- For the colloquial term alum meaning "graduate," see Alumnus
An alumnus according to the American Heritage Dictionary is "a graduate or former student of a school, college, or university." In addition, an alumna An alumnus (pl. alumni) according to the American Heritage Dictionary is "a graduate (JC) or former student of a school, college, or university."...
.
Alum is both a specific
chemical compoundA chemical compound is a pure chemical substance consisting of two or more different chemical elements that can be separated into simpler substances by chemical reactions. Chemical compounds have a unique and defined chemical structure; they consist of a fixed ratio of atoms that are held together...
and a class of chemical compounds. The specific compound is the hydrated aluminium potassium sulfate with the
formulaA chemical formula or molecular formula is a way of expressing information about the atoms that constitute a particular chemical compound....
KAl(SO
4)
2.12H
2O. The wider class of compounds known as alums have the related
stoichiometryStoichiometry is the calculation of quantitative relationships of the reactants and products in a balanced chemical reaction...
,
AB(SO
4)
2.12H
2O.
Crystal chemistry of the alums
Double
sulfateIn inorganic chemistry, a sulfate is a salt of sulfuric acid.-Chemical properties:...
s with the general formula
A2SO
4·
B2(SO
4)
3·24H
2O, are known where
A is a monovalent cation such as
sodiumSodium is a metallic element with a symbol Na and atomic number 11. It is a soft, silvery-white, highly reactive metal and is a member of the alkali metals within "group 1"...
,
potassiumPotassium is the chemical element with the symbol K , atomic number 19, and atomic mass 39.0983. Potassium was first isolated from potash...
,
rubidiumRubidium is a chemical element with the symbol Rb and atomic number 37. Rb is a soft, silvery-white metallic element of the alkali metal group....
,
caesiumCaesium or cesium is the chemical element with the symbol Cs and atomic number 55. It is a soft, silvery-gold alkali metal with a melting point of , which makes it one of only five metals that are liquid at or near room temperature...
, or
thallium(I)Thallium is a chemical element with the symbol Tl and atomic number 81. This soft gray malleable poor metal resembles tin but discolors when exposed to air. Approximately 60-70% of thallium production is used in the electronics industry, and the rest is used in the pharmaceutical industry and in...
, or a compound cation such as
ammoniumThe ammonium cation is a positively charged polyatomic cation of the chemical formula NH. It has a formula weight of 18.05 and is formed by the protonation of ammonia...
(NH
4+),
methylammoniumMethylamine is the organic compound with a formula of CH3NH2. This colourless gas is a derivative of ammonia, wherein one H atom is replaced by a methyl group. It is the simplest primary amine. It is sold as a solution in methanol, ethanol, THF, and water, or as the anhydrous...
(CH
3NH
3+),
hydroxylammoniumHydroxylamine is a reactive chemical with formula NH2OH. It can be considered a hybrid of ammonia and water due to parallels it shares with each...
(HONH
3+) or hydrazinium (N
2H
5+),
B is a trivalent metal ion, such as
aluminiumAluminium or aluminum is a silvery white and ductile member of the boron group of chemical elements. It has the symbol Al; its atomic number is 13. It is not soluble in water under normal circumstances....
,
chromiumChromium is a chemical element which has the symbol Cr and atomic number 24, first element in Group 6. It is a steely-gray, lustrous, hard metal that takes a high polish and has a high melting point. It is also odourless, tasteless, and malleable...
,
titaniumTitanium is a chemical element with the symbol Ti and atomic number 22. Sometimes called the “space age metal”, it has a low density and is a strong, lustrous, corrosion-resistant transition metal with a silver color.Titanium can be alloyed with iron, aluminium, vanadium, molybdenum, among other...
,
manganeseManganese is a chemical element, designated by the symbol Mn. It has the atomic number 25. It is found as a free element in nature , and in many minerals...
,
vanadiumVanadium is the chemical element with the symbol V and atomic number 23. It is a soft, silvery grey, ductile transition metal. The formation of an oxide layer stabilizes the metal against oxidation. Andrés Manuel del Río discovered vanadium in 1801 by analyzing the mineral vanadinite, and named it...
,
iron (III)Iron is a metallic chemical element with the symbol Fe and atomic number 26. Iron is a group 8 and period 4 element and is therefore classified as a transition metal. Iron and iron alloys are by far the most common metals and the most common ferromagnetic materials in everyday use...
,
cobalt(III)Cobalt is a hard, lustrous, gray metal, a chemical element with symbol Co and atomic number 27. Although cobalt-based colors and pigments have been used since ancient times for making jewelry and paints, and miners have long used the name kobold ore for some minerals, the free metallic cobalt was...
,
galliumGallium is a chemical element that has the symbol Ga and atomic number 31. Elemental gallium does not occur in nature, but as the Ga salt, in trace amounts in bauxite and zinc ores. A soft silvery metallic poor metal, elemental gallium is a brittle solid at low temperatures. As it liquefies...
,
molybdenumMolybdenum , is a Group 6 chemical element with the symbol Mo and atomic number 42. The free element, which is a silvery metal, has the sixth-highest melting point of any element. It readily forms hard, stable carbides, and for this reason it is often used in high-strength steel alloys...
,
indiumIndium is a chemical element with chemical symbol In and atomic number 49. This rare, soft, malleable and easily fusible post-transition metal is chemically similar to aluminium or gallium but more closely resembles zinc .Indium's current primary application is to form transparent electrodes from...
,
rutheniumRuthenium is a chemical element that has the symbol Ru and atomic number 44. A rare transition metal of the platinum group of the periodic table, ruthenium is found associated with platinum ores and used as a catalyst in some platinum alloys....
,
rhodiumRhodium is a chemical element that is a rare, silvery-white, hard transition metal and a member of the platinum group. Rhodium is found in platinum ores and is used in alloys with platinum and as a catalyst. It is abbreviated to Rh and has atomic number 45...
, or
iridiumIridium is the chemical element with atomic number 77, and is represented by the symbol Ir. A very hard, brittle, silvery-white transition metal of the platinum family, iridium is the second densest element and is the most corrosion-resistant metal, even at temperatures as high as 2000 °C...
. The specific combinations of univalent cation, trivalent cation, and anion depends on the sizes of the
ionAn ion is an atom or molecule where the total number of electrons is not equal to the total number of protons, giving it a net positive or negative electrical charge...
s. For example, unlike the other
alkali metalThe alkali metals are a series of chemical elements forming Group 1 of the periodic table: lithium , sodium , potassium , rubidium , caesium , and francium ....
s the smallest one,
lithiumLithium is a soft, silver-white metal that belongs to the alkali metal group of chemical elements. It is represented by the symbol Li, and it has the atomic number three. Under standard conditions it is the lightest metal and the least dense solid element. Like all alkali metals, lithium is highly...
, does not form alums, and there is only one known sodium alum. In some cases,
solid solutionA solid solution is a solid-state solution of one or more solutes in a solvent. Such a mixture is considered a solution rather than a compound when the crystal structure of the solvent remains unchanged by addition of the solutes, and when the mixture remains in a single homogeneous phase.This...
s of alums occur.
Alums crystallize in one of three different crystal structures. These classes are called α-, β- and γ-alums.
Applications
Alums are useful for a range of industrial processes. They are soluble in water; have an
astringentAn astringent substance is a chemical that tends to shrink or constrict body tissues, usually locally after topical medicinal application. The word "astringent" derives from Latin adstringere, meaning "to bind fast"...
, acid, and sweetish taste; react
acidAn acid is traditionally considered any chemical compound that, when dissolved in water, gives a solution with a hydrogen ion activity greater than in pure water, i.e. a pH less than 7.0...
to
litmusLitmus or litmus test may refer to:* Litmus test , a common pH test* Litmus , a test case management tool maintained by Mozilla* "Litmus" , an episode in the first season of the television series...
; and
crystalA crystal or crystalline solid is a solid material whose constituent atoms, molecules, or ions are arranged in an orderly repeating pattern extending in all three spatial dimensions. The scientific study of crystals and crystal formation is crystallography...
lize in regular
octahedraIn geometry, an octahedron is a polyhedron with eight faces. A regular octahedron is a Platonic solid composed of eight equilateral triangles, four of which meet at each vertex.It is a 3-dimensional cross polytope.-Dimensions:...
. When heated they liquefy; and if the heating is continued, the
water of crystallizationWater of crystallization is water that occurs in crystals but is not covalently bonded to a host molecule or ion. The term is archaic and predates modern structural inorganic chemistry, coming from an era when the relationships between stoichiometry and structure were poorly understood. ...
is driven off, the salt froths and swells, and at last an amorphous powder remains.
Potassium alumPotassium alum or potash alum is the potassium double sulfate of aluminium. Its chemical formula is KAl2 and it is commonly found in its dodecahydrate form as KAl2·12. It is commonly used in water purification, leather tanning, fireproof textiles, and baking powder...
is the common alum of commerce, although
soda alumSodium alum, Na2SO4·Al23·24H2O, is a form of alum, which mainly occurs in nature as the mineral mendozite. It is very soluble in water, and is extremely difficult to purify...
, ferric alum, and
ammonium alumAmmonium alum or ammonium aluminium sulfate dodecahydrate is a white crystalline double sulfate of aluminium, used in water purification, in vegetable glues, in porcelain cements, in natural deodorants and in tanning, dyeing and in fireproofing textiles.No human or ecological toxicity...
are manufactured.
Aluminium sulfateAluminium sulfate or Aluminium Sulphate, written as Al23 or Al2O12S3 Aluminium sulfate is an industrial chemical used as a flocculating agent in the purification of drinking water and waste water treatment plants, and also in paper...
is sometimes called alum in informal contexts, but this usage is not regarded as technically correct. Its properties are quite different from those of the set of alums formally described above.
Alum in antiquity - Pliny's writings
The word "alumen" occurs in
PlinyGaius Plinius Secundus , better known as Pliny the Elder, was an author, naturalist, and natural philosopher as well as naval and army commander of the early Roman Empire and personal friend of the emperor Vespasian...
's
Natural History. In the 52nd chapter of his 35th book, he gives a detailed description. By comparing this with the account of stupteria given by Dioscorides in the 123rd chapter of his 5th book, it is obvious that the two are identical. Pliny informs us that alumen was found naturally in the earth. He calls it
salsugoterrae. Different substances were distinguished by the name of "alumen"; but they were all characterized by a certain degree of astringency, and were all employed in dyeing and medicine, the light-colored alumen being useful in brilliant dyes, the dark-colored only in dyeing black or very dark colors. One species was a liquid, which was apt to be adulterated; but when pure it had the property of blackening when added to
pomegranateThe pomegranate is a fruit-bearing deciduous shrub or small tree growing to between five and eight meters tall. The pomegranate is native to Southwest Asia and has been cultivated in the Caucasus since ancient times...
juice. This property seems to characterize a solution of iron
sulfateIn inorganic chemistry, a sulfate is a salt of sulfuric acid.-Chemical properties:...
in water; a solution of ordinary (potassium) alum would possess no such property. Pliny says that there is another kind of alum that the
GreekAncient Greece is the civilisation belonging to the period of Greek history lasting from the Greek Dark Ages ca. 1100 BC and the Dorian invasion, to 146 BC and the Roman conquest of Greece after the Battle of Corinth. It is generally considered to be the seminal culture which provided the...
s call
schistos. It forms in white threads upon the surface of certain stones. From the name schistos, and the mode of formation, it appears that this species was the salt which forms spontaneously on certain salty minerals, as alum
slateSlate is a fine-grained, foliated, homogeneous metamorphic rock derived from an original shale-type sedimentary rock composed of clay or volcanic ash through low grade regional metamorphism. The result is a foliated rock in which the foliation may not correspond to the original sedimentary...
and bituminous
shaleShale is a fine-grained, clastic sedimentary rock composed of flakes of clay minerals and tiny fragments of other minerals, especially quartz and calcite. The ratio of clay to other minerals is variable...
, and which consists chiefly of sulfates of iron and aluminium. Possibly in certain places the iron sulfate may have been nearly wanting, and then the salt would be white, and would answer, as Pliny says it did, for dyeing bright colors. Several other species of alumen are described by Pliny, but we are unable to make out to what minerals he alludes.
The alumen of the ancients, then, was not always the same as the alum of the moderns. They certainly knew how to produce alum from
aluniteAlunite, or alumstone, is a sulfate mineral that was first observed in the 15th century in Monti della Tolfa, north of Rome, where it was mined for the manufacture of alum. First called aluminilite by J.C. Delametherie in 1707, this name was contracted by François Beudant in 1824 to...
, as this process is archaeologically attested on the island Lesbos. This site was abandoned in the 7th century but dates back at least to the 2nd century AD. Native alumen from Melos appears to have been a mixture mainly of alunogen (Al
2(SO
4)
3.17H
2O) with alum and other minor sulfates. The western desert of Egypt was a major source of alum substitutes in antiquity. These evaporites were mainly FeAl
2(SO
4)
4.22H
2O, MgAl
2(SO
4)
4.22H
2O, NaAl(SO
4)
2.6H
2O, MgSO
4.7H
2O and Al
2(SO
4)
3.17H
2O. Any contamination with iron sulfate was greatly disliked as this darkened and dulled dye colours. They were acquainted with a variety of substances of varying degrees of purity by the names of misy, sory, and chalcanthum. As alum and
green vitriolIron sulfate or ferrous sulfate is the chemical compound with the formula , known since ancient times as copperas...
were applied to a variety of substances in common, and as both are distinguished by a sweetish and astringent taste, writers, even after the discovery of alum, do not seem to have discriminated the two salts accurately from each other. In the writings of the
alchemistsAlchemy is both a philosophy and a practice with an aim of achieving ultimate wisdom as well as immortality, involving the improvement of the alchemist as well as the making of several substances described as possessing unusual properties...
we find the words misy, sory, chalcanthum applied to alum as well as to iron sulfate; and the name atramentum sutorium, which ought to belong, one would suppose, exclusively to green vitriol, applied indifferently to both. Various
mineralA mineral is a naturally occurring solid formed through geological processes that has a characteristic chemical composition, a highly ordered atomic structure, and specific physical properties. A rock, by comparison, is an aggregate of minerals and/or mineraloids, and need not have a specific...
s are employed in the manufacture of alum, the most important being alunite or alum-stone, alum
schistThe schists form a group of medium-grade metamorphic rocks, chiefly notable for the preponderance of lamellar minerals such as micas, chlorite, talc, hornblende, graphite, and others. Quartz often occurs in drawn-out grains to such an extent that a particular form called quartz schist is produced...
,
bauxiteBauxite is the most important aluminium ore. It consists largely of the minerals gibbsite Al
3, boehmite γ-AlO, and diaspore α-AlO, together with the iron oxides goethite and hematite, the clay mineral kaolinite and small amounts of anatase TiO
2...
and
cryoliteCryolite is an uncommon mineral identified with the once large deposit at Ivigtût on the west coast of Greenland, which ran out in 1987....
.
Alchemical and later discoveries and uses
The presence of
sulfuric acidSulfuric acid, , is a strong mineral acid. It is soluble in water at all concentrations. Sulfuric acid has many applications, and is one of the top products of the chemical industry. World production in 2001 was 165 million tonnes, with an approximate value of US$8 billion...
in potassium alum was known to the
alchemistsAlchemy is both a philosophy and a practice with an aim of achieving ultimate wisdom as well as immortality, involving the improvement of the alchemist as well as the making of several substances described as possessing unusual properties...
. In the 18th century, J. H. Pott and
Andreas Sigismund MarggrafAndreas Sigismund Marggraf was a German chemist and pioneer of analytical chemistry from Berlin, Brandenburg. He discovered the formic and phosphoric acids, and in 1746 he isolated zinc by heating calamine and carbon in 1746. In 1747 he announced his discovery of sugar in beets and devised a...
demonstrated that alumina was another constituent. Pott in his
Lithogeognosia showed that the
precipitatePrecipitation is the formation of a solid in a solution during a chemical reaction. When the reaction occurs, the solid formed is called the precipitate, and the liquid remaining above the solid is called the supernate...
obtained when an alkali is poured into a
solutionIn chemistry, a solution is a homogeneous mixture composed of two or more substances. In such a mixture, a solute is dissolved in another substance, known as a solvent. Gases may dissolve in liquids, for example, carbon dioxide or oxygen in water. Liquids may dissolve in other liquids. Gases can...
of alum is quite different from
limeLime is a general term for calcium-containing inorganic materials, in which carbonates, oxides and hydroxides predominate. Strictly speaking, lime is calcium oxide or calcium hydroxide. It is also the name for a single mineral of the CaO composition, occurring very rarely...
and
chalkChalk is a soft, white, porous sedimentary rock, a form of limestone composed of the mineral calcite. It forms under relatively deep marine conditions from the gradual accumulation of minute calcite plates shed from micro-organisms called coccolithophores. It is common to find flint and chert...
, with which it had been confounded by
G.E. StahlGeorg Ernst Stahl , was a German chemist and physician.He was born at Ansbach. Having graduated in medicine at the University of Jena in 1683, he became court physician to Duke Johann Ernst of Sachsen Weimar in 1687...
. Marggraf showed that alumina is one of the constituents of alum, but that this earth possesses peculiar properties, and is one of the ingredients in common
clayClay is a naturally occurring material composed primarily of fine-grained minerals, which show plasticity through a variable range of water content, and which can be hardened when dried and/or fired...
. He also showed that crystals of alum cannot be obtained by dissolving alumina in sulfuric acid and evaporating the solutions, but when a solution of
potashPotash is the common name given to potassium carbonate and various mined and manufactured salts that contain the element potassium in water-soluble form. In some rare cases, potash can be formed with traces of organic materials such as plant remains....
or
ammoniaAmmonia is a compound of nitrogen and hydrogen with the formula NH
3. It is normally encountered as a gas with a characteristic pungent odor. Ammonia contributes significantly to the nutritional needs of terrestrial organisms by serving as a precursor to foodstuffs and fertilizers...
is dropped into this liquid, it immediately deposits perfect crystals of alum.
Torbern BergmanTorbern Olof Bergman was a Swedish chemist and mineralogist noted for his 1775 Dissertation on Elective Attractions, containing the largest chemical affinity tables ever published...
also observed that the addition of potash or ammonia made the solution of alumina in sulfuric acid crystallize, but that the same effect was not produced by the addition of soda or of
limeLime is a general term for calcium-containing inorganic materials, in which carbonates, oxides and hydroxides predominate. Strictly speaking, lime is calcium oxide or calcium hydroxide. It is also the name for a single mineral of the CaO composition, occurring very rarely...
, and that potassium sulfate is frequently found in alum.
After
M.H. KlaprothMartin Heinrich Klaproth was a German chemist.Klaproth was born in Wernigerode. During a large portion of his life he followed the profession of an apothecary...
had discovered the presence of potassium in
leuciteLeucite is a rock-forming mineral composed of potassium and aluminium tectosilicate K[AlSi2O6]. Crystals have the form of cubic icositetrahedra but, as first observed by Sir David Brewster in 1821, they are not optically isotropic, and are therefore pseudo-cubic...
and
lepidoliteLepidolite Lepidolite Lepidolite (KLi2Al(Al,Si)3O10(F,OH)2 is a lilac-gray or rose-colored phyllosilicate mineral of the mica group that is a secondary source of lithium. It is associated with other lithium-bearing minerals like spodumene in pegmatite...
, it occurred to
L.N. VauquelinLouis Nicolas Vauquelin , was a French pharmacist and chemist.-Early life:Vauquelin was born at Saint-André-d'Hébertot in Normandy, France. His first acquaintance with chemistry was gained as laboratory assistant to an apothecary in Rouen , and after various vicissitudes he obtained an introduction...
that it was probably an ingredient likewise in many other
mineralA mineral is a naturally occurring solid formed through geological processes that has a characteristic chemical composition, a highly ordered atomic structure, and specific physical properties. A rock, by comparison, is an aggregate of minerals and/or mineraloids, and need not have a specific...
s. Knowing that alum cannot be obtained in crystals without the addition of
potashPotash is the common name given to potassium carbonate and various mined and manufactured salts that contain the element potassium in water-soluble form. In some rare cases, potash can be formed with traces of organic materials such as plant remains....
, he began to suspect that this alkali constituted an essential ingredient in the salt, and in 1797 he published a dissertation demonstrating that alum is a
double saltDouble salts are salts containing more than one cation or anion. They form when more than one salt is dissolved in a liquid and when together they crystallize in a regular pattern. A well-known double salt is alum containing two cations and a sulfate anion...
, composed of sulfuric acid, alumina, and potash. Soon after,
J.A. ChaptalJean-Antoine Claude, comte Chaptal de Chanteloup was a French chemist and statesman.-Early life:Born in Saint-Pierre-de-Nogaret, Lozère, as the son of an apothecary, he studied chemistry at the University of Montpellier, obtaining his doctorate in 1777, when he settled in Paris...
published the analysis of four different kinds of alum, namely, Roman alum, Levant alum, British alum and alum manufactured by himself. This analysis led to the same result as Vauquelin.
Early uses in industry
EgyptiansEgyptians is the name of the nationality and Mediterranean North African ethnic group native to Egypt....
reportedly used the coagulant alum as early as 1500 B.C. to reduce the visible cloudiness (turbidity) in the water. Alum was imported into England mainly from the Middle East, and, from the late 15th century onwards, the Papal States for hundreds of years. Its use there was as a
dyeA dye can generally be described as a colored substance that has an affinity to the substrate to which it is being applied. The dye is generally applied in an aqueous solution, and may require a mordant to improve the fastness of the dye on the fiber....
-fixer (
mordantA mordant is a substance used to set dyes on fabrics or tissue sections 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 stains in cell or tissue preparations. A mordant is always a polyvalent metal ion...
) for
woolWool is a fibrous protein derived from the specialized skin cells called follicles. The wool is taken from animals in the Caprinae family, principally sheep, but the hair of certain species of other mammals including: goats, llamas, and rabbits may also be called wool...
(which was one of England's primary industries, the value of which increased significantly if dyed). These sources were unreliable, however, and there was a push to develop a source in England especially as imports from the Papal States were ceased following the excommunication of King Henry VIII. With state financing, attempts were made throughout the 16th century, but without success until early on in the 17th century. An industry was founded in
YorkshireYorkshire is a historic county of northern England and the largest in the British Isles. Because of its great size, functions were increasingly undertaken over time by its subdivisions, which have been subject to periodic reform. Throughout these changes, Yorkshire has continued to be recognised as...
to process the shale which contained the key ingredient,
aluminium sulfateAluminium sulfate or Aluminium Sulphate, written as Al23 or Al2O12S3 Aluminium sulfate is an industrial chemical used as a flocculating agent in the purification of drinking water and waste water treatment plants, and also in paper...
, and made an important contribution to the
Industrial RevolutionThe Industrial Revolution was a period from the 18th to the 19th century where major changes in agriculture, manufacturing, mining, and transport had a profound effect on the socioeconomic and cultural conditions in the United Kingdom. The changes subsequently spread throughout Europe, North...
. One of the oldest historic sites for the production of alum from shale and human urine are the Peak alum works in
RavenscarRavenscar is a coastal village in the Scarborough district of North Yorkshire, England, approximately north of Scarborough. Raven Hall Hotel , was once owned by Dr. Francis Willis, physician of King George III, and it is rumoured that King George stayed there during his treatment...
, North Yorkshire.
Alum (known as
turti in local Indian languages) was also used for water treatment by Indians for hundreds of years.
Alum from alunite
In order to obtain alum from
aluniteAlunite, or alumstone, is a sulfate mineral that was first observed in the 15th century in Monti della Tolfa, north of Rome, where it was mined for the manufacture of alum. First called aluminilite by J.C. Delametherie in 1707, this name was contracted by François Beudant in 1824 to...
, it is calcined and then exposed to the action of air for a considerable time. During this exposure it is kept continually moistened with water, so that it ultimately falls to a very fine powder. This powder is then lixiviated with hot water, the
liquor decanted, and the alum allowed to crystallize. The alum schists employed in the manufacture of alum are mixtures of iron
pyriteThe mineral pyrite, or iron pyrite, is an iron sulfide with the formula FeS
2. This mineral's metallic luster and pale-to-normal, brass-yellow hue have earned it the nickname fool's gold due to its resemblance to gold...
, aluminium
silicateA silicate is a compound containing an ion in which one or more central silicon atoms are surrounded by electronegative ligands. This definition is broad enough to include species such as hexafluorosilicate , [SiF6]2−, but the silicate species that are encountered most often...
and various bituminous substances, and are found in upper
BavariaBavaria , with an area of and almost 12.5 million inhabitants, is located in the southeast of Germany and is the largest state of Germany by area, forming almost 20% of the total land area of Germany...
,
BohemiaBohemia is a historical region in central Europe, occupying the western two-thirds of the traditional Czech Lands, currently the Czech Republic...
,
BelgiumThe Kingdom of Belgium is a country in northwest Europe. It is a founding member of the European Union and hosts its headquarters, as well as those of other major international organizations, including NATO...
, and
ScotlandScotland is a country that is part of the United Kingdom. Occupying the northern third of the island of Great Britain, it shares a border with England to the south and is bounded by the North Sea to the east, the Atlantic Ocean to the north and west, and the North Channel and Irish Sea to the...
. These are either roasted or exposed to the
weatheringWeathering is the decomposition of Earth's rocks, soils and minerals through direct contact with the planet's atmosphere. Weathering occurs in situ, or "with no movement", and thus should not be confused with erosion, which involves the movement of rocks and minerals by agents such as water, ice,...
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, decanted from any sediment, and finally mixed with the calculated quantity of potassium sulfate (or if ammonium alum is required, with ammonium sulfate), well agitated, and the alum is thrown down as a finely-divided precipitate of alum meal. If much iron should be present in the shale then it is preferable to use potassium chloride in place of potassium sulfate.
Alum from clays or bauxite
In the preparation of alum from
clayClay is a naturally occurring material composed primarily of fine-grained minerals, which show plasticity through a variable range of water content, and which can be hardened when dried and/or fired...
s or from
bauxiteBauxite is the most important aluminium ore. It consists largely of the minerals gibbsite Al
3, boehmite γ-AlO, and diaspore α-AlO, together with the iron oxides goethite and hematite, the clay mineral kaolinite and small amounts of anatase TiO
2...
, 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 and mixed with acid potassium sulfate and allowed to crystallize. When
cryoliteCryolite is an uncommon mineral identified with the once large deposit at Ivigtût on the west coast of Greenland, which ran out in 1987....
is used for the preparation of alum, it is mixed with
calcium carbonateCalcium carbonate is a chemical compound with the chemical formula CaCO
3. It is a common substance found in rock in all parts of the world, and is the main component of shells of marine organisms, snails, pearls, and eggshells. Calcium carbonate is the active ingredient in agricultural...
and heated. By this means, sodium aluminate is formed; it is then extracted with water and
precipitatedPrecipitation is the formation of a solid in a solution during a chemical reaction. When the reaction occurs, the solid formed is called the precipitate, and the liquid remaining above the solid is called the supernate...
either by
sodium bicarbonateSodium bicarbonate or sodium hydrogen carbonate is the chemical compound with the formula NaHCO3. Sodium bicarbonate is a white solid that is crystalline but often appears as a fine powder. It has a slight alkaline taste resembling that of washing soda . It is a component of the...
or by passing a current of
carbon dioxideCarbon dioxide is a chemical compound composed of two oxygen atoms covalently bonded to a single carbon atom. It is a gas at standard temperature and pressure and exists in Earth's atmosphere in this state...
through the solution. The precipitate is then dissolved in sulfuric acid, the requisite amount of potassium sulfate added and the solution allowed to crystallize.
Soda alum
Sodium alum, Na
2SO
4·Al
2(SO
4)
3·24H
2O, mainly occurs in nature as the mineral
mendoziteMendozite is a sulfate mineral, one of the alum series with formula: NaAl2·11H2O.It was discovered in Mendoza Province, Argentina in 1868, it occurs in evaporites and is unlikely to be found anywhere except in extremely dry conditions.-References:* *...
. It is very soluble in water, and is extremely difficult to purify. In the preparation of this salt, it is preferable to mix the component solutions in the cold, and to evaporate them at a temperature not exceeding 60 °C. 100 parts of water dissolve 110 parts of sodium alum at 0 °C, and 51 parts at 16 °C. Soda alum is used in the
acidulentAcidulated describes something that has been rendered acid or sour.* Acidulated drops are an old-fashioned candy similar to modern lemon and lime drops....
of food as well as in the manufacture of baking powder.
Ammonium alum
Ammonia alum, NH
4Al(SO
4)
2·12H
2O, a white crystalline double sulfate of aluminium, is used in water purification, in vegetable glues, in porcelain cements, in natural deodorants (though potassium alum is more commonly used), in tanning, dyeing and in fireproofing textiles.
Chrome alum
Chrome alumChrome alum or Chromium potassium sulfate is the potassium double sulfate of chromium. Its chemical formula is KCr2 and it is commonly found in its dodecahydrate form as KCr2·12...
, K
2Cr(SO
4)
2·12H
2O, a dark violet crystalline double sulfate of chromium and potassium, was used in
tanningTanning is the process of making leather, which does not easily decompose, from the skins of animals, which do. Often this uses tannin, an acidic chemical compound. Coloring may occur during tanning....
.
Alum solubility
The solubility of the various alums in water varies greatly, sodium alum being readily soluble in water, while
caesiumCaesium or cesium is the chemical element with the symbol Cs and atomic number 55. It is a soft, silvery-gold alkali metal with a melting point of , which makes it one of only five metals that are liquid at or near room temperature...
and
rubidiumRubidium is a chemical element with the symbol Rb and atomic number 37. Rb is a soft, silvery-white metallic element of the alkali metal group....
alums are only sparingly soluble. The various solubilities are shown in the following table.
- At temperature T, 100 parts water dissolve:
T |
Ammonium Alum |
Potassium Alum |
Rubidium Alum |
Caesium Alum |
0 °C |
2.62 |
3.90 |
0.71 |
0.19 |
10 °C |
4.50 |
9.52 |
1.09 |
0.29 |
50 °C |
15.9 |
44.11 |
4.98 |
1.235 |
80 °C |
35.20 |
134.47 |
21.60 |
5.29 |
100 °C |
70.83 |
357.48 |
|
|
Selenate-containing alums
Alums are also known that contain selenium in place of sulfur. They are called selenium- or selenate-alums.
Cosmetic
- Alum in block form (usually potassium alum
Potassium alum or potash alum is the potassium double sulfate of aluminium. Its chemical formula is KAl2 and it is commonly found in its dodecahydrate form as KAl2·12. It is commonly used in water purification, leather tanning, fireproof textiles, and baking powder...
) is used as an aftershave, rubbed over the wet, freshly shaved face.
- Styptic pencils containing aluminium sulfate or potassium aluminium sulfate are used as astringent
An astringent substance is a chemical that tends to shrink or constrict body tissues, usually locally after topical medicinal application. The word "astringent" derives from Latin adstringere, meaning "to bind fast"...
s to prevent bleeding from small shaving cuts.
- Alum was used as a base in skin whiteners and treatments during the late 16th Century. A recipe for one such compound was given thus :
"For the Freckles which one getteth by the heat of the Sun: Take a little Allom beaten small, temper amonst it a well brayed white of an egg, put it on a milde fire, stirring it always about that it wax not hard, and when it casteth up the scum, then it is enough, wherewith anoint the Freckles the space of three dayes: if you will defend your self that you get no Freckles on the face, then anoint your face with the whites of eggs." —Christopher Wirzung, General Practise of Physicke, 1654.
- Alum may be used in depilatory waxes used for the removal of body hair, or applied to freshly waxed skin as a soothing agent.
- In the 1950s, men sporting crewcut or flattop hairstyles sometimes applied alum to their front short hairs as an alternative to pomade. When the hair dried, it would stay up all day.
- Alum's antibacterial properties contribute to its traditional use as an underarm deodorant
Deodorants are substances applied to the body mainly to reduce body odor which is caused by the bacterial breakdown of perspiration. A subgroup of deodorants are "antiperspirants", which prevent odor and reduce sweat produced by parts of the body...
. It has been used for this purpose in Europe; Mexico; Thailand, where it is called Sarn-Som; throughout Asia; and in the Philippines, where it is called Tawas. Today, potassium alum is sold commercially for this purpose as a "deodorant crystal," often in a protective plastic case.
Medicinal
- Alum is used in many subunit vaccines as an adjuvant
In immunology, an adjuvant is an agent that may stimulate the immune system and increase the response to a vaccine, without having any specific antigenic effect in itself. The word “adjuvant” comes from the Latin word adjuvare, meaning to help or aid...
to enhance the body's response to immunogens. Such vaccines include hepatitis A, hepatitis B and DTaP.
- Alum in powder or crystal form, or in styptic pencils, is sometimes applied to cuts to prevent or treat infection.
- Powdered alum is commonly cited as a home remedy for canker sores.
- Preparations containing alum are used by pet owners to stem bleeding associated with animal injuries caused by improper nail clipping.
- Alum is listed as an ingredient of some brands of toothpaste or toothpowder.
Culinary
- Alum powder, found in the spice section of many grocery stores, may be used in pickling recipes as a preservative to maintain fruit and vegetable crispness.
- Alum is used as the acidic component of some commercial baking powders.
- Alum was used by bakers in England during the 1800s to make bread whiter. White bread was demanded by the middle class. In 1875, the Sale of Food and Drugs Act prevented this and other adulterations.
As a Flame Retardant
- Solutions containing alum may be used to treat cloth, wood and paper materials to increase their resistance to fire.
- Alum is also a component of foamite, used in fire extinguishers to smother chemical and oil fires.
As a Chemical Flocculant
- Alum is used to clarify water by catching the very fine suspended particles in a gel-like precipitate of aluminium hydroxide. This sinks to the bottom of the containing vessel and can be removed in a variety of ways.
- Alum may be used to increase the viscosity
Viscosity is a measure of the resistance of a fluid which is being deformed by either shear stress or extensional stress. In everyday terms , viscosity is "thickness." Thus, water is "thin," having a lower viscosity, while honey is "thick," having a higher viscosity...
of a ceramic glazeGlaze is a layer or coating of a vitreous substance which has been fired to fuse to a ceramic object to color, decorate, strengthen or waterproof it.-Use:...
suspensionIn chemistry, a suspension is a heterogeneous fluid containing solid particles that are sufficiently large for sedimentation. Usually they must be larger than 1 micrometre. The internal phase is dispersed throughout the external phase through mechanical agitation, with the use of certain...
; this makes the glaze more readily adherent and slows its rate of sedimentationSedimentation is the tendency for particles in suspension or molecules in solution to settle out of the fluid in which they are entrained, and come to rest against a wall...
.
- Alum is an ingredient in some recipes for homemade modeling compounds intended for use by children. (These are often called "play clay" or "play dough" for their similarity to "Play-Doh
Play-Doh is a modeling compound used by children for art and craft projects at home and in school. Composed of flour, water, salt, and other ingredients, the product was first manufactured in Cincinnati as a wallpaper cleaner in the years following World War II...
", a trademarked product marketed by American toy manufacturer HasbroHasbro is a U.S.-based, multinational toy and boardgame company. It is one of the largest toy makers in the world. The corporate headquarters is located in Pawtucket, Rhode Island, United States. The majority of its products, however, are manufactured overseas.-History:In 1923, two brothers—Henry...
).
Alum in comedy
Much use was made of the supposed properties of alum as a comedy gag in films, primarily in the 1920s and 1930's. In a typical situation it would be introduced into foodstuffs (by accident or intent) with ingestion causing the victim's mouth to assume a tight pucker. Speech was usually difficult or impossible. In animation,
cartoon physicsCartoon physics is a joking reference to the fact that animation allows regular laws of physics to be ignored in humorous ways for dramatic effects...
could magnify the effect- the victim's head might shrink and/or the voice alter to a shrill squeak.
See The Three Stooges "No Census No Feeling" (1940).
Related compounds
In addition to the alums, which are dodecahydrates, double sulfates and selenates of univalent and trivalent cations occur with other degrees of hydration. These materials may also be referred to as alums, including the undecahydrates such as
mendoziteMendozite is a sulfate mineral, one of the alum series with formula: NaAl2·11H2O.It was discovered in Mendoza Province, Argentina in 1868, it occurs in evaporites and is unlikely to be found anywhere except in extremely dry conditions.-References:* *...
and
kaliniteKalinite is a mineral form of aluminium potassium sulfate, AlK2·11H2O.This compound is used as potassium alum in a wide variety of processes such as the manufacture of dyes, explosives, and porcelain cement, tanning, hardening gelatin and water purification....
, hexahydrates such as
guanidiniumGuanidine is a crystalline compound of strong alkalinity formed by the oxidation of guanine. It is used in the manufacture of plastics and explosives. It is found in urine as a normal product of protein metabolism. The molecule was first synthesized in 1861 by the oxidative degradation of an...
(CH
6N
3+) and
dimethylammoniumDimethylamine is an organic compound with the formula 2NH. This secondary amine is a colorless, flammable liquified gas with an ammonia- or fish-like odor. Dimethylamine is generally encountered as a solution in water at concentrations up to around 40%...
((CH
3)
2NH
2+) "alums", tetrahydrates such as goldichite, monohydrates such as thallium plutonium sulfate and anhydrous alums (yavapaiites). These classes include differing, but overlapping, combinations of ions.
A pseudo alum is a double sulfate of the typical formula ASO
4·B
2(SO
4)
3·22H
2O, where A is a divalent metal ion, such as
cobaltCobalt is a hard, lustrous, gray metal, a chemical element with symbol Co and atomic number 27. Although cobalt-based colors and pigments have been used since ancient times for making jewelry and paints, and miners have long used the name kobold ore for some minerals, the free metallic cobalt was...
(wupatkiite),
manganeseManganese is a chemical element, designated by the symbol Mn. It has the atomic number 25. It is found as a free element in nature , and in many minerals...
(apjohnite),
magnesiumMagnesium is a chemical element with the symbol Mg, atomic number 12 and common oxidation number +2. It is an alkaline earth metal and the eighth most abundant element in the earth's crust by mass, although ninth in the Universe as a whole...
(pickingerite) or
ironIron is a metallic chemical element with the symbol Fe and atomic number 26. Iron is a group 8 and period 4 element and is therefore classified as a transition metal. Iron and iron alloys are by far the most common metals and the most common ferromagnetic materials in everyday use...
(
halotrichiteHalotrichite, also known as feather alum, is a highly hydrated sulfate of aluminium and iron. It is formed by the weathering and decomposition of pyrite commonly near or in volcanic vents. Its chemical formula is FeAl24⋅22H2O. It forms fibrous monoclinic...
or feather alum), and B is a trivalent metal ion.
A
Tutton saltTutton's salts are a family of salts with the formula M2M'2.6 or M2M'2.6 . These materials are "double salts," which means that they contain two different cations, M+ and M'2+...
is a double sulfate of the typical formula A
2SO
4·BSO
4·6H
2O, where A is a univalent cation, and B a divalent metal ion.