How to Prepare Potash Alum in the Laboratory?
Preparation of Potash Alum is essential in chemistry and helps students understand various practical and theoretical applications related to this important double salt and its role in laboratory experiments, water purification, and the dyeing industry.
What is Preparation of Potash Alum in Chemistry?
A potash alum is a type of double salt with the chemical formula KAl(SO4)2·12H2O. It is also known as potassium aluminium sulphate or ‘fitkari’ in India. This concept appears in chapters related to types of salts, crystallization, and double salts, making it a foundational part of your chemistry syllabus.
Molecular Formula and Composition
The molecular formula of potash alum is KAl(SO4)2·12H2O. It consists of potassium (K+), aluminium (Al3+) and sulphate ions (SO42−) in a 1:1:2 ratio along with 12 molecules of water of crystallization. This compound belongs to the double salt class.
Preparation and Synthesis Methods
In the laboratory, preparation of potash alum involves crystallizing a concentrated solution containing equimolar amounts of potassium sulphate and aluminium sulphate with the help of dilute sulphuric acid.
Industrial preparation can use bauxite or even scrap aluminium to generate aluminium sulphate, mixed with potassium sulphate. The final mixture crystallizes on cooling, giving pure potash alum.
Physical Properties of Potash Alum
Potash alum is a colourless, crystalline solid with a characteristic sour taste. The crystals are transparent and have a typical octahedral shape. It is highly soluble in water and slightly acidic (turns blue litmus red). Melting point lies around 92–95°C (decomposes, losing water of crystallization).
Chemical Properties and Reactions
Potash alum shows properties typical of double salts: it dissociates completely in aqueous solution to give all its constituent ions. When heated, it melts and eventually decomposes to give alumina, potassium sulphate, sulphur dioxide and water.
Frequent Related Errors
- Confusing potash alum with other alums like ammonium or sodium alum.
- Writing an incorrect chemical formula or omitting the water of crystallization.
- Forgetting to add dilute sulphuric acid, leading to incomplete dissolution of aluminium sulphate.
- Disturbing the solution during crystallization resulting in poor crystal formation.
Uses of Potash Alum in Real Life
Potash alum is widely used for water purification (removes suspended particles), as an astringent after shaving, in fabric dyeing, and in medicine as a styptic. Its ability to cause flocculation makes it essential in municipal water treatment plants.
Relation with Other Chemistry Concepts
Potash alum connects with crystallization (as a practical method for purifying salts), with double salts in inorganic chemistry, and is frequently compared with simple salts in types of salts discussions.
Step-by-Step Reaction Example
1. Prepare two separate solutions: potassium sulphate in water and aluminium sulphate in water (add a few drops of dilute sulphuric acid).2. Filter both solutions to remove any insoluble particles.
3. Mix the two clear solutions in a china dish.
4. Heat gently with stirring until the solution is concentrated (reaches the crystallization point).
5. Remove from heat and cool the dish over cold water. Crystals of potash alum will form.
6. Filter and wash the crystals with ice-cold water. Dry between filter papers.
Balanced Chemical Equation:
K2SO4 (aq) + Al2(SO4)3 (aq) + 24 H2O (l) → 2 KAl(SO4)2·12H2O (s)
Lab or Experimental Tips
Always add dilute sulphuric acid when dissolving aluminium sulphate to prevent hydrolysis and improve solubility. Avoid touching the hot solution directly—use tongs.
For best crystals, cool the saturated solution slowly and don’t disturb the dish. Vedantu educators stress the importance of careful filtration for clear solutions and slow cooling for better crystallization.
Try This Yourself
- Write the IUPAC name of potash alum.
- Test the acidity of its aqueous solution using litmus paper.
- List two real-world uses of potash alum outside the lab.
- Draw an octahedral crystal shape as commonly seen in potash alum.
Final Wrap-Up
We explored preparation of potash alum—its formula, stepwise crystallization method, physical/chemical properties, and practical significance in everyday life. For detailed video lessons, exam tips, and live support, visit more learning resources on Vedantu and check related Chemistry topics for a complete understanding.
Crystallization | Alum: Definition, Types and Uses
FAQs on Preparation of Potash Alum: Lab Procedure, Formula, and Uses
1. What is the chemical formula of potash alum?
Potash alum has the chemical formula KAl(SO4)2·12H2O. It is a double salt containing potassium sulfate, aluminium sulfate, and water of crystallization.
2. How is potash alum prepared in the laboratory?
Potash alum preparation in the lab uses the crystallization method. The key steps are:
- Dissolve measured amounts of potassium sulfate and aluminium sulfate in hot water separately.
- Combine both solutions and add dilute sulphuric acid.
- Stir well, then allow the solution to cool.
- Potash alum crystals will form. Filter and dry them.
3. What are the uses of potash alum?
Potash alum is widely used due to its chemical properties. Main uses include:
- Water purification as a coagulant
- Dyeing and textile printing
- Medicinal uses as an astringent and antiseptic
- Pickling and food preservation
4. What is the appearance of potash alum crystals?
Potash alum crystals are colourless, transparent, and typically have a distinct octahedral shape. They dissolve easily in water and feel cool to the touch.
5. Why is dilute sulphuric acid added during potash alum preparation?
Dilute sulphuric acid is added to:
- Increase the solubility of the reactants, especially aluminium sulfate
- Prevent hydrolysis of aluminium ions
- Facilitate the efficient crystallization of potash alum
6. Can scrap aluminium be used for preparing potash alum?
Yes, scrap aluminium can be used. First, dissolve the scrap aluminium in sulphuric acid to form aluminium sulfate. Then, follow the standard potash alum preparation steps with potassium sulfate and crystallize the product.
7. What observations are made during the crystallization of potash alum?
During crystallization, you will observe:
- Formation of colourless, octahedral crystals
- The solution cools and becomes supersaturated
- Small, glistening alum crystals appear as water evaporates
8. How is potash alum different from other alums?
Potash alum contains potassium ions (K+) as the monovalent cation. Other alums may have sodium or ammonium ions instead. The chemical formula and some uses can differ accordingly.
9. What safety precautions should be observed during potash alum preparation?
Key safety precautions include:
- Wear protective gloves and goggles
- Handle dilute sulphuric acid with care
- Avoid inhaling fumes
- Prevent skin and eye contact with chemicals
- Work in a well-ventilated area
10. What is a double salt? How does potash alum demonstrate this property?
Double salts are compounds formed by the combination of two different salts, which crystallize together but dissociate into their respective ions in water. Potash alum shows double salt behavior as it gives tests for K+, Al3+, and SO42− ions in solution.
11. What are the balanced chemical equations for potash alum preparation?
The main reaction steps are:
- 2Al + 3H2SO4 → Al2(SO4)3 + 3H2
- K2SO4 + Al2(SO4)3 + 24H2O → 2KAl(SO4)2·12H2O
12. How is potash alum solution tested for double salt property?
To confirm the double salt nature of potash alum:
- Test for K+ ions using flame test (lilac color)
- Test for Al3+ with sodium hydroxide (white precipitate forms)
- Test for SO42− with barium chloride (white precipitate of BaSO4)
