Aluminium sulfate is a salt with the

formula In science, a formula is a concise way of expressing information symbolically, as in a mathematical formula or a ''chemical formula''. The informal use of the term ''formula'' in science refers to the general construct of a relationship betwe ...

. 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 and

wastewater treatment plant Wastewater treatment is a process which removes and eliminates contaminants from wastewater. It thus converts it into an effluent that can be returned to the water cycle. Once back in the water cycle, the effluent creates an acceptable impact on ...

s, and also in paper manufacturing. The anhydrous form occurs naturally as a rare

mineral In geology and mineralogy, a mineral or mineral species is, broadly speaking, a solid substance with a fairly well-defined chemical composition and a specific crystal structure that occurs naturally in pure form.John P. Rafferty, ed. (2011): Mi ...

millosevichite Millosevichite is a rare sulfate mineral with the chemical formula Al2(SO4)3. Aluminium is often substituted by iron. It forms finely crystalline and often porous masses. It was first described in 1913 for an occurrence in Grotta dell'Allume, Po ...

, found for example 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

hydrate In chemistry, a hydrate is a substance that contains water or its constituent elements. The chemical state of the water varies widely between different classes of hydrates, some of which were so labeled before their chemical structure was understo ...

s, of which the hexadecahydrate and octadecahydrate are the most common. The heptadecahydrate, whose formula can be written as , occurs naturally as the mineral

alunogen Alunogen (from French ''alun'', "alum"), also called feather alum and hair salt is a colourless to white (although often coloured by impurities, such as iron substituting for aluminium) fibrous to needle-like aluminium sulfate mineral. It has the ...

. Aluminium sulfate is sometimes called alum or papermaker's alum in certain industries. However, the name "

alum An alum () is a type of chemical compound, usually a hydrated double salt, double sulfate salt (chemistry), salt of aluminium with the general chemical formula, formula , such that is a valence (chemistry), monovalent cation such as potassium ...

" is more commonly and properly used for any double sulfate salt with the generic formula , where ''X'' is a monovalent

cation An ion () is an atom or molecule with a net electrical charge. The charge of an electron is considered to be negative by convention and this charge is equal and opposite to the charge of a proton, which is considered to be positive by convent ...

such as

potassium Potassium is a chemical element; it has Symbol (chemistry), symbol K (from Neo-Latin ) and atomic number19. It is a silvery white metal that is soft enough to easily cut with a knife. Potassium metal reacts rapidly with atmospheric oxygen to ...

ammonium Ammonium is a modified form of ammonia that has an extra hydrogen atom. It is a positively charged (cationic) polyatomic ion, molecular ion with the chemical formula or . It is formed by the protonation, addition of a proton (a hydrogen nucleu ...

Production

In the laboratory

Aluminium sulfate may be made by adding

aluminium hydroxide Aluminium hydroxide, , is found as the mineral gibbsite (also known as hydrargillite) and its three much rarer polymorphs: bayerite, doyleite, and nordstrandite. Aluminium hydroxide is amphoteric, i.e., it has both basic and acidic propert ...

, , to

sulfuric acid Sulfuric acid (American spelling and the preferred IUPAC name) or sulphuric acid (English in the Commonwealth of Nations, Commonwealth spelling), known in antiquity as oil of vitriol, is a mineral acid composed of the elements sulfur, oxygen, ...

, : : or by heating aluminium in a sulfuric acid solution: :

From alum schists

The alum

schist Schist ( ) is a medium-grained metamorphic rock generally derived from fine-grained sedimentary rock, like shale. It shows pronounced ''schistosity'' (named for the rock). This means that the rock is composed of mineral grains easily seen with a l ...

s employed in the manufacture of aluminium sulfate are mixtures of

iron pyrite The mineral pyrite ( ), or iron pyrite, also known as fool's gold, is an iron sulfide with the chemical formula Fe S2 (iron (II) disulfide). Pyrite is the most abundant sulfide mineral. Pyrite's metallic luster and pale brass-yellow hue ...

aluminium silicate Aluminum silicate (or aluminium silicate) is a name commonly applied to chemical compounds which are derived from aluminium oxide, Al2O3 and silicon dioxide, SiO2 which may be anhydrous or hydrated, naturally occurring as minerals or synthetic. ...

and various

bituminous Bitumen ( , ) is an immensely viscous constituent of petroleum. Depending on its exact composition, it can be a sticky, black liquid or an apparently solid mass that behaves as a liquid over very large time scales. In American English, the m ...

substances, and are found in upper

Bavaria Bavaria, officially the Free State of Bavaria, is a States of Germany, state in the southeast of Germany. With an area of , it is the list of German states by area, largest German state by land area, comprising approximately 1/5 of the total l ...

Bohemia Bohemia ( ; ; ) is the westernmost and largest historical region of the Czech Republic. In a narrow, geographic sense, it roughly encompasses the territories of present-day Czechia that fall within the Elbe River's drainage basin, but historic ...

Belgium Belgium, officially the Kingdom of Belgium, is a country in Northwestern Europe. Situated in a coastal lowland region known as the Low Countries, it is bordered by the Netherlands to the north, Germany to the east, Luxembourg to the southeas ...

, and

Scotland Scotland is a Countries of the United Kingdom, country that is part of the United Kingdom. It contains nearly one-third of the United Kingdom's land area, consisting of the northern part of the island of Great Britain and more than 790 adjac ...

. These are either roasted or exposed to the

weathering Weathering is the deterioration of rocks, soils and minerals (as well as wood and artificial materials) through contact with water, atmospheric gases, sunlight, and biological organisms. It occurs '' in situ'' (on-site, with little or no move ...

action of the air. In the roasting process,

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 Relative density, also called specific gravity, is a dimensionless quantity defined as the ratio of the density (mass of a unit volume) of a substance to the density of a given reference material. Specific gravity for solids and liquids is nea ...

1.16 is prepared. This solution is allowed to stand for some time (in order that any

calcium sulfate Calcium sulfate (or calcium sulphate) is an inorganic salt with the chemical formula . It occurs in several hydrated forms; the anhydrous state (known as anhydrite) is a white crystalline solid often found in evaporite deposits. Its dihydrate ...

and basic

iron(III) sulfate Iron(III) sulfate or ferric sulfate (British English: sulphate instead of sulfate) is a family of inorganic compounds with the formula Fe2(SO4)3(H2O)n. A variety of hydrates are known, including the most commonly encountered form of "ferric sulf ...

may separate), and is then evaporated until

iron(II) sulfate Iron(II) sulfate or ferrous sulfate (British English: sulphate instead of sulfate) denotes a range of salts with the formula Fe SO4·''x''H2O. These compounds exist most commonly as the heptahydrate (''x'' = 7), but several values for ...

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.

From clays or bauxite

In the preparation of aluminium sulfate from

clay Clay is a type of fine-grained natural soil material containing clay minerals (hydrous aluminium phyllosilicates, e.g. kaolinite, ). Most pure clay minerals are white or light-coloured, but natural clays show a variety of colours from impuriti ...

s or from

bauxite Bauxite () is a sedimentary rock with a relatively high aluminium content. It is the world's main source of aluminium and gallium. Bauxite consists mostly of the aluminium minerals gibbsite (), boehmite (γ-AlO(OH)), and diaspore (α-AlO(OH) ...

, the material is gently

calcined Calcination is thermal treatment of a solid chemical compound (e.g. mixed carbonate ores) whereby the compound is raised to high temperature without melting under restricted supply of ambient oxygen (i.e. gaseous O2 fraction of air), generally fo ...

, then mixed with sulfuric acid and water and heated gradually to boiling; if concentrated acid is used no external heat is generally required as the formation of aluminium sulfate is

exothermic In thermodynamics, an exothermic process () is a thermodynamic process or reaction that releases energy from the system to its surroundings, usually in the form of heat, but also in a form of light (e.g. a spark, flame, or flash), electricity (e ...

. It is allowed to stand for some time, and the clear solution is drawn off.

From cryolite

When

cryolite Cryolite ( Na3 Al F6, sodium hexafluoroaluminate) is a rare mineral identified with the once-large deposit at Ivittuut on the west coast of Greenland, mined commercially until 1987. It is used in the reduction ("smelting") of aluminium, in pest ...

is used as the ore, it is mixed with

calcium carbonate Calcium carbonate is a chemical compound with the chemical formula . It is a common substance found in Rock (geology), rocks as the minerals calcite and aragonite, most notably in chalk and limestone, eggshells, gastropod shells, shellfish skel ...

and heated. By this means,

sodium aluminate Sodium aluminate is an inorganic chemical that is used as an effective source of aluminium hydroxide for many industrial and technical applications. Pure sodium aluminate (anhydrous) is a white crystalline solid having a formula variously given as ...

is formed; it is then extracted with water and

precipitated In an aqueous solution, precipitation is the "sedimentation of a solid material (a precipitate) from a liquid solution". The solid formed is called the precipitate. In case of an inorganic chemical reaction leading to precipitation, the chemic ...

either by

sodium bicarbonate Sodium bicarbonate ( IUPAC name: sodium hydrogencarbonate), commonly known as baking soda or bicarbonate of soda (or simply “bicarb” especially in the UK) is a chemical compound with the formula NaHCO3. It is a salt composed of a sodium cat ...

or by passing a current of

carbon dioxide Carbon dioxide is a chemical compound with the chemical formula . It is made up of molecules that each have one carbon atom covalent bond, covalently double bonded to two oxygen atoms. It is found in a gas state at room temperature and at norma ...

through the solution. The precipitate is then dissolved in sulfuric acid.

Uses

Aluminium sulfate is sometimes used in the human food industry as a firming agent, where it takes on

E number E numbers, short for Europe numbers, are codes for substances used as food additives, including those found naturally in many foods, such as vitamin C, for use within the European Union (EU) and European Free Trade Association (EFTA). Commonly ...

E520, and in animal feed as a

bactericide A bactericide or bacteriocide, sometimes abbreviated Bcidal, is a substance which kills bacteria. Bactericides are disinfectants, antiseptics, or antibiotics. However, material surfaces can also have bactericidal properties based solely on their p ...

. In the United States, the

FDA The United States Food and Drug Administration (FDA or US FDA) is a federal agency of the Department of Health and Human Services. The FDA is responsible for protecting and promoting public health through the control and supervision of food ...

lists it as "

generally recognized as safe Generally recognized as safe (GRAS) is a United States Food and Drug Administration (FDA) designation that a chemical or substance added to food is considered safe by experts under the conditions of its intended use. An ingredient with a GRAS d ...

" with no limit on concentration. Aluminium sulfate may be used as a

deodorant A deodorant is a substance applied to the body to prevent or mask body odor caused by bacterial breakdown of perspiration, for example in the armpits, groin, or feet. A subclass of deodorants, called antiperspirants, prevents sweating itself, t ...

, an

astringent An astringent (sometimes called adstringent) is a chemical that shrinks or constricts body tissues. The word derives from the Latin '' adstringere'', which means "to bind fast". Astringency, the dry, puckering or numbing mouthfeel caused by t ...

, or as a

styptic An antihemorrhagic () agent is a substance that promotes hemostasis (a process which stops bleeding). It may also be known as a hemostatic (also spelled haemostatic) agent. Antihemorrhagic agents used in medicine have various mechanisms of action: ...

for superficial shaving wounds. Aluminium sulfate is used as a

mordant A mordant or dye fixative is a substance used to set (i.e., bind) dyes on fabrics. It does this by forming a coordination complex with the dye, which then attaches to the fabric (or tissue). It may be used for dyeing fabrics or for intensifying ...

dyeing Dyeing is the application of dyes or pigments on textile materials such as fibers, yarns, and fabrics with the goal of achieving color with desired color fastness. Dyeing is normally done in a special solution containing dyes and particular ...

and printing textiles. It is a common

vaccine A vaccine is a biological Dosage form, preparation that provides active acquired immunity to a particular infectious disease, infectious or cancer, malignant disease. The safety and effectiveness of vaccines has been widely studied and verifi ...

adjuvant In pharmacology, an adjuvant is a drug or other substance, or a combination of substances, that is used to increase the efficacy or potency of certain drugs. Specifically, the term can refer to: * Adjuvant therapy in cancer management * Anal ...

and works "by facilitating the slow release of

antigen In immunology, an antigen (Ag) is a molecule, moiety, foreign particulate matter, or an allergen, such as pollen, that can bind to a specific antibody or T-cell receptor. The presence of antigens in the body may trigger an immune response. ...

from the vaccine depot formed at the site of

inoculation Inoculation is the act of implanting a pathogen or other microbe or virus into a person or other organism. It is a method of artificially inducing immunity against various infectious diseases. The term "inoculation" is also used more generally ...

." Aluminium sulfate is used in

water purification Water purification is the process of removing undesirable chemicals, biological contaminants, suspended solids, and gases from water. The goal is to produce water that is fit for specific purposes. Most water is purified and disinfected for hu ...

and for

chemical phosphorus removal Chemical phosphorus removal is a wastewater treatment method, where phosphorus is removed using salts of aluminum (e.g. alum or polyaluminum chloride), iron (e.g. ferric chloride), or calcium (e.g. lime). Phosphate forms precipitates with the metal ...

from

wastewater Wastewater (or waste water) is water generated after the use of freshwater, raw water, drinking water or saline water in a variety of deliberate applications or processes. Another definition of wastewater is "Used water from any combination of do ...

. 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 Coagulation, also known as clotting, is the process by which blood changes from a liquid to a gel, forming a thrombus, blood clot. It results in hemostasis, the cessation of blood loss from a damaged vessel, followed by repair. The process of co ...

flocculation In colloidal chemistry, flocculation is a process by which colloidal particles come out of Suspension (chemistry), suspension to sediment in the form of floc or flake, either spontaneously or due to the addition of a clarifying agent. The actio ...

. Research suggests that in Australia, aluminium sulfate used in this way in drinking water treatment is the primary source of

hydrogen sulfide Hydrogen sulfide is a chemical compound with the formula . It is a colorless chalcogen-hydride gas, and is toxic, corrosive, and flammable. Trace amounts in ambient atmosphere have a characteristic foul odor of rotten eggs. Swedish chemist ...

gas in

sanitary sewer A sanitary sewer is an underground pipe or tunnel system for transporting sewage from houses and commercial buildings (but not stormwater) to a sewage treatment plant or disposal. Sanitary sewers are a type of gravity sewer and are part of ...

systems. An improper and excess application incident in 1988 polluted the water supply of

Camelford Camelford () is a town and civil parish in north Cornwall, England, United Kingdom, situated in the River Camel valley northwest of Bodmin Moor. The town is approximately north of Bodmin and is governed by Camelford Town Council. The ward pop ...

Cornwall Cornwall (; or ) is a Ceremonial counties of England, ceremonial county in South West England. It is also one of the Celtic nations and the homeland of the Cornish people. The county is bordered by the Atlantic Ocean to the north and west, ...

. Aluminium sulfate has been used as a method of

eutrophication Eutrophication is a general term describing a process in which nutrients accumulate in a body of water, resulting in an increased growth of organisms that may deplete the oxygen in the water; ie. the process of too many plants growing on the s ...

remediation for shallow lakes. It works by reducing the

phosphorus Phosphorus is a chemical element; it has Chemical symbol, symbol P and atomic number 15. All elemental forms of phosphorus are highly Reactivity (chemistry), reactive and are therefore never found in nature. They can nevertheless be prepared ar ...

load in the lakes. When dissolved in a large amount of neutral or slightly alkaline water, aluminium sulfate produces a gelatinous precipitate of

, 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 Hydrolysis (; ) is any chemical reaction in which a molecule of water breaks one or more chemical bonds. The term is used broadly for substitution reaction, substitution, elimination reaction, elimination, and solvation reactions in which water ...

to form the aluminium hydroxide precipitate and a dilute

solution. An example of what changing the pH level of soil can do to plants is visible when looking at ''

Hydrangea macrophylla ''Hydrangea macrophylla'' is a species of flowering plant in the family (biology), family Hydrangeaceae, native plant, native and endemism, endemic to Japan. It is a deciduous shrub growing to tall by or more broad with large heads of pink or ...

''. 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. In the construction industry, it is used as waterproofing agent and accelerator in

concrete Concrete is a composite material composed of aggregate bound together with a fluid cement that cures to a solid over time. It is the second-most-used substance (after water), the most–widely used building material, and the most-manufactur ...

. Another use is a foaming agent in

fire fighting foam Firefighting foam is a foam used for fire suppression. Its role is to cool the fire and to coat the fuel, preventing its contact with oxygen, thus achieving suppression of the combustion. Firefighting foam was invented by the Moldovan engineer a ...

. It can also be very effective as a

molluscicide Molluscicides () are chemicals that kill molluscs. They are also known as snail baits, snail pellets, or slug pellets. These pesticides against molluscs are usually used in agriculture or gardening, in order to control Pest (organism)#Gastropods, ...

, killing

spanish slug The Spanish slug (''Arion vulgaris'', but formerly widely referred to as ''Arion lusitanicus'' owing to a misidentification) is an air-breathing land slug, a terrestrial molluscs, terrestrial pulmonate gastropod mollusc, mollusk in the family Ari ...

Mordant A mordant or dye fixative is a substance used to set (i.e., bind) dyes on fabrics. It does this by forming a coordination complex with the dye, which then attaches to the fabric (or tissue). It may be used for dyeing fabrics or for intensifying ...

aluminium triacetate Aluminium triacetate, formally named aluminium acetate, is a chemical compound with composition . Under standard conditions it appears as a white, water-soluble solid that decomposes on heating at around 200 °C. The triacetate hydrolyse ...

and

aluminium sulfacetate Aluminium sulfacetate is a mixture of aluminium salts dissolved in water with formula . Uses It is an evenly balanced mixture of aluminium sulfate and aluminium acetate. It can be used as a mordant, which is a substance used to set dyes on fabr ...

can be prepared from aluminium sulfate, the product formed being determined by the amount of

lead(II) acetate Lead(II) acetate is a white crystalline chemical compound with a slightly sweet taste. Its chemical formula is usually expressed as or , where Ac represents the acetyl group. Like many other lead compounds, it causes lead poisoning. Lead aceta ...

used: : :

Chemical reactions

The compound decomposes to γ-alumina and

sulfur trioxide Sulfur trioxide (alternative spelling sulphur trioxide) is the chemical compound with the formula SO3. It has been described as "unquestionably the most conomicallyimportant sulfur oxide". It is prepared on an industrial scale as a precursor to ...

when heated between 580 and 900 °C. It combines with water forming hydrated salts of various compositions. Aluminium sulfate reacts with

to which foam stabilizer has been added, producing

for fire-extinguishing foams: : The

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 Oxygen is a chemical element; it has chemical symbol, symbol O and atomic number 8. It is a member of the chalcogen group (periodic table), group in the periodic table, a highly reactivity (chemistry), reactive nonmetal (chemistry), non ...

, smothering the

fire Fire is the rapid oxidation of a fuel in the exothermic chemical process of combustion, releasing heat, light, and various reaction Product (chemistry), products. Flames, the most visible portion of the fire, are produced in the combustion re ...

. Chemical foam was unsuitable for use on

polar solvent A solvent (from the Latin language, Latin ''wikt:solvo#Latin, solvō'', "loosen, untie, solve") is a substance that dissolves a solute, resulting in a Solution (chemistry), solution. A solvent is usually a liquid but can also be a solid, a gas ...

s such as

alcohol Alcohol may refer to: Common uses * Alcohol (chemistry), a class of compounds * Ethanol, one of several alcohols, commonly known as alcohol in everyday life ** Alcohol (drug), intoxicant found in alcoholic beverages ** Alcoholic beverage, an alco ...

, 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 A fire extinguisher is a handheld active fire protection device usually filled with a dry or wet chemical used to extinguish or control small fires, often in emergencies. It is not intended for use on an out-of-control fire, such as one which ha ...

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.

References

Footnotes

Notations

* *

External links

International Chemical Safety Card 1191

Aluminum and health

Government of Canada Fact Sheets and Frequently Asked Questions: Aluminum Salts
{{Authority control Aluminium compounds Sulfates Molluscicides Water treatment Adjuvants E-number additives