Alum: Properties, Types, Amazing Applications

Alum is an inorganic compound consisting of water molecules, aluminum, other metals besides aluminum, and sulfates. Alum, in its hydrated state, is classified as a double salt. There are various forms of alum, including Potash Alum, Soda Alum, Ammonium Alum, and Chrome Alum. The chemical formula commonly used to represent Alum is XAl(SO4)2.12H2O.

Typically, when discussing alum, it commonly refers to potassium alum, a hydrated compound of potassium aluminum sulfate with the chemical formula KAl(SO4)2.12H2O. Nevertheless, any compounds exhibiting the empirical formula AB(SO4)2.12H2O are classified as alum.

Alum: Properties, Applications, Effects

Occasionally, alum can be observed in its crystalline state, although it is predominantly marketed and distributed in powdered form. Potassium alum is a finely powdered substance that is commonly available for purchase alongside culinary spices or pickling components. Additionally, it is marketed and available in the form of a sizable crystal, commonly referred to as a “deodorant rock,” specifically designed for underarm application.

Interesting Science Videos

Discovery of Alum

  • The earliest recorded utilization of alum can be traced back to linguistic studies conducted in Mesopotamia, where it was employed as a mordant for madder dye.
  • The utilization of alum in Egypt appears to have commenced during the conclusion of the eighteenth and twenty-first dynasties, which occurred between approximately 1500 BC and 1000 BC. Its purpose at that time was primarily for dyeing.
  • According to Herodotus, a historian from the fifth century BC, it has been documented that the ancient Egyptians employed alum in the process of mummifying deceased individuals.
  • During the Islamic Middle Ages, alum became the predominant mordant used in the dye industry for affixing dyes onto fabrics.
  • The Chad region primarily exported its goods to the markets of Egypt and Morocco, and subsequently to Europe. Additional sources of relatively lower significance have been identified in Egypt and Yemen.
  • During the early 18th century, G.E. Stahl suggested that the reaction between sulfuric acid and limestone resulted in the formation of a type of alum. 
  • The error was promptly rectified by J.H. Pott and A.S. Marggraf, who demonstrated that the precipitate formed when an alkali is added to a solution of alum, known as alumina, is distinct from lime and chalk, and is in fact a constituent of ordinary clay.
  • Furthermore, Marggraf successfully showcased his capability to generate impeccable crystals that exhibited the distinctive attributes of alum. This was achieved through the dissolution of alumina in sulfuric acid, followed by the addition of potash or ammonia into the concentrated solution.

Synthesis of Alum

  • There are several notable types of alum, with potassium, sodium, and ammonium being among the most significant.
  • Additionally, these compounds are commonly manufactured on an industrial scale through the combination of aluminum sulfate and a monovalent sulfate cation.
  • The product is generated through the precipitation process from a solution in water.
  • To illustrate, the production of potassium alum involves the dissolution of aluminum sulfate and potassium sulfate in water.
  • During the process of evaporation, the alum undergoes crystallization and separates from the solution.
  • Aluminum sulfate is frequently obtained through the process of treating minerals such as bauxite, cryolite, or alum schist with sulfuric acid.
  • Additionally, it can be acquired through the treatment of a combination of aluminum sulfate and sulfuric acid with ammonia.

Properties of Alum

  • The majority of alumni typically exhibit a flavor profile characterized by astringency and acidity.
  • Alum has no discernible odor or color, frequently presenting themselves in the crystalline powder state, typically appearing white in color.
  • Alum compounds exhibit high solubility in water, particularly when exposed to elevated temperatures.
  • The crystals typically adopt a regular octahedral shape upon precipitation from aqueous solutions.
  • Alum is typically found in the crystalline form, which is white and transparent.
  • The boiling point of the substance is approximately 200°C.
  • The melting point of the substance is recorded to be 92.5°C.
  • The density of the substance is 1.725 gm/cm3.
  • The substances exhibit an acidic reaction, resulting in the formation of litmus.
  • The aforementioned compounds exhibit a pleasant taste sensation.

Types of Alum

There are different forms of alum some of which are included here:

Ammonium Alum

  • This particular type is commonly known as ammonium sulfate alum. The more commonly used term for it is AAS.
  • The chemical formula for potash alum is NH3Al(SO4)2·12H2O. The molar mass of ammonium alum is 132.14 g/mol.
  • Additionally, it is found in a crystalline white form that emits a metallic odor reminiscent of water.
  • Ammonium alum is utilized in various industries, including tanning, textile dyeing, flame retardant textile production, porcelain cement and vegetable glue manufacturing, water purification, and certain deodorant formulations.

Potash Alum

  • The singular form of alum is commonly referred to as potassium alum or potash alum.
  • The chemical formula for potash alum is KAl(SO₄)₂·12H₂O, and it is commonly referred to as potassium aluminum sulfate in chemical nomenclature.
  • The commonly encountered type of alum is known as potash alum or potassium alum. The molar mass of the potash alum is 258.192 g/mol.
  • The substance can manifest in a crystalline white form and emits an odor reminiscent of metallic water. A Potash alum is occasionally referred to as a white alum.
  • The type of alum commonly found in grocery stores is used for pickling and can also be found in baking powder.

Chrome Alum

  • The commonly used abbreviation for chromium alum is CAS.
  • This compound is commonly referred to as chromium alum and is represented by the chemical formula KCr(SO4)2·12H2O.
  • This compound is commonly referred to as chromium aluminum sulfate and possesses a molar mass of 283.22 g/mol.
  • Similar to other alumni, it exhibits a metallic odor in its aqueous state, predominantly manifesting as purple crystalline structures.
  • Chrome alum is a deep violet substance that is frequently utilized in tanning procedures.
  • Furthermore, it can be employed as an additive to various alum compounds, leading to the formation of lavender or purple crystals.

Soda Alum

  • Soda Alum, also referred to as sodium alum or by its abbreviated form SAS, is a ubiquitous compound commonly found in baking powder and utilized as an acidulant in the domain of culinary practices.
  • The aforementioned compound is identified by its chemical nomenclature as sodium aluminum sulfate exhibiting a molar mass of 458.28 g/mol.
  • The material materializes in a crystalline formation of a light tint, emitting an olfactory perception reminiscent of liquid metal.
  • The molecular constitution of sodium alum may be concisely articulated as NaAl(SO₄)₂·12H₂O.
  • Sodium alum, a compound extensively utilized in the gastronomic domain, exhibits its efficacy in the realm of leavening agents, specifically in the context of baking powder. Additionally, it serves as an acidulant in diverse food applications, further enhancing its multifaceted utility.

Selenate Alum

  • Selenate Alum effectively displaces sulfur in chemical reactions. In this particular manifestation of alum, the anionic species that is observed is selenate, as opposed to the more commonly encountered sulfate.
  • It possesses the characteristic of exhibiting strong oxidizing capabilities.
  • The molecular formula denoting the compound known as selenate alum is Al2O12Se3, with a corresponding molecular mass of 482.9 g/mol.
  • Selenate Alum, a compound renowned for its multifaceted applications, finds utility in the realm of culinary arts as a constituent of baking powder, while also serving as an acidulant in the realm of gastronomy.
AlumChemical NameChemical FormulaMolar MassAppearance
Ammonium AlumAmmonium Aluminum SulfateNH3Al(SO4)2·12H2O132.14 g/molIt appears in crystalline white form and smells like metallic water
Potash AlumPotassium
KAl(SO₄)₂·12H₂O258.192 g/molIt appears in crystalline white form and smells like metallic water
Chrome AlumChromium
KCr(SO4)2·12H2O283.22 g/molIt appears in crystalline purple form and smells like metallic water
Soda AlumSodium Aluminum
NaAl(SO₄)₂·12H₂O458.28 g/molIt appears in crystalline white form and smells like metallic water
Selenate AlumSelenium
Al2O12Se3 482.9 g/molIt appears in crystalline white form and smells like metallic water

Alum Water Treatment

The process of alum water treatment is commonly undertaken in order to remediate water that has been contaminated. These compounds function as coagulants. It finds application within the Coagulation-Flocculation procedure employed for the treatment of contaminated water.

The water treatment technique that is frequently utilized prior to sedimentation and filtration is employed to enhance the effectiveness of the treatment process in removing particles.


It elicits a perturbation in the charges of the particles, thus resulting in a state of destabilization. Coagulants with charges opposing those displayed by the suspended solids are introduced into the water system to neutralize the negative charges found on dispersed non-settable solids, such as clay and organic substances.

Al2(SO4)3.18H2O + 6 HCO3− → 2 Al(OH)3 + 6 CO2 + 18 H2 + 3 SO4 -2

After the charge is neutralized, the small suspended particles demonstrate the ability to adhere to each other. The entities produced by this specific mechanism are known as microflows. These microflows have dimensions slightly larger than their individual particles, making them impossible to see without assistance.

Application of Alum

There are different industries that apply the alum for a wide range of applications some of them are discussed here:

Used in Cosmetics Industry

  • Within the realm of cosmetic applications, alum is employed as an astringent agent. In its aqueous state, this compound finds utility as an ingredient in emulsions, particularly those formulated for individuals with sebaceous skin types.
  • There has been a recent increase in the use of alum as a deodorant and antiperspirant. Although it may have a lower potency compared to aluminum chloride hexahydrate, this product still demonstrates effective performance.
  • The product operates by generating plugs within the pores, leading to a partial closure that impedes the perspiration process.
  • Furthermore, it has the effect of reducing the pH level, thereby creating an environment where microorganisms find it more difficult to thrive. A reduction in the presence of microorganisms results in a decrease in unpleasant odors. The powder can be compressed into spherical shapes, referred to as “crystals” or other forms, or alternatively, it can be dissolved in water for application as a deodorant.
  • The application of sticks and chunks is commonly employed for wound treatment following the act of shaving

Miscellaneous Uses

  • As previously mentioned, it is commonly employed in the coagulation and flocculation procedures of water treatment.
  • Alum is commonly employed in the baking and pickling processes.
  • Serves as an acidulating agent in culinary applications.
  • Utilized as a desiccant in the textile industry.
  • This substance is employed during the leather’s tanning procedure.
  • Alum compounds exhibit anti-inflammatory properties and are frequently employed in oral gargling procedures to alleviate inflammation and discomfort in the gums.
  • It is commonly included as an ingredient in certain toothpaste formulations.
  • This substance is utilized as an antiseptic agent and adjuvant in vaccines.

Effects of Alum

  • While alum is generally considered safe, it can pose risks if mishandled or used improperly.
  • Just like sand, sugar, or salt, the process of sanding has the potential to cause ocular harm. There is a restricted quantity of reported cases in which youngsters have encountered ocular exposure to alum.
  • Therefore, it is crucial to consider all medications, irrespective of their source (whether natural or synthetic), as potentially dangerous and take measures to prevent youngsters from accessing them.


  • Klaproth, M. H. (1801). Analytical Essays Towards Promoting the Chemical Knowledge of Mineral Substances. London, UK: T. Cadell, Jr. & W. Davies

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Jyoti Bashyal

Jyoti Bashyal, a graduate of the Central Department of Chemistry, is an avid explorer of the molecular realm. Fueled by her fascination with chemical reactions and natural compounds, she navigates her field's complexities with precision and passion. Outside the lab, Jyoti is dedicated to making science accessible to all. She aspires to deepen audiences' understanding of the wonders of various scientific subjects and their impact on the world by sharing them with a wide range of readers through her writing.

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