide gas in sanitary sewer systems.[8] An improper and excess application incident in 1988 polluted the water supply of Camelford in Cornwall.

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 insoluble.

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 Hydrangea macrophylla. The gardener can add aluminium sulfate to the soil to reduce the pH which in turn will result in the flowers of the Hydrangea turning a different color (blue). The aluminium is what makes the flowers blue; at a higher pH, the aluminium is not available to the plant.[9]

In the construction industry, it is used as waterproofing agent and accelerator in concrete. Another use is a foaming agent in fire fighting foam.

In limnology (lake & pond science), Alum is often effectively employed to control levels of soluble ortho-phosphate (PO4) and to agglomerate suspended dirt particles for improved clarification. The product can be applied as a granular product or as "Liquid Alum" which is water + dissolved ALum (48.6% Alum dry basis). pH must be monitored in lakes with valuable fish when considering an Alum treatment. Alum is quite acidic and a lake with low pH (below 7) or a lake lacking a natural carbonate buffering capacity may find that the pH falls below 6 and to level when fish as harmed or killed. In such cases, a buffering agent - such as lime or Soda Ash or Sodium Bicarbonate - may be applied simultaneously with the Alum to prevent a precipitous drop in pH.

It can also be very effective as a molluscicide,[10] killing spanish slugs.

Mordants aluminium triacetate and aluminium sulfacetate can be prepared from aluminium sulfate, the product formed being determined by the amount of lead(II) acetate used:[11]

  +   3 Pb(CH
  →   2 Al(CH
  +   3 PbSO
  +   2 Pb(CH
  →   Al
  +   2 PbSO

Chemical reactionsEdit

The compound decomposes to γ−alumina and sulfur trioxide when heated between 580 and 900 °C. It combines with water forming hydrated salts of various compositions.

Aluminium sulfate reacts with sodium bicarbonate to which foam stabilizer has been added, producing carbon dioxide for fire-extinguishing foams:

Al2(SO4)3 + 6 NaHCO3 → 3 Na2SO4 + 2 Al(OH)3 + 6 CO2

The carbon dioxide is trapped by the foam stabilizer and creates a thick foam which will float on top of hydrocarbon fuels and seal off access to atmospheric oxygen, smothering the fire. Chemical foam was unsuitable for use on polar solvents such as alcohol, as the fuel would mix with and break down the foam blanket. The carbon dioxide generated also served to propel the foam out of the container, be it a portable fire extinguisher or fixed installation using hoselines. Chemical foam is considered obsolete in the United States and has been replaced by synthetic mechanical foams, such as AFFF which have a longer shelf life, are more effective, and more versatile, although some countries such as Japan and India continue to use it.[citation needed]



  1. ^ Pradyot Patnaik. Handbook of Inorganic Chemicals. McGraw-Hill, 2002, ISBN 0-07-049439-8
  2. ^ a b c NIOSH Pocket Guide to Chemical Hazards. "#0024". National Institute for Occupational Safety and Health (NIOSH).
  3. ^ Global Health and Education Foundation (2007). "Conventional Coagulation-Flocculation-Sedimentation". Safe Drinking Water is Essential. National Academy of Sciences. Archived from the original on 2007-10-07. Retrieved 2007-12-01.
  4. ^ Kvech S, Edwards M (2002). "Solubility controls on aluminum in drinking water at relatively low and high pH". Water Research. 36 (17): 4356–4368. doi:10.1016/S0043-1354(02)00137-9. PMID 12420940.
  5. ^ Austin, George T. (1984). Shreve's Chemical process industries (5th ed.). New York: McGraw-Hill. p. 357. ISBN 9780070571471. Archived from the original on 2014-01-03.
  6. ^ a b Chisholm 1911, p. 767.
  7. ^ a b "Compound Summary for CID 24850 - Aluminum Sulfate Anhydrous". PubChem.
  8. ^ Ilje Pikaar, Keshab R. Sharma, Shihu Hu, Wolfgang Gernjak, Jürg Keller, Zhiguo Yuan (2014). "Reducing sewer corrosion through integrated urban water management". Science. 345 (6198): 812–814. Bibcode:2014Sci...345..812P. doi:10.1126/science.1251418. PMID 25124439. Archived from the original on 2014-08-18. Retrieved 2014-08-25.CS1 maint: Uses authors parameter (link)
  9. ^ Kari Houle (2013-06-18). "Blue or Pink - Which Color is Your Hydrangea". University of Illinois Extension. Retrieved 2018-09-03.CS1 maint: Uses authors parameter (link)
  10. ^ Council, British Crop Protection; Society, British Ecological; Biologists, Association of Applied (1994). Field margins: integrating agriculture and conservation : proceedings of a symposium organised by the British Crop Protection Council in association with the British Ecological Society and the Association of Applied Biologists and held at the University of Warwick, Coventry on 18-20 April 1994. British Crop Protection Council. ISBN 9780948404757.
  11. ^ Georgievics, Von (2013). The Chemical Technology of Textile Fibres – Their Origin, Structure, Preparation, Washing, Bleaching, Dyeing, Printing and Dressing. Read Books. ISBN 9781447486121. Archived from the original on 2017-12-05.


External linksEdit