Potassium alum, potash alum, or potassium aluminium sulfate is a chemical compound defined as 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 crystallizes in an octahedral structure in neutral solution and cubic structure in an alkali solution with space group Pa and lattice parameter of 12.18 Å. The compound is the most important member of the generic class of compounds called alums, and is often called simply alum.
Potassium alum is commonly used in water purification, leather tanning, dyeing, fireproof textiles, and baking powder as E number E522. It also has cosmetic uses as a deodorant, as an aftershave treatment and as a styptic for minor bleeding from shaving.
History Historically, potassium alum was used extensively in the wool industrySee Henry VII of England trade section. Henry broke the Pope's monopoly by financing shipping bootstrapping a trading system with the Ottoman Empires mines from Classical antiquity, during the Middle Ages, and well into 19th century as a mordant or dye fixative in the process of turning wool into dyed bolts of cloth.
Antiquity
Egypt Potassium alum was also known to the Ancient Egyptians, who obtained it from evaporites in the Western desert and reportedly used it as early as 1500 BCE to reduce the visible cloudiness (turbidity) in the water.
Mesopotamia According to the expert on Middle Eastern history of chemistry Martin Levey, potassium alum is one of the few compounds known to the ancients that can be found relatively pure in nature, as well as one of only a few chemicals used in Mesopotamian chemical technology that can be identified with certainty. Both native and imported potassium alum was used. Together with other agents, potassium alum was used in glass-making, tanning, and in the dyeing of cloth, wood, and possibly hair. A tanning process using potassium alum is described in tablets from the first millennium BCE. When Levey wrote his article in 1958, no description of the dyeing process had been found, so it is not known how potassium alum was used in it. In Mesopotamian medicine potassium alum was used extensively, for example against itch, jaundice, some eye condition, and unidentified ailments.
According to Levey, potassium alum was used in "classical times" as a flux when soldering copper, in the fireproofing of wood, and in the separation of silver and gold, but that there is no evidence that these uses existed in Mesopotamia.
Greece The production of potassium alum from alunite is archaeologically attested on the island Lesbos.A. Archontidou 2005, "Un atelier de preparation de l'alun a partir de l'alunite dans l'isle de Lesbos" in L'alun de Mediterranée. ed P. Borgard et al. This site was abandoned in the 7th century but dates back at least to the 2nd century CE.
Rome Potassium alum was described under the name alumen or salsugoterrae by Pliny,Alumen, and the Several Varieties of it; Thirty-eight Remedies. , Pliny the Elder, The Natural History, book 35, chapter 52; on the Perseus Digital Library at Tufts University. Last accessed 27 December 2011. and it is clearly the same as the stypteria (στυπτηρία) described by Dioscorides.Dioscorides, book 5, chapter 123. However, the name alum and other names applied to this substance — like misy, sory, chalcanthum, and atramentum sutorium — were often applied to other products with vaguely similar properties or uses, such as iron sulfate or "green vitriol".
India and China Potassium alum is mentioned in Ayurvedic texts namely Charak Samhita, Sushurta Samhita, and Ashtanga Hridaya with the name such as sphaṭika kṣāra, phitkari or saurashtri. It is used in traditional Chinese medicine with the name mingfan .
Middle Ages In the 13th and 14th centuries, alum (from alunite) was a major import from Phocaea (Gulf of Smyrna in Byzantium) by Genoans and Venetians (and was a cause of war between Genoa and Venice) and later by Florence. After the fall of Constantinople, alunite (the source of alum) was discovered at Tolfa in the Papal States (1461). The textile dyeing industry in Bruges, and many locations in Italy, and later in England, required alum to stabilize the dyes onto the fabric (make the dyes "fast") and also to brighten the colors."Color in Relation to the Political and Economic History of the Western World" by Sidney M Edelstein, Proceedings of the Perkin Centennial, American Association of Textile Chemists and Colorists, September 1956"Worldly Goods: A New History of the Renaissance", Lisa Jardine, 1996, Norton&Co, pages 114–116
Modern era
England Potassium 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 dye-fixer (mordant) for 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 ceased following the excommunication of Henry VIII.
With state financing, attempts were made throughout the 16th century, but without success until the early 17th century. An industry was founded in Yorkshire to process the shale, which contained the key ingredient, aluminium sulfate, and made an important contribution to the Industrial Revolution. One of the oldest historic sites for the production of alum from shale and human urine are the Peak alum works in Ravenscar, North Yorkshire. By the 18th century, the landscape of northeast Yorkshire had been devastated by this process, which involved constructing stacks of burning shale and fuelling them with firewood continuously for months. The rest of the production process consisted of quarrying, extraction, steeping of shale ash with seaweed in urine, boiling, evaporating, crystallisation, milling and loading into sacks for export. Quarrying ate into the cliffs of the area, the forests were felled for charcoal and the land polluted by sulfuric acid and ash.
Identification of the formula In the early 1700s, Georg Ernst Stahl claimed that reacting sulfuric acid with limestone produced a sort of alum.George Ernst Stahl (1703), Specimen Beccherianum . Johann Ludwig Gleditsch, Leipzig.George Ernst Stahl (1723), Ausführliche Betrachtung und zulänglicher Beweiss von den Saltzen, daß diesselbe aus einer zarten Erde, mit Wasser innig verbunden, bestehen (Detailed treatment and adequate proof of salts, that they consist of a subtile earth intimately bound with water) Wäysenhaus, Halle. The error was soon corrected by Johann Pott and Andreas 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.Johann Heinrich Pott (1746), Chymische Untersuchungen, welche fürnehmlich von der Lithogeognosia oder Erkäntniß und Bearbeitung der gemeinen einfacheren Steine und Erden ingleichen von Feuer und Licht handeln , volume 1, p. 32.Andreas Sigismund Marggraf (1754), "Expériences faites sur la terre d'alun" (Experiments made on the earth of alum), Mémoires de l'Académie des sciences et belles-lettres de Berlin, pp. 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.Marggraf (1754) "Expériences qui concernent la régénération de l'alun de sa propre terre, l'après avoir séparé par l'acide vitriolique; avec quelques compositions artificielles de l'alun par moyen d'autres terres, et dudit acide" . Mémoires de l'Académie des sciences et belles-lettres de Berlin, pp. 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.Torbern Bergman (1767), "IX. De confectione Aluminis" . In Opuscula physica et chemica, I. G. Müller, Leipzig, 1788), volume 1, pp. 306–307.
At the time, potassium ("potash") was believed to be exclusively found on plants. However, in 1797, Martin Klaproth discovered the presence of potassium in the minerals leucite and lepidolite.Martin Heinrich Klaproth (1797), Beiträge zur Chemischen Kenntniss Der Mineralkörper (Contributions to [our] chemical knowledge of mineral substances). Decker and Co., Posen, and Heinrich August Rottmann, Berlin; pp. 45–46 and https://babel.hathitrust.org/cgi/pt?id=nyp.33433066422746;view=1up;seq=213 p. 193.Martin Heinrich Klaproth (1801), Analytical Essays Towards Promoting the Chemical Knowledge of Mineral Substances. T. Cadell, Jr. & W. Davies, London. His finding of potassium in leucite appears on pp. 353–354.: "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 to belong to the , and has, on this account, been called . ... This discovery, which I think of great importance, cannot fail to occasion considerable changes in the systems of natural history ...". The discovery of potassium in lepidolite is mentioned on p. 472.
Louis Vauquelin then conjectured that potassium was likewise an ingredient in many other minerals. Given Marggraf and Bergman's experiments, he suspected that this alkali constituted an essential ingredient of natural alum. In 1797 he published a dissertation demonstrating that alum is a double salt, composed of sulfuric acid, alumina, and potash.Vauquelin (1797), "Sur la nature de l'Alun du commerce, sur l'existence de la potasse dans ce sel, et sur diverses combinaisons simples ou triples de l'alumine avec l'acide sulfurique" (On the nature of commercial alum, on the existence of potash in this salt, and on various simple or triple compounds of alumina with sulfuric acid). In Annales de Chimie et de Physique, 1st series, volume 22, pages 258–279. In the same journal volume, Jean-Antoine Chaptal published the analysis of four different kinds of alum, namely, Roman alum, Levant alum, British alum and alum manufactured by himself,Jean-Antoine Chaptal (1797), "Comparée des quatre principales sortes d'Alun connues dans le commerce; et Observations sur leur nature et leur usage" (Comparison of the four main types of commercial alum; and observations on their nature and use). In Annales de Chimie et de Physique, 1st series, volume 22, pages 280–296. confirming Vauquelin's results.
Characteristics
Potassium alum crystallizes in regular octahedra with flattened corners and is very soluble in water. The solution is slightly acidic and is astringent to the taste. Neutralizing a solution of alum with potassium hydroxide will begin to cause the separation of , which redissolves with excess hydroxide.J. Strähle, E. Schweda (2006), Jander-Blasius, Handbuch der analytischen und präparativen anorganischen Chemie, S. Hirzel Verlag, 16th edition, p. 420
When heated to nearly a red heat, it gives a porous, friable mass, which is known as "burnt alum". It fuses at in its own water of crystallization.
Natural occurrence Potassium alum dodecahydrate occurs in nature as a sulfate mineral called alum-(K), typically as encrustations on rocks in areas of weathering and oxidation of sulfide minerals and potassium-bearing minerals.
In the past, potassium alum has been obtained from alunite (), mined from sulfur-containing volcanic sediments.Bottomley (2010) p. 35. Alunite is an associate and likely potassium and aluminium source. It has been reported at Vesuvius, Italy; east of Springsure, Queensland; in Alum Cave, Tennessee; Alum Gulch, Santa Cruz County, Arizona and the Philippine island of Cebu.