Aluminium sulfate
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Aluminium sulfate, Al2(SO4)3, is an industrial chemical used as a flocculating agent in the purification of drinking water[3][4] and waste water treatment plants, and also in paper manufacturing.
Aluminium sulfate is sometimes incorrectly referred to as alum but alums are closely related compounds typified by KAl(SO4)2.12H2O. 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 alunogenite.
Contents
Preparation
Aluminium sulfate may be made by aluminium hydroxide, Al(OH)3, in sulfuric acid, H2SO4:
- 2 Al(OH)3 + 3 H2SO4 → Al2(SO4)3·6H2O
Uses
Aluminium Sulfate is used in water purification and as a mordant in dyeing and printing textiles. In water purification, it causes impurities to coagulate which are removed as the particulate settles to the bottom of the container or more easily filtered. This process is called coagulation or flocculation.
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 the Hydrangea macrophylla. The gardener can add Aluminum sulfate to the soil to reduce the pH level which in turn will result in the flowers of the Hydrangea turning a different color.
Aluminium sulfate is the active ingredient of some antiperspirants; however, beginning in 2005 the US Food and Drug Administration no longer recognized it as a wetness reducer.
Aluminium sulfate is usually found in baking powder, where there is controversy over its use due to concern regarding the safety of adding aluminum to the diet.
In construction industry it is used as waterproofing agent and accelerator in concrete. Another use is a foaming agent in fire fighting foam.
It is also used in styptic pencils, and pain relief from stings and bites; it is the active ingredient in popular pain relief products such as Stingose.
It can also be very effective as a molluscicide, killing spanish slugs.
Chemical Reactions
The compound decomposes to γ−alumina and sulfite when heated between 580 and 900°C. It combines with water forming hydrated salts of various compositions.
Produces calcium aluminate with the evolution of sulfate when calcined with calcium sulfate:[ref. needed]
- Al2(SO4)3 + CaSO4 → Ca(AlO2)2 + 4 SO3[ref. needed]
Aluminium sulfate reacts with sodium bicarbonate to which foam stabilizer has been added, typically made from saponin, licorice root, or hydrolyzed animal protein in aqueous solution, forming fire-extinguishing foams, producing carbon dioxide
- Al2(SO4)3 + 6 NaHCO3 → 2 Al(OH)3 + 3 Na2SO4 + 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 counties such as Japan and India continue to use it.
References
Footnotes
- ↑ 1.0 1.1 1.2 CRC Handbook of Chemistry and Physics, 62nd ed.; Weast, Robert C., Ed.; CRC Press: Boca Raton, FL, 1981; p B-74. ISBN 0-8493-0462-8.
- ↑ HSNO Chemical Classification Information Database, <http://www.ermanz.govt.nz/Chemicals/ChemicalDisplay.aspx?SubstanceID=9476> (accessed 25 November 2009), New Zealand Environmental Risk Management Authority.
- ↑ Conventional Coagulation-Flocculation-Sedimentation, <http://drinking-water.org/html/en/Treatment/Coagulation-Flocculation-technologies.html> (accessed 1 December 2007), Safe Drinking Water is Essential; National Academy of Sciences, 2007.
- ↑ Kvech, S.; Edwards, M. Solubility controls on aluminum in drinking water at relatively low and high pH. Water Res. 2002, 36 (17), 4356–68. DOI: 10.1016/S0043-1354(02)00137-9.
Notations
- Pauling, Linus (1970). General Chemistry. W.H. Freeman: San Francisco. ISBN 0-486-65622-5.
External links
- International Chemical Safety Card 1191
- NLM Hazardous Substances Data Bank entry for Aluminum sulfate
- NIOSH Pocket Guide to Chemical Hazards 0024
- IPCS Environmental Health Criteria 194: Aluminium
- FAO/WHO Joint Expert Committee on Food Additives (JECFA) monograph: Aluminium (WHO Food Additives Series 24)
- Aluminum and health
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