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{{Short description|Family of double sulfate salts of aluminium}}{{About|the class of double sulfates of aluminium|the specific representative compound|potassium alum|the chromium analogs|chrome alum|papermakers’ alum|aluminium sulfate|a graduate or former student of an educational institution|alumni|other uses}}(File:Kaaliumi maarjase monikristallid2.JPG|thumb|upright=1.5|Crystal of potassium alum, {{chem|KAl|(|SO|4|)|2|·12H|2|O}})An alum ({{IPAc-en|ˈ|æ|l|É™|m}}) is a type of chemical compound, usually a hydrated double sulfate salt of aluminium with the general formula {{chem|{{mvar|X}}Al|(|SO|4|)|2|·12 H|2|O}}, such that {{mvar|X}} is a monovalent cation such as potassium or ammonium. By itself, “alum” often refers to potassium alum, with the formula {{chem|KAl|(|SO|4|)|2|·12 H|2|O}}. Other alums are named after the monovalent ion, such as sodium alum and ammonium alum.The name “alum” is also used, more generally, for salts with the same formula and structure, except that aluminium is replaced by another trivalent metal ion like chromium{{sup|III}}, and/or sulfur is replaced by another chalcogen like selenium. The most common of these analogs is chrome alum {{chem|K|Cr|(|SO|4|)|2|·12 H|2|O}}.In most industries, the name “alum” (or “papermaker’s alum“) is used to refer to aluminium sulfate, {{chem|Al|2| (SO|4|)|3|·{{mvar|n}} {{chem|H|2|O}}}}, which is used for most industrial flocculation{{rp|page=766}} (the variable {{mvar|n}} is an integer whose size depends on the amount of water absorbed into the alum). For medicine, the word “alum” may also refer to aluminium hydroxide gel used as a vaccine adjuvant.

History

Alum found at archaeological sites

The western desert of Egypt was a major source of alum substitutes in antiquity. These evaporites were mainly {{chem|Fe|Al|2|(SO|4|)|4|·22 H|2|O}}, {{chem|Mg|Al|2|(SO|4|)|4|·22 H|2|O}}, {{chem|NaAl(SO|4|)|2|·6 H|2|O}}, {{chem|Mg|SO|4|·7H|2|O}} and {{chem|Al|2|(SO|4|)|3|·17 H|2|O}}.{{refn|name=Picon-etal-2005-lAlun|CONFERENCE }} The Ancient Greek Herodotus mentions Egyptian alum as a valuable commodity in The Histories.The production of potassium alum from alunite is archaeologically attested on the island Lesbos.{{refn|name=Archontidou-2005-alunMedit|CONFERENCE , }}The site was abandoned during the 7th century CE, but dates back at least to the 2nd century CE. Native alumen from the island of Melos appears to have been a mixture mainly of alunogen ({{chem|Al|2|(SO|4|)|3|·17 H|2|O}}) with potassium alum and other minor sulfates.{{refn|name=Hall-Jones-2005-alunMedit|CONFERENCE , }}

Alumen in Pliny and Dioscorides

A detailed description of a substance termed alumen occurs in the Roman Pliny the Elder’s Natural History.By comparing Pliny’s description with the account of stypteria (στυπτηρία) given by Dioscorides, it is obvious the two are identical. Pliny informs us that a form of alumen was found naturally in the earth, and terms it salsugoterrae.Pliny wrote that different substances were distinguished by the name of alumen, but they were all characterised by a certain degree of astringency, and were all employed for dyeing and medicine. Pliny wrote that there is another kind of alum that the ancient Greeks term schiston, and which “splits into filaments of a whitish colour”. From the name schiston and the mode of formation, it seems that this kind was the salt that forms spontaneously on certain salty minerals, as alum slate and bituminous shale, and consists mainly of sulfates of iron and aluminium.{{citation needed|date=May 2009}} One kind of alumen was a liquid, which was apt to be adulterated; but when pure it had the property of blackening when added to pomegranate juice. This property seems to characterize a solution of iron{{sup|II}} sulfate in water; a solution of ordinary (potassium) alum would possess no such property. Contamination with iron sulfate was greatly disliked as this darkened and dulled dye colours. In some places the iron sulfate may have been lacking, so the salt would be white and would be suitable, according to Pliny, for dyeing bright colors.Pliny describes several other types of alumen but it is not clear as to what these minerals are. The alumen of the ancients, then, was not always potassium alum, not even an alkali aluminum sulfate.{{rp|pages= 766–767}}

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 distinguished the two salts accurately. 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 indiscriminately to both.{{citation needed|date=June 2023}}Alum was the most common mordant (substance used to set dyes on fabrics) used by the dye industry, especially in Islamic countries, during the middle ages. It was the main export of the Chad region, from where it was transported to the markets of Egypt and Morocco, and then to Europe. Less significant sources were found in Egypt and Yemen.

Modern understanding of the alums

During the early 1700s, G. E. Stahl claimed that reacting sulfuric acid with limestone produced a sort of alum.{{efn|CVII.   Vitriolum, Creta præcipitari potest, ut omissa metallica sua substantia, aluminosum evadat. Ausführliche Betrachtung und zulänglicher Beweiss von den Saltzen, daß diesselbe aus einer zarten Erde, mit Wasser innig verbunden, bestehen ... wie aus Kreide und Vitriole-Spiritu, ein rechter Alaun erwächset: ... }}{{rp|pages=41–66}}Marggraf also showed that perfect crystals with properties of alum can be obtained by dissolving alumina in sulfuric acid and adding potash or ammonia to the concentrated solution.{{rp|page=766}}{{rp|page=31–40}} In 1767, Torbern Bergman observed the need for potassium or ammonium sulfates to convert aluminium sulfate into alum, while sodium or calcium would not work.{{efn|After acknowledging that Marggraf had noticed that potash caused alum to crystallize from a solution of alumina and sulfuric acid, Bergman adds The composition of common alum was determined finally by Louis Vauquelin in 1797. As soon as Martin Klaproth discovered the presence of potassium in leucite and lepidolite,{{efn|“On the contrary, I was surprised in an unexpected manner, by discovering in it another constituent part, consisting of a substance, the existence of which, certainly, no one person would have conjectured within the limits of the mineral kingdom ... This constituent part of leucite ... is no other than pot-ash, which, hitherto, has been thought exclusively to belong to the vegetable kingdom, and has, on this account, been called vegetable alkali. This discovery, which I think of great importance, cannot fail to occasion considerable changes in the systems of natural {{nobr|history, ... .” — M. H. Klaproth (1801)}}}}Vauquelin demonstrated that common alum is a double salt, 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.The most important alums – potassium, sodium, and ammonium – are produced industrially. Typical recipes involve combining aluminium sulfate and the sulfate monovalent cation. The aluminium sulfate is usually obtained by treating minerals like alum schist, bauxite and cryolite with sulfuric acid.{{rp|page=767|}}

Types

(File:Potassium alum octahedral like crystal.jpg|thumb|Crystal of potassium alum.)Aluminium-based alums are named by the monovalent cation. Unlike the other alkali metals, lithium does not form alums; a fact attributed to the small size of its ion.The most important alums are

• Potassium alum, {{chem|KAl(SO|4|)|2|·12 H|2|O}}, also called “potash alum” or simply “alum”
• Sodium alum, {{chem|NaAl(SO|4|)|2|·12 H|2|O}}, also called “soda alum” or “SAS”
• Ammonium alum, {{chem|NH|4|Al(SO|4|)|2|·12 H|2|O}}

Chemical properties

Aluminium-based alums have a number of common chemical properties. They are soluble in water, have a sweetish taste, react as acid 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.{{rp|page=766|}} 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 and Lawrence Bragg, and were used to develop the phase retrieval technique isomorphous replacement.

Solubility

The solubility of the various alums in water varies greatly, sodium alum being soluble readily in water, while caesium and rubidium alums are only slightly soluble. The various solubilities are shown in the following table.{{rp|page=767}}At temperature {{mvar|T}}, 100 parts water dissolve: