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 | |
| Names | |
|---|---|
| IUPAC name Potassium alum[2] | |
Other names
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| Identifiers | |
| |
3D model (JSmol) | |
| ChEBI | |
| ECHA InfoCard | 100.112.464 |
| E number | E522(acidity regulators, ...) |
| UNII |
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| |
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| Properties | |
| KAl(SO4)2·12H2O | |
| Molar mass | 258.192 g/mol (anhydrous) 474.37 g/mol (dodecahydrate) |
| Appearance | White crystals |
| Odor | Watery metallic |
| Density | 1.725 g/cm3 |
| Melting point | 92 to 95 °C (198 to 203 °F; 365 to 368 K) |
| Boiling point | Decomposes at 200[3] °C (392 °F; 473 K) |
| 6.01 g/100 g (20°C)[4] 109 g/100 g (90 °C) | |
| Solubility in other solvents | Insoluble in acetone |
Refractive index (nD) | 1.4564 |
| Hazards | |
| NFPA 704 (fire diamond) | |
Except where otherwise noted, data are given for materials in theirstandard state (at 25 °C [77 °F], 100 kPa). | |
Potassium alum,potash alum, orpotassium aluminium sulfate is achemical compound defined as the doublesulfate ofpotassium andaluminium, with chemical formula KAl(SO4)2. It is commonly encountered as thedodecahydrate, KAl(SO4)2·12H2O. It crystallizes in an octahedral structure in neutral solution and cubic structure in an alkali solution with space group Pa3 and lattice parameter of 12.18 Å.[5] The compound is the most important member of the generic class of compounds calledalums, and is often called simplyalum.[6]
Potassium alum is commonly used inwater purification,leather tanning,dyeing,[7] fireprooftextiles, andbaking powder asE numberE522. It also has cosmetic uses as a deodorant, as an aftershave treatment and as astyptic for minor bleeding from shaving.[8][9]
Historically, potassium alum was used extensively in the wool industry[10] fromClassical antiquity, during theMiddle Ages, and well into 19th century as amordant or dye fixative in the process of turning wool intodyedbolts of cloth.[citation needed]
Potassium alum was also known to theAncient Egyptians, who obtained it fromevaporites in the Western desert and reportedly used it as early as 1500 BCE to reduce the visible cloudiness (turbidity) in the water.[citation needed]
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 inMesopotamian chemical technology that can be identified with certainty.[11] Both native and imported potassium alum was used.[11] Together with other agents, potassium alum was used inglass-making,tanning, and in thedyeing of cloth, wood, and possibly hair.[11] A tanning process using potassium alum is described in tablets from the first millennium BCE.[11] 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. InMesopotamian medicine potassium alum was used extensively, for example against itch,jaundice, some eye condition, and unidentified ailments.[11]
According to Levey, potassium alum was used in "classical times" as aflux whensoldering 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.[11]
The production of potassium alum fromalunite is archaeologically attested on the islandLesbos.[12] This site was abandoned in the 7th century but dates back at least to the 2nd century CE.
Potassium alum was described under the namealumen orsalsugoterrae byPliny,[13] and it is clearly the same as thestypteria (στυπτηρία) described byDioscorides.[14] However, the namealum and other names applied to this substance — likemisy,sory,chalcanthum, andatramentum sutorium — were often applied to other products with vaguely similar properties or uses, such asiron sulfate or "green vitriol".[15][full citation needed]
Potassium alum is mentioned in Ayurvedic texts namely Charak Samhita, Sushurta Samhita, and Ashtanga Hridaya with the name such assphaṭika kṣāra,phitkari orsaurashtri. It is used intraditional Chinese medicine with the namemingfan .
In the 13th and 14th centuries, alum (from alunite) was a major import fromPhocaea (Gulf of Smyrna in Byzantium) by Genoans and Venetians (and was a cause of war betweenGenoa andVenice) and later byFlorence. After the fall ofConstantinople, alunite (the source of alum) was discovered atTolfa in the Papal States (1461). The textile dyeing industry inBruges, 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.[16][17]
Potassium alum was imported into England mainly from theMiddle East, and, from the late 15th century onwards, thePapal States for hundreds of years. Its use there was as adye-fixer (mordant) forwool (which was one of England's primary industries, the value of which increased significantly if dyed).[citation needed] 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 theexcommunication ofHenry VIII.[18]
With state financing, attempts were made throughout the 16th century, but without success until the early 17th century. An industry was founded inYorkshire to process theshale, which contained the key ingredient,aluminium sulfate, and made an important contribution to theIndustrial Revolution. One of the oldest historic sites for the production of alum from shale andhuman urine are the Peak alum works inRavenscar, North Yorkshire. By the 18th century, the landscape of northeast Yorkshire had been devastated by this process, which involved constructing 100-foot (30 m) 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.[19]
In the early 1700s, Georg Ernst Stahl claimed that reacting sulfuric acid with limestone produced a sort of alum.[20][21] The error was soon corrected byJohann Pott andAndreas Marggraf, who showed that theprecipitate obtained when an alkali is poured into asolution of alum, namelyalumina, is quite different fromlime andchalk, and is one of the ingredients in commonclay.[22][23]
Marggraf also showed that perfect crystals with properties of alum can be obtained by dissolving alumina insulfuric acid and addingpotash orammonia to the concentrated solution.[24][25] In 1767,Torbern Bergman observed the need for potassium or ammonium sulfates to convertaluminium sulfate into alum, while sodium or calcium would not work.[24][26]
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 mineralsleucite andlepidolite.[27][28]
Louis Vauquelin then conjectured that potassium was likewise an ingredient in many otherminerals. 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 adouble salt, composed of sulfuric acid, alumina, and potash.[29] 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,[30] confirming Vauquelin's results.[24]
Potassium alum crystallizes in regularoctahedra with flattened corners and is very soluble in water. The solution is slightly acidic and isastringent to the taste. Neutralizing a solution of alum withpotassium hydroxide will begin to cause the separation of aluminaAl(OH)
3.[citation needed]
When heated to nearly a red heat, it gives a porous, friable mass, which is known as "burnt alum". It fuses at 92 °C (198 °F) in its ownwater of crystallization.[citation needed]
Potassium alum dodecahydrate occurs in nature as asulfate mineral calledalum-(K), typically as encrustations on rocks in areas ofweathering andoxidation ofsulfide minerals and potassium-bearing minerals.[citation needed]
In the past, potassium alum has been obtained fromalunite (KAl(SO
4)
2·2Al(OH)
3), mined from sulfur-containing volcanic sediments.[31] Alunite is an associate and likely potassium and aluminium source.[1][32] It has been reported atVesuvius,Italy; east ofSpringsure,Queensland; in Alum Cave,Tennessee; Alum Gulch,Santa Cruz County, Arizona and thePhilippine island ofCebu.
In order to obtain alum fromalunite, it iscalcined 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.[citation needed]
Theundecahydrate also occurs as the fibrous mineralkalinite (KAl(SO
4)
2·12H
2O).[33]
Potassium alum historically was mainly extracted fromalunite.
Potassium alum is now produced industrially by addingpotassium sulfate to a concentrated solution ofaluminium sulfate.[34] The aluminium sulfate is usually obtained by treating minerals like alumschist,bauxite andcryolite with sulfuric acid.[35] If much iron should be present in the sulfate then it is preferable to usepotassium chloride in place of potassium sulfate.[35]
Potassium alum is used in medicine mainly as anastringent (orstyptic) andantiseptic.
Styptic pencils are rods composed of potassium alum or aluminum sulfate, used topically to reduce bleeding in minor cuts (especially fromshaving) and abrasions,nosebleeds, andhemorrhoids, and to relieve pain from stings and bites.[citation needed] Potassium alum blocks are rubbed over the wet skin after shaving.[9]
Potassium alum is also used topically to removepimples andacne, and to cauterizeaphthous ulcers in the mouth andcanker sores, as it has a significant drying effect to the area and reduces the irritation felt at the site.[37][38] It has been used to stop bleeding in cases ofhemorrhagic cystitis[39] and is used in some countries as a cure forhyperhidrosis.[citation needed]
It is used in dentistry (especially in gingival retraction cords) because of itsastringent andhemostatic properties.[citation needed]
Potassium andammonium alum are the active ingredients in someantiperspirants anddeodorants, acting by inhibiting the growth of thebacteria responsible forbody odor. Alum's antiperspirant and antibacterial properties[40][41] contribute to its traditional use as an underarmdeodorant.[13] It has been used for this purpose in Europe, Mexico, Thailand (where it is calledsarn-som), throughout Asia and in the Philippines (where it is calledtawas). Today, potassium or ammonium alum is sold commercially for this purpose as a "deodorant crystal".[42][43][8] Beginning in 2005 the USFood and Drug Administration no longer recognized it as a wetness reducer, but it is still available and used in several other countries, primarily in Asia.[citation needed]
Potassium alum was the majorimmunologic adjuvant used to increase the efficacy ofvaccines, and has been used since the 1920s.[44] But it has been almost completely replaced byaluminium hydroxide andaluminium phosphate in commercial vaccines.[45]
Alum may be used in depilatory waxes used for the removal ofbody 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 hair, as an alternative topomade, to keep the hair standing up.[citation needed]
Potassium alum may be an acidic ingredient ofbaking powder to provide a secondleavening phase at high temperatures (althoughsodium alum is more commonly used for that purpose).[citation needed] For example, potassium alum is frequently used in leavening ofyoutiao, a traditional Chinese fried bread, throughout China.[46]
Alum was used by bakers in England during the 1800s to make bread whiter. This was theorized by some, includingJohn Snow, to causerickets.[47][48] TheSale of Food and Drugs Act 1875 (38 & 39 Vict. c. 63) prevented this and other adulterations.[49]
Potassium alum, under the name "alum powder", is found in the spice section of many grocery stores in theUS. Its chief culinary use is inpickling recipes, to preserve and add crispness to fruit and vegetables.[50]
Potassium alum is used as afire retardant to render cloth, wood, and paper materials less flammable.[34]
Potassium alum is used inleathertanning,[51] in order to remove moisture from thehide and prevent rotting.[citation needed] Unliketannic acid, alum doesn't bind to the hide and can be washed out of it.[citation needed]
Alum has been used since antiquity asmordant to form a permanent bond betweendye and natural textile fibers likewool.[52] It is also used for this purpose inpaper marbling.[53]
Potassium alum has been used since remote antiquity for purification of turbid liquids.[54] It is still widely used in the purification of water for drinking and industrial processes water, treatment ofeffluents and post-storm treatment of lakes to precipitate contaminants.[55]
Between 30 and 40ppm of alum[54][56] for household wastewater, often more for industrial wastewater,[57] is added to the water so that the negatively charged colloidal particles clump together into "flocs", which then float to the top of the liquid,settle to the bottom of the liquid, or can be more easilyfiltered from the liquid, prior to further filtration and disinfection of the water.[34] Like other similar salts, it works by neutralizing theelectrical double layer surrounding very fine suspended particles, allowing them to join into flocs.
The same principle is exploited when using alum to increase theviscosity of aceramic glazesuspension; this makes the glaze more readily adherent and slows its rate ofsedimentation.[citation needed]
Aluminum hydroxide from potassium alum serves as a base for the majority oflake pigments.[58]
Alum solution has the property of dissolving steels while not affecting aluminium orbase metals. Alum solution can be used to dissolve steel tool bits that have become lodged in machined castings.[59][60]
In traditionalJapanese art, alum andanimal glue were dissolved in water, forming a liquid known asdousa (ja:礬水), and used as an undercoat for papersizing.[citation needed]
Alum is an ingredient in some recipes for homemade modeling compounds, often called "play clay" or "play dough", intended for use by children.[citation needed]
Potassium alum was formerly used as a hardener forphotographic emulsions (films and papers), usually as part of thefixer. It has now been replaced in that use by other chemicals.
Potassium alum may be a weakirritant to the skin.[61]
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This article incorporates text from a publication now in thepublic domain: Chisholm, Hugh, ed. (1911). "Alum".Encyclopædia Britannica. Vol. 1 (11th ed.). Cambridge University Press. pp. 766–767.