Structure, Properties, Uses of Alums

Alum is a mineral chemical compound made up of aluminium, water molecules, other metals and sulphates. Alum is nothing but a hydrated double sulphate aluminium salt. An alum’s universal chemical formula is XAl(SO4)2·12H2O. Alums can occur as minerals such as alunite and leucite.

Alum crystals can be found in grocery stores, medical stores, and e-commerce websites, among other places. White fitkari, red fitkari, or white alum stones and red alum stones are common names for these chemical compounds. The cost of alum is relatively cheaper than any other chemical compound. 

Structure and Color of Alums

The hydrated salt of potassium aluminium sulphate is known as alum. Because it contains two distinct cations – in this case, potassium and aluminium – that crystallize together in a single solid, it’s called a “double salt.” aluminium (grey) and potassium (purple) ions, as well as tetrahedral sulphate (SO42-) counterions, are illustrated in the alum crystal structure below.

The crystal is a hydrate, and the red atoms bonded to the metals represent oxygen molecules from water molecules. (Hydrogens are not depicted for clarity.) A single metal ion is surrounded by six water molecules in an octahedral coordination sphere formed by metal atoms. (The potassium atoms’ site symmetry is slightly bent from that of a real octahedron.) KAl[SO4]2*12H2O is the formula for this chemical. It’s vital to remember that this substance is a salt, which means it dissolves completely into individual ions in solution.

Types of Alum

Here are the different types of alum that you should know;

1. Potash Alum – Alum is sometimes identified as potassium alum or potash alum in its pure form. Potash alum has the chemical formula KAl(SO4)2.12H2O and the chemical name potassium aluminium sulphate. Potash alum, often known as potassium alum, is the most common alum. Potash alum has a molar mass of 258.192 g/mol. It appears as a white crystal and has a metallic water odour. White alum is another name for potash alum.

2. Soda Alum – SAS, often known as sodium alum is a frequent ingredient being used as a food acidulant. It has a molar mass of 458.28 g/mol and the chemical name sodium aluminium sulphate. It comes in the form of a white crystal with a metallic water odour. NaAl(SO4)2.12H2O is the chemical formula for sodium alum.

3. Ammonium Alum – It is the name for this form of alum. AAS is the most frequent name for it. NH3Al (SO4)2.12H2O is the chemical formula for ammonium alum. Molar mass of ammonia alum is 132.14 g/mol. It’s also available as a white crystal with a metallic water odour.

4. Chrome Alum – The abbreviation CAS stands for chromium alum. It has the chemical formula KCr(SO4)2.12H2O and is also known as chromium alum. It has a molar mass of 283.22 g/mol and is also known as chromium aluminium sulphate. It has a metallic water odor, like other alums, but is mostly found as purple crystals.

5. Selenate Alum – In this case, selenium replaces Sulphur. Instead of sulphate, the anion present in this type of alum is selenate. It has the ability to be a powerful oxidizing agent. Selenate alum has the chemical formula Al2O12Se3 and it’s molecular mass is 482.9 g/mol.

Properties of Alum

• Alums are very water-soluble (especially in hot water)

• These chemicals have a sweet flavour.

• When precipitated from aqueous solutions, they usually crystallize in the typical octahedral form.

• When heated, the alum crystals liquefy.

• Alums are typically found in a white, clear crystalline form.

• Its boiling point is approximately 200°C.

• It has a melting point of 92.5 °C.

• It has a density of 1.725 grams per cubic centimetre.

• When litmus is applied to them, they react acidic.

Alum Water Treatment

Alum is a nontoxic liquid that is commonly used in water treatment plants to clarify drinking water. It helps to purify and disinfect contaminated water. Alum is utilized as a coagulant in the coagulation-flocculation process of contaminated water in this use case scenario. Prior to the sedimentation and filtering procedure, the water is treated with alum. This is mostly done to improve a treatment procedure and ability to remove undesirable particles.

Coagulation – The charges on the particles become destabilized during the coagulation process. Coagulants are introduced to the water that has opposing charges to the suspended particles. This is done to neutralize negative charges on non-settable dispersion particles such as clay and some organic compounds.

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

The small-suspended particles stick together when the charge is neutralized.

Uses of Alum

Some of the common uses of alum are;

• In the baking and pickling processes, alum is employed.

• Used in the leather tanning process.

• As previously noted, it is frequently employed in the coagulation and flocculation of water.

• Alum in cooking acts as an acidulating agent.

• In the textile industry, it’s used as a drying agent.

• It is utilized in vaccinations as an antiseptic and adjuvant.

• Baking powder contains an acidic component.

• Alums are anti-inflammatory substances that are often utilized in the gargling procedure to relieve inflammation and pain in the teeth and gums. Some toothpastes contain it as an ingredient.

Conclusion 

Water molecules, aluminium, various metals other than aluminium, and sulphates make up alum, which is an inorganic substance. In its hydrated state, alum is a double salt. Liquid alum, on the other hand, helps to purify drinking water. Some of the popular examples of alum are ammonium, soda, potash, and chrome alum.