[
]
Alum described in medieval texts
Alum and green vitriol (iron sulfate) both have sweetish and astringent taste, and they had overlapping uses. Therefore, through the Middle Ages, alchemists and other writers do not seem to have discriminated the two salts accurately from each other. In the writings of the alchemists we find the words ''misy'', ''sory'', and ''chalcanthum'' applied to either compound; and the name ''atramentum sutorium'', which one might expect to belong exclusively to green vitriol, applied indifferently to both.
Alum was the most common mordant (substance used to set dyes on fabrics) used in the dye industry in the Islamic middle ages. It was the main export of the Chad
Chad (; ar, تشاد , ; french: Tchad, ), officially the Republic of Chad, '; ) is a landlocked country at the crossroads of North and Central Africa. It is bordered by Libya to the north, Sudan to the east, the Central African Republic ...
region, from where it was transported to the markets of Egypt and Morocco, and then on to Europe. Other, less significant, sources were found in Egypt and Yemen.[
]
Modern understanding of the alums
In the early 1700s, G.E. Stahl claimed that reacting sulfuric acid with limestone produced a sort of alum.[ The error was soon corrected by J.H. Pott and A.S. Marggraf, who showed that the precipitate obtained when an alkali is poured into a solution of alum, namely alumina, is quite different from lime and chalk, and is one of the ingredients in common clay.][
Marggraf also showed that perfect crystals with properties of alum can be obtained by dissolving alumina in ]sulfuric acid
Sulfuric acid (American spelling and the preferred IUPAC name) or sulphuric acid ( Commonwealth spelling), known in antiquity as oil of vitriol, is a mineral acid composed of the elements sulfur, oxygen and hydrogen, with the molecular formu ...
and adding potash or ammonia to the concentrated solution.[ In 1767, Torbern Bergman observed the need for potassium or ammonium sulfates to convert ]aluminium sulfate
Aluminium sulfate is a salt with the chemical formula, formula aluminium, Al2sulfate, (SO4)3. It is soluble in water and is mainly used as a Coagulation (water treatment), coagulating agent (promoting particle collision by neutralizing charge) in ...
into alum, while sodium or calcium would not work.[
:
The composition of common alum was finally determined by Vauquelin in 1797. As soon as Klaproth discovered the presence of potassium in leucite and lepidolite,][
Vauquelin demonstrated that common alum is a ]double salt
A double salt is a salt that contains two or more different cations or anions. Examples of double salts include alums (with the general formula ) and Tutton's salts (with the general formula ). Other examples include potassium sodium tartrate, ammo ...
, composed of sulfuric acid, alumina, and potash.[ In the same journal volume, Chaptal published the analysis of four different kinds of alum, namely, Roman alum, Levant alum, British alum, and an alum manufactured by himself,][ confirming Vauquelin's result.][
]
Production
Some alums occur as minerals, the most important being alunite
Alunite is a hydroxylated aluminium potassium sulfate mineral, formula K Al3( S O4)2(O H)6. It was first observed in the 15th century at Tolfa, near Rome, where it was mined for the manufacture of alum. First called ''aluminilite'' by J.C. Del ...
.
The most important alums – potassium, sodium, and ammonium – are produced industrially. Typical recipes involve combining aluminium sulfate
Aluminium sulfate is a salt with the chemical formula, formula aluminium, Al2sulfate, (SO4)3. It is soluble in water and is mainly used as a Coagulation (water treatment), coagulating agent (promoting particle collision by neutralizing charge) in ...
and the sulfate monovalent cation.[ The aluminium sulfate is usually obtained by treating minerals like alum schist, bauxite and cryolite with sulfuric acid.][
]
Types
Aluminium-based alums are named by the monovalent cation. Unlike the other alkali metal
The alkali metals consist of the chemical elements lithium (Li), sodium (Na), potassium (K),The symbols Na and K for sodium and potassium are derived from their Latin names, ''natrium'' and ''kalium''; these are still the origins of the names ...
s, lithium does not form alums; a fact attributed to the small size of its ion.
The most important alums are
* Potassium alum, , also called "potash alum" or simply "alum"
* Sodium alum
Sodium aluminium sulfate is the inorganic compound with the chemical formula NaAl(SO4)2·12H2O (sometimes written Na2SO4·Al2(SO4)3·24H2O). Also known as soda alum, sodium alum, or SAS, this white solid is used in the manufacture of baking powd ...
, , also called "soda alum" or "SAS"
* Ammonium alum
Ammonium aluminium sulfate, also known as ammonium alum or just alum (though there are many different substances also called "alum"), is a white crystalline double sulfate usually encountered as the dodecahydrate, formula (NH4)Al(SO4)2·12H2O. It ...
,
Chemical properties
Aluminium-based alums have a number of common chemical properties. They are soluble in water, have a sweetish taste, react as acid
In computer science, ACID ( atomicity, consistency, isolation, durability) is a set of properties of database transactions intended to guarantee data validity despite errors, power failures, and other mishaps. In the context of databases, a sequ ...
by turning blue litmus to red, and crystallize in regular octahedra. In alums each metal ion is surrounded by six water molecules. When heated, they liquefy, and if the heating is continued, the water of crystallization is driven off, the salt froths and swells, and at last an amorphous powder remains.[ They are astringent and acidic.
]
Crystal structure
Alums crystallize in one of three different crystal structures. These classes are called α-, β- and γ-alums. The first X-ray crystal structures of alums were reported in 1927 by James M. Cork
James Murle Cork (July 9, 1894, Yale, Michigan – November 27, 1957, Ann Arbor, Michigan) was an American physicist, known for his research in nuclear physics and nuclear spectroscopy.
Biography
He graduated in 1911 from Yale High School in Yale, ...
and Lawrence Bragg, and were used to develop the phase retrieval
Phase retrieval is the process of algorithmically finding solutions to the phase problem. Given a complex signal F(k), of amplitude , F (k), , and phase \psi(k):
::F(k) = , F(k), e^ =\int_^ f(x)\ e^\,dx
where ''x'' is an ''M''-dimensional spatia ...
technique isomorphous replacement Multiple isomorphous replacement (MIR) is historically the most common approach to solving the phase problem in X-ray crystallography studies of proteins. For protein crystals this method is conducted by soaking the crystal of a sample to be analyze ...
.[
]
Solubility
The solubility of the various alums in water varies greatly, sodium alum being readily soluble in water, while caesium
Caesium (IUPAC spelling) (or cesium in American English) is a chemical element with the symbol Cs and atomic number 55. It is a soft, silvery-golden alkali metal with a melting point of , which makes it one of only five elemental metals that a ...
and rubidium
Rubidium is the chemical element with the symbol Rb and atomic number 37. It is a very soft, whitish-grey solid in the alkali metal group, similar to potassium and caesium. Rubidium is the first alkali metal in the group to have a density higher ...
alums are only sparingly soluble. The various solubilities are shown in the following table.[
At temperature , 100 parts water dissolve:
:
]
Uses
Aluminium-based alums have been used since antiquity, and are still important in many industrial processes. The most widely used alum is potassium alum. It has been used since antiquity as a flocculant to clarify turbid liquids, as a mordant in dyeing, and in tanning. It is still widely used in water treatment, in medicine, for cosmetics (in deodorant), in food preparation (in baking powder and pickling), and to fire-proof paper and cloth.
Alum is also used as a styptic
An antihemorrhagic (antihæmorrhagic) agent is a substance that promotes hemostasis (stops bleeding). It may also be known as a hemostatic (also spelled haemostatic) agent.
Antihemorrhagic agents used in medicine have various mechanisms of action: ...
, in styptic pencils available from pharmacists, or as an alum block, available from barber shops and gentlemen's outfitters, to stem bleeding from shaving nicks;[ and as an astringent. An alum block can be used directly as a perfume-free deodorant (antiperspirant), and unprocessed mineral alum is sold in Indian bazaars for just that purpose. Throughout ]Island Southeast Asia
Maritime Southeast Asia comprises the countries of Brunei, Indonesia, Malaysia, the Philippines, Singapore, and East Timor. Maritime Southeast Asia is sometimes also referred to as Island Southeast Asia, Insular Southeast Asia or Oceanic Sout ...
, potassium alum is most widely known as ''tawas'' and has numerous uses. It is used as a traditional antiperspirant and deodorant, and in traditional medicine for open wounds and sores. The crystals are usually ground into a fine powder before using.[
In the Victorian era, alum was used along with other substances like plaster of Paris to adulterate certain food products, particularly bread. It was used to make lower-grade flour appear whiter, allowing the producers to spend less on whiter flour. Because it retains water, it would make the bread heavier, meaning that merchants could charge more for it in their shops. The amount of alum present in each loaf of bread could reach levels that would be toxic to humans and cause chronic diarrhea, which could lead to death in young children.][Phillips, Suzanne, director]
''The Hidden Killers of the Victorian Home''
Sterling Documentaries, 27 Jan. 2018. Accessed 9 Oct. 2021.
Alum is used as a mordant in traditional textiles;[ and in Indonesia and the Philippines, solutions of ''tawas'', salt, borax, and organic pigments were used to change the color of gold ornaments.][ In the Philippines, alum crystals were also burned and allowed to drip into a basin of water by '' babaylan'' (shamans) for ]divination
Divination (from Latin ''divinare'', 'to foresee, to foretell, to predict, to prophesy') is the attempt to gain insight into a question or situation by way of an occultic, standardized process or ritual. Used in various forms throughout histor ...
. It is also used in other rituals in the animistic anito
''Anito'', also spelled ''anitu'', refers to ancestor spirits, nature spirits, and deities in the indigenous Philippine folk religions from the precolonial age to the present, although the term itself may have other meanings and associati ...
religions of the islands.[
In traditional ]Japanese art
Japanese art covers a wide range of art styles and media, including ancient pottery, sculpture, ink painting and calligraphy on silk and paper, ''ukiyo-e'' paintings and woodblock prints, ceramics, origami, and more recently manga and anime. It ...
, alum and animal glue were dissolved in water, forming a liquid known as ''dousa'' ( ja, 礬水), and used as an undercoat for paper sizing
Sizing or size is a substance that is applied to, or incorporated into, other materials—especially papers and textiles—to act as a protective filler or glaze. Sizing is used in papermaking and textile manufacturing to change the absorption ...
.
Alum in the form of potassium aluminium sulphate
Potassium alum, potash alum, or potassium aluminium sulfate is a chemical compound: the double sulfate of potassium and aluminium, with chemical formula KAl(SO4)2. It is commonly encountered as the dodecahydrate, KAl(SO4)2·12H2O. It crystall ...
or ammonium aluminium sulfate
Ammonium aluminium sulfate, also known as ammonium alum or just alum (though there are many different substances also called " alum"), is a white crystalline double sulfate usually encountered as the dodecahydrate, formula (NH4)Al(SO4)2·12H2O. I ...
in a concentrated bath of hot water is regularly used by jewelers and machinists to dissolve hardened steel drill bits that have broken off in items made of aluminum, copper, brass, gold (any karat), silver (both sterling and fine) and stainless steel. This is because alum does not react chemically to any significant degree with any of these metals, but will corrode carbon steel. When heat is applied to an alum mixture holding a piece of work that has a drill bit stuck in it, if the lost bit is small enough, it can sometimes be dissolved / removed within hours.[
]
Related compounds
Many trivalent metals are capable of forming alums. The general form of an alum is , where is an alkali metal
The alkali metals consist of the chemical elements lithium (Li), sodium (Na), potassium (K),The symbols Na and K for sodium and potassium are derived from their Latin names, ''natrium'' and ''kalium''; these are still the origins of the names ...
or ammonium
The ammonium cation is a positively-charged polyatomic ion with the chemical formula or . It is formed by the protonation of ammonia (). Ammonium is also a general name for positively charged or protonated substituted amines and quaternary a ...
, is a trivalent metal, and often is 12. The most important example is chrome alum
Chrome alum or Chromium(III) potassium sulfate is the potassium double sulfate of chromium. Its chemical formula is KCr(SO4)2 and it is commonly found in its dodecahydrate form as KCr(SO4)2·12(H2O). It is used in leather tanning.
Production a ...
, , a dark violet crystalline double sulfate of chromium and potassium, was used in tanning.
In general, alums are formed more easily when the alkali metal atom is larger. This rule was first stated by Locke in 1902,[ who found that if a trivalent metal does not form a caesium alum, it neither will form an alum with any other alkali metal or with ammonium.
]
Selenate-containing alums
''Selenium'' or ''selenate alums'' are also known that contain selenium in place of sulfur
Sulfur (or sulphur in British English) is a chemical element with the symbol S and atomic number 16. It is abundant, multivalent and nonmetallic. Under normal conditions, sulfur atoms form cyclic octatomic molecules with a chemical formula ...
in the sulfate anion, making selenate () instead.[ They are strong oxidizing agents.
]
Mixed alums
In some cases, solid solutions of alums with different monovalent and trivalent cations may occur.
Other hydrates
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 mendozite
Mendozite is a sulfate mineral, one of the alum series, with formula NaAl(SO4)2·11H2O. It is a hydrated form of sodium aluminium sulfate (soda alum).
It was discovered in western Argentina in 1868, probably near San Juan. The exact location has ...
and kalinite
Kalinite is a mineral composed of hydrated potassium aluminium sulfate (a type of alum). It is a fibrous monoclinic alum, distinct from isometric potassium alum,American Mineralogist (1923) 8:15 named in 1868. Its name comes from ''kalium'' (d ...
, hexahydrates such as guanidinium and dimethylammonium "alums", tetrahydrates such as goldichite, monohydrates such as thallium plutonium sulfate and anhydrous alums ( yavapaiites). These classes include differing, but overlapping, combinations of ions.
Other double sulfates
A pseudo alum is a double sulfate of the typical formula , where
: is a divalent metal ion, such as
:: cobalt ( wupatkiite), manganese (apjohnite
Apjohnite ( IMA symbol: Apj) is a manganese aluminium sulfate mineral with the chemical formula MnAl(SO)·22HO. It was named after Trinity College Dublin professor James Apjohn. Its type locality is Maputo Province, Mozambique
Mozambique ( ...
), magnesium ( pickingerite) or iron ( halotrichite or feather alum), and
: is a trivalent metal ion.[
Double sulfates with the general formula are also known, where
: 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
:: sodium, potassium, rubidium
Rubidium is the chemical element with the symbol Rb and atomic number 37. It is a very soft, whitish-grey solid in the alkali metal group, similar to potassium and caesium. Rubidium is the first alkali metal in the group to have a density higher ...
, caesium
Caesium (IUPAC spelling) (or cesium in American English) is a chemical element with the symbol Cs and atomic number 55. It is a soft, silvery-golden alkali metal with a melting point of , which makes it one of only five elemental metals that a ...
, or thallium, or a compound cation such as ammonium
The ammonium cation is a positively-charged polyatomic ion with the chemical formula or . It is formed by the protonation of ammonia (). Ammonium is also a general name for positively charged or protonated substituted amines and quaternary a ...
(), methylammonium (), hydroxylammonium () or hydrazinium ()
: is a trivalent metal ion, such as
:: aluminium, chromium
Chromium is a chemical element with the symbol Cr and atomic number 24. It is the first element in group 6. It is a steely-grey, lustrous, hard, and brittle transition metal.
Chromium metal is valued for its high corrosion resistance and hardne ...
, titanium, manganese, vanadium
Vanadium is a chemical element with the symbol V and atomic number 23. It is a hard, silvery-grey, malleable transition metal. The elemental metal is rarely found in nature, but once isolated artificially, the formation of an oxide layer ( pas ...
, iron, cobalt, gallium
Gallium is a chemical element with the symbol Ga and atomic number 31. Discovered by French chemist Paul-Émile Lecoq de Boisbaudran in 1875, Gallium is in group 13 of the periodic table and is similar to the other metals of the group (aluminiu ...
, molybdenum
Molybdenum is a chemical element with the symbol Mo and atomic number 42 which is located in period 5 and group 6. The name is from Neo-Latin ''molybdaenum'', which is based on Ancient Greek ', meaning lead, since its ores were confused with lea ...
, indium, ruthenium, rhodium, or iridium.[
Analogous selenates also occur. The possible combinations of univalent cation, trivalent cation, and anion depends on the sizes of the ions.
A ]Tutton salt
Tutton's salts are a family of salts with the formula M2M'(SO4)2(H2O)6 (sulfates) or M2M'(SeO4)2(H2O)6 (selenates). These materials are double salts, which means that they contain two different cations, M+ and M'2+ crystallized in the same regular ...
is a double sulfate of the typical formula , where is a univalent cation, and a divalent metal ion.
Double sulfates of the composition , where
: is a univalent cation and
: is a divalent metal ion are referred to as langbeinites, after the prototypical potassium magnesium sulfate.
See also
* Alunite
Alunite is a hydroxylated aluminium potassium sulfate mineral, formula K Al3( S O4)2(O H)6. It was first observed in the 15th century at Tolfa, near Rome, where it was mined for the manufacture of alum. First called ''aluminilite'' by J.C. Del ...
* List of minerals
* Gum bichromate – photo prints and other similar processes use alums, sometimes as colloid
A colloid is a mixture in which one substance consisting of microscopically dispersed insoluble particles is suspended throughout another substance. Some definitions specify that the particles must be dispersed in a liquid, while others extend ...
(gelatin, albumen) hardeners
Footnotes
References
External links
*
{{Authority control
*
Sulfates
Sulfate minerals
Traditional medicine
Astringent flavors