Alums: Properties, Production and Uses

Alum crystals

1. The History of Alum

Alum, recognized as a double salt of potassium and aluminum sulfates, was known among the ancient Greeks and Romans as both an astringent and a mordant for wool dyeing. It found diverse application, extending to skin processing, preservation of both animal and human remains, and for fireproofing wood.

In the 16th century, Paracelsus was the pioneer in distinguishing alum from iron vitriol. In the 18th century, Chaptal and Vauquelin established that alum could be produced from aluminum sulfate and potassium sulfate and that potassium ion could be replaced by ammonium.

During the Middle Ages, the alum industry was important and relied on materials like alum stone (alunite) or alum shale. Alum stone contained the necessary components in the correct proportions, and its processing involved roasting and leaching.

By roasting, aging, and leaching alum shale, an aluminum solution can be produced and when treated with alkali forms alum precipitation.

The significance of industrial alum production diminished with the emergence of economically viable methods to produce high-purity aluminum sulfate, where the aluminum content had practical importance.

Many alum production techniques now hold limited or historical value. Raw materials like clay, alkali-containing silicates, especially bauxite, were employed, and digestion with alkali or acid was conducted.

Today, alums are exclusively produced from aluminum hydroxide, derived from bauxite by the Bayer process.

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2. General Properties of Alums

1. General Formula of Alums: Alums are crystalline double salts with the formula (cation 1)+(cation 2)3+(anion2-)2 • 12 H2O. The most common alums contain trivalent aluminum cations and sulfate anions, denoted as M+Al3+(SO42-)2 • 12 H2O.

2. Component Replacement: The components of alums can be replaced while maintaining the crystalline structure and water content. Alums with different trivalent metal cations can be iron, chromium, cobalt, manganese, titanium, vanadium, gallium, indium, scandium, rhodium, and iridium. The monovalent component can be alkali metals, ammonium, alkylammonium, arylammonium, or thallium.

3. Crystal Structure: Alums crystals are octahedral or cubic  with strong light refraction.

4. Properties: Alums possess an astringent taste and exhibit properties such as rot-proofing and protein precipitation.

5. Chemical Behavior: When dissolved in water, alums display chemical properties inherent to their components. In very dilute solutions, physical properties like color, electrical conductivity, and freezing-point depression are additive. However, at higher concentrations, complex formations such as [Al(SO4)2 • (H2O)2] can occur.

6. Solubilities: Solubilities of alums in water decrease from sodium to cesium.

7. Heating Effects: Alums lose their water of crystallization partially or completely when heated. Yet, they can reabsorb some of this water under normal temperature and humidity conditions.

3. Potassium Aluminum Sulfate

Potassium aluminum sulfate, is also known as potassium alum has the chemical formula KAl(SO4)2 • 12 H2O and the molecular weight of 474.4 g/mol. This compound and ammonium aluminum sulfate are the longest-known aluminum compounds.

Potassium alum can be found in nature as an efflorescence on alum shale and in volcanic regions on trachyte and lava as feather alum.

In its natural state, potassium alum forms large, colorless, transparent octahedral crystals that have a melting point of 92.5 °C due to their inherent water of crystallization.

On the Mohs scale, these crystals have a hardness of 2. They are capable of absorbing long-wavelength infrared radiation almost entirely while remaining transparent to visible light.

Certain substances, including hydroxides, carbonates, borates, carbamides, metals, and organic dyes, facilitate the formation of basic aluminum sulfate by binding free sulfuric acid within the mother liquor. Under these conditions, the cubic form, also known as cubic or Roman alum, becomes more prevalent.

Potassium alum remains stable under typical air humidity conditions. Dehydration initiates only below 30 °C, and a loss of nine moles of water occurs at 65 °C. K2SO4, γ-Al2O3, and 3 K2SO4 • Al2(SO4)3 are formed at 780 °C, while K2SO4, α-Al2O3, and K2O • 12 Al2O3 are generated at 1400 °C.

When heated beyond its melting point, potassium alum undergoes dehydration, transforming into calcined alum (alum ustum), represented as KAl(SO4)2. At elevated temperatures, SO3 is released.

In terms of solubility, potassium alum is soluble in dilute acids but almost insoluble in anhydrous ethanol, acetone, and methyl acetate. Its solubility in water increases notably with temperature, facilitating easier purification by recrystallization compared to other aluminum salts. This process, in particular, helps in the removal of iron sulfate.

Potassium alum readily forms mixed crystals with ammonium sulfate.

Another variation is basic potassium aluminum sulfate, K[Al(OH)2]3(SO4)2 • 3/2 H2O. This compound occurs naturally and it can be produced synthetically as an amorphous, relatively insoluble powder by heating aluminum sulfate, water, and an excess of potassium sulfate.

Additionally, another type of basic potassium aluminum sulfate, K[Al3(OH)6(SO4)2], containing less water, is found in nature as alum stone (alunite).

3.1. Production of Potassium Alum

The aluminum hydroxide is first dissolved in water to form a solution of aluminum sulfate. The sulfuric acid is then added to the aluminum sulfate solution, and the mixture is heated at a pressure of 5-6 bar. The reaction between the aluminum hydroxide and sulfuric acid produces a melt of aluminum sulfate.

The aluminum sulfate melt is then led into a copper container, where a stoichiometric quantity of potassium sulfate is added. The solution is heated to a temperature of about 100 °C for 2-3 hours and is adjusted to a specific gravity of 40-44°Bé with mother liquor. The solution is then filtered to remove any insoluble material.

The filtered solution is then left in crystallization boxes for 10 days. During this time, the aluminum sulfate crystallizes out of the solution. The alum crystals are then removed from the boxes and washed with water.

The washed alum crystals are then dried at a temperature of 50-60 °C. The dried alum crystals are then sieved to remove any impurities. The pure alum crystals are then packed in paper bags that are lined with polyethylene.

The production of potassium alum can also be carried out by reacting aluminum sulfate with potassium hydroxide. The aluminum sulfate is first dissolved in water to form a solution of aluminum hydroxide.

The potassium hydroxide is then added to the aluminum hydroxide solution, and the mixture is heated to a temperature of 5-6 bar. The reaction between the aluminum hydroxide and potassium hydroxide produces a melt of potassium alum.

The potassium alum melt is then processed in the same way as the aluminum sulfate melt.

The production of potassium alum is a relatively simple process. However, it is important to control the reaction conditions carefully in order to produce a high-quality product.

3.2. Uses of Potassium Alum

Potassium alum has a variety of uses, including:

  • Tanning skins: Potassium alum is used in the tanning of skins to make leather. It helps to remove the hair from the skin and to make the skin more pliable.
  • Mordant in dyeing: Potassium alum is used as a mordant in dyeing. A mordant is a substance that helps the dye to bind to the fabric. Potassium alum helps to make the dye color more permanent.
  • Coagulating agent for latex: Potassium alum is used as a coagulating agent for latex. Latex is a milky fluid that is made from the sap of rubber trees. Potassium alum helps to coagulate the latex, which makes it easier to handle and process.
  • Astringent and protein-precipitating properties: Potassium alum has astringent and protein-precipitating properties. This means that it can shrink tissues and precipitate proteins. Potassium alum is used in the pharmaceutical and cosmetics industries for these properties. For example, it is used in styptic pencils to stop bleeding.
  • Hardening agent and setting accelerator for gypsum: Potassium alum is used as a hardening agent and setting accelerator for gypsum. Gypsum is a mineral that is used to make plaster and concrete. Potassium alum helps to make the gypsum set more quickly and to make it stronger.
  • Purifying water: Potassium alum was formerly used to purify water. It helps to remove impurities from the water and to make it more clear. However, potassium alum is no longer used for this purpose because it is not as effective as other methods.
  • Sizing paper: Potassium alum was formerly used to size paper. Sizing is the process of coating paper with a substance to make it more water-resistant. However, potassium alum is no longer used for this purpose because it is not as effective as other methods.

In the paper industry, aluminum sulfate is designated traditionally, although incorrectly, as an alum. This is because aluminum sulfate was originally made by treating potassium alum with sulfuric acid. However, aluminum sulfate is now made by a different process, and it does not contain any potassium.

4. Ammonium Aluminum Sulfate

Ammonium alum is a double salt of ammonium and aluminum sulfates, NH4Al(SO4)2 • 12 H2O. It is a white, odorless, and tasteless solid that crystallizes in large, colorless, transparent octahedra. Ammonium alum has a melting point of 93.5 °C and a density of 1.64 g/cm3. It is soluble in water and dilute acids, but insoluble in absolute alcohol.

Ammonium alum occurs naturally as shermigite. Crystals of ammonium alum that are doped with other alums show birefringence. This means that they have different refractive indices for different directions of light.

The solubility of ammonium alum in water is similar to that of potassium alum. Ammonium alum can form a continuous series of mixed crystals with potassium alum.

Ammonium alum is slightly soluble in dilute acids and glycerol. It is insoluble in absolute ethanol.

In aqueous solution, ammonium alum is neutral. This means that it does not have a pH of either acidic or alkaline.

The data on the loss of water on heating ammonium alum are not consistent. Some studies have shown that the water of crystallization is released in three stages. First, 21 mol of water are released to give the hydrate with 21 mol water.

Then, 3 mol of water are released to give the hydrate with 3 mol water. Finally, the anhydrous product is formed. Other studies have shown that the water of crystallization is released in two stages. First, 12 mol of water are released to give the hydrate with 3 mol water. Then, the anhydrous product is formed.

Above 193 °C, ammonium alum begins to decompose with the release of ammonia. On glowing above 1000 °C, sulfur trioxide is lost, leaving an aluminum oxide residue.

4.1. Production of Ammonium Alum

Ammonium alum is typically produced by dissolving aluminum hydroxide in sulfuric acid and adding ammonium sulfate. The resulting solution is then evaporated to crystallize the ammonium alum. This process is analogous to the production of potassium alum.

To obtain a high-purity ammonium alum, which is required for some applications such as the production of synthetic gems, very pure starting materials must be used. The iron oxide content of the ammonium alum should be less than 0.0001%.

Ammonium alum can also be produced by reacting ammonia gas with aluminum sulfate and sulfuric acid. This process is less common, but it can be used to produce ammonium alum with a high iron oxide content.

Ammonium alum is an intermediate in the “aloton” process, which is a method for producing aluminum hydroxide and aluminum oxide. This process was used in Germany and the United States prior to 1945, but it is no longer widely used.

4.2. Uses of Ammonium Alum

Ammonium alum is used in a variety of applications, including:

  • Tanning: Ammonium alum is used to dress furs in tanning. It helps to shrink the fur and make it more pliable.
  • Production of aluminum oxide particles: Ammonium alum can be used to produce very fine aluminum oxide particles, which are used for polishing metallographic surfaces.
  • Disinfectant: In some countries outside of Europe, ammonium alum is used as a disinfectant. It can be used to kill bacteria and other microorganisms.
  • Baking powder: Ammonium alum is an ingredient in baking powder. It helps to leaven baked goods by releasing carbon dioxide gas.
  • Production of synthetic gems: Ammonium alum is used as a starting material for the production of synthetic gems, such as rubies and sapphires. When heated to 1000°C, ammonium alum decomposes to form aluminum oxide, which is the main component of these gems.

In Europe, ammonium alum is not used in large quantities. However, it is more widely used in the United States, where it is an important ingredient in baking powder. Ammonium alum is also gaining importance as a starting material for the production of synthetic gems.

5. Sodium Aluminum Sulfate

Sodium aluminum sulfate (NaAl(SO4)2·12 H2O), also known as soda alum, is an inorganic compound with a white crystalline appearance. It is soluble in water, but insoluble in absolute alcohol. Sodium alum melts at 61 °C and occurs naturally as the mineral mendozite.

The data on the thermal dehydration of sodium alum is not consistent. Some studies have shown that it loses water in a stepwise manner, while others have shown that it loses water in a single step.

The two main challenges in obtaining sodium alum free from iron are:

  • It is difficult to produce sodium alum powder (very fine crystalline form) commercially.
  • Sodium alum is very soluble in water, which makes it difficult to remove the iron impurities.

Due to these challenges, sodium alum has not gained the same importance as other alums. In Europe, its use has been abandoned. However, it is still used in relatively large quantities (about 3,000 tons per year) in the United States, mainly in baking powder.

Here are some additional details about the properties of sodium alum:

  • The molar mass of sodium alum is 242.10 g/mol.
  • The density of sodium alum is 1.67 g/cm3.
  • The melting point of sodium alum is 61 °C.
  • The boiling point of sodium alum is 330 °C.
  • The solubility of sodium alum in water is 39.1 g/100 mL (at 20 °C).

5.1. Production of Sodium aluminum sulfate

Sodium alum is produced in the United States by adding a clear solution of sodium sulfate to aluminum sulfate. The solution is diluted to 30 Baumé and heated. A sludge of potassium sulfate, sodium silicate, and caustic soda is then added to improve the purity of the product.

The mixture is pumped into a stirred vessel and mixed for several hours. During this stage, the ratio of aluminum sulfate to sodium sulfate is adjusted to the stoichiometric amount.

The melt is then pumped into an evaporator and concentrated until it solidifies to a hard cake when poured into a cooling tank. The sodium alum cake is then heated and ground to the desired size (99% through a sieve of 100 mesh).

6. Toxicology

Alum solutions are known for their astringent effects, which means that they can cause the contraction of tissues. This can be useful in some applications, such as styptic pencils. However, alum solutions can also be harmful if they are inhaled or ingested in large amounts.

A TLV of 2 mg/m3 was established for water-soluble aluminum salts. TLV stands for Threshold Limit Value, which is the level of a substance in the air that is considered to be safe for most people over an eight-hour workday.

This means that inhaling 2 mg of water-soluble aluminum salts per cubic meter of air is not expected to cause any health problems for most people.

However, it is important to note that the TLV is just an average value. Some people may be more sensitive to the effects of alum solutions than others. It is also important to consider the length of exposure.

Inhaling or ingesting even small amounts of alum solutions over a long period of time can lead to health problems.

Here are some of the health problems that can be caused by alum solutions:

  • Eye irritation
  • Skin irritation
  • Respiratory irritation
  • Nausea
  • Vomiting
  • Diarrhea
  • Kidney problems
  • Liver problems
  • Central nervous system problems



Potassium alum, also known as potash alum or potassium aluminum sulfate, is a chemical compound with the formula KAl(SO4)2 • 12 H2O. It is a double salt of potassium sulfate and aluminum sulfate, commonly used for its astringent and mordant properties.

Potassium alum is generally considered safe when used in appropriate amounts and applications. It has been used in various industries for centuries. However, as with any chemical compound, excessive or improper use should be avoided.

While both potassium alum and aluminum are chemical elements, they are not the same. Potassium alum is a specific compound composed of potassium, aluminum, sulfur, and oxygen. Aluminum, on the other hand, is an element with the symbol Al and is commonly used in various alloys and products.

Potassium alum has diverse applications. It has historically been used in tanning, dyeing, and water purification due to its astringent and coagulating properties. Additionally, it finds use in cosmetics, pharmaceuticals, and as a styptic agent to stop bleeding. Its significance in various industries has evolved over time.

Ammonium alum, or ammonium aluminum sulfate, is a compound with the formula NH4Al(SO4)2 • 12 H2O. Similar to potassium alum, it has been utilized for its astringent and coagulating properties in various industries.

Ammonium alum can be produced through the reaction of aluminum hydroxide with sulfuric acid, followed by the addition of ammonium sulfate. The process involves precise steps to obtain the desired product while controlling impurities.

Ammonium alum serves roles similar to potassium alum, finding application in tanning, dyeing, and water treatment. It is also used as a starting material for producing high-purity aluminum oxide required for synthesizing gemstones like corundum.

Sodium alum, also known as sodium aluminum sulfate, has been employed in various applications including fur dressing, metallographic surface polishing, and limited usage as a disinfectant in specific regions. In the United States, it is utilized in baking powder production.


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