To Prepare a Pure Sample of Potash Alum

In medicines, potassium alum is primarily used as an astringent (or styptic) and antiseptic. Styptic pencils are rods made of potassium alum or aluminium sulphate that are used topically to reduce bleeding in minor cuts and abrasions, nosebleeds, and haemorrhoids and relieve pain from stings and bites. After shaving, potassium alum blocks are rubbed over wet skin.

Potassium alum is also used topically to treat pimples and acne, as well as to cauterise aphthous ulcers in the mouth and canker sores because it has a drying effect on the area and reduces irritation. It has been used to stop bleeding in cases of hemorrhagic cystitis and treat hyperhidrosis in some countries.

Table of Contents

• Aim of the Experiment

• Apparatus Required

• Theory

• Procedure

• Observations

• Result

• Precautions

Aim of the Experiment

To prepare potash alum and submit it for inspection.

Apparatus Required

• Two Beakers 

• China-Dish 

• Funnel 

• Funnel-Stand 

• Glass-Rod 

• Wash-Bottle 

• Tripod Stand 

• Wire-Gauze 

• Potassium Sulphate

• Aluminium Sulphate 

• Dilute Sulphuric Acid

Theory

Potash alum is made by dissolving an equimolar mixture of hydrated aluminium sulphate and potassium sulphate in a small amount of water containing a little sulphuric acid and then crystallising the resulting solution, which produces octahedral crystals of potash alum.

Procedure

• Take a beaker with a capacity of 250 mL. After washing, add 2.5 g potassium sulphate crystals to it. Pour in about 20 mL of water. Stir the crystals to dissolve them. If necessary, warm.

• Transfer 10 g aluminium sulphate crystals to the other 250 ml beaker after washing it with water. To prevent aluminium sulphate hydrolysis, add about 20 mL of water and 1 mL of dilute sulphuric acid. Five minutes on high heat If the milkiness persists, filter the solution.

• Place the china dish on a wire gauze over a burner and combine the two solutions. Using a glass rod, stir the solution. Concentrate the solution until it reaches the crystallisation point. Place the dish on top of a beaker filled with cold water.

• Potash alum crystals separate quickly. Remove the mother liquor and rinse the crystals with a small amount of ice-cold water.

• Place the crystals between filter paper pads or spread them over a porous plate to dry.

Observations

Result

Potash alum crystals are octahedral in shape.

Precautions

• To get good crystals, slowly cool the solution.

• Do not disturb the solution when cooling.

Lab Manual Questions

1. How do you test the purity of alum?

Ans. The first test is to find the melting temperature of the compound and compare this value with the accepted (published) value for alum (92.5°C). The second test determines the water of hydration present in the alum crystals. The third test is a chemical test to determine the per cent sulfate in your sample of alum.

2. What happens when you add KOH to alum?

Ans. A wispy, gelatinous precipitate will form upon adding a small amount of KOH to the aqueous alum solution. Further addition of KOH will cause the precipitate to re-dissolve.

3. What happens when alum is mixed with water as a coagulant?

Ans. When alum is added to water, it reacts with the alkalinity present in water. It leads to the formation of a sticky gelatinous precipitate of Aluminium hydroxide, which attracts fine suspended impurities in water over its surface and gets easily settled in the following sedimentation process.

4. Why is alum the most preferred coagulant?

Ans. Alum is preferred over other coagulants because it reduces the taste and odour in addition to turbidity and produces lighter flocs that can be broken easily. The ferric sulphate and ferric chloride so formed are called Chlorinated Copperas.

Viva Questions

1. Which process can purify the sample of potash alum?

Ans. Crystallisation can be used to purify the sample of potash alum. 

2. Why do we use distilled water in potash alum?

Ans. The alum used for purifying water is known as potash alum. Potash alum is generally used to remove the hardness of the water. Alum precipitates the insoluble colloidal materials in the water and kills or lowers the total bacterial content of the water.

3. How do you separate alum from water?

Ans. In crystallisation, the alum is heated. During heating, the pure alum separates from its solution of impurities in the form of a crystal.

4. Why is potash alum called double salt?

Ans. Potash alum is double potassium sulphate of aluminium, i.e. a double salt, as it contains more than one cation. 

5. Where is alum found?

Ans. Alum is a mineral found in nature in both pure and impure forms. It is a mineral salt. It is obtained from the soil ore found in Nepal, Bihar, Punjab, and the Kathiawar area. 

6. What is the role of alum in water purification?

Ans. Aluminium sulfate or alum is used as a flocculant to remove colour and turbidity from water supplies. The alum promotes the coagulation of fine particles. Once coagulated, they sediment down.

7. Is alum basic or acidic?

Ans. The aqueous solution of potash alum is acidic because it is a double salt of strong acid.

8. What happens when alum is heated?

Ans. On heating potash alum, it losses water of crystallisation. When heated they liquefy and if the heating is continued, the water of crystallization is driven off. 

9. Is alum a mineral?

Ans. Yes. Alum is a hydrous potassium aluminium sulphate mineral.

Practical Based Questions

1. Potash alum formula is

(a) $${{K}_{2}{S}{O}_{4}{.}{Al}_{2}{(}{S}{O}_{4}{)}_{3}{.}{24}{H}_{2}{O}}$$

(b) $${{K}{H}{S}{O}_{4}{.}{Al}_{2}{(}{S}{O}_{4}{)}_{3}{.}{24}{H}_{2}{O}}$$

(c) $${{K}_{2}{S}{O}_{4}{.}{Al}_{2}{(}{S}{O}_{4}{)}{3}{.}{21}{H}_{2}{O}}$$

(d) None of the above

Answer: (a)

2. What is the common name of the potash alum?

(a) Alum

(b) Fitkari

(c) Both (a) and (b)

(d) None of the above

Answer: (c)

3. What is the colour of the potash alum?

(a) White

(b) Colourless

(c) Off-white

(d) None of the above

Answer: (b) 

4. What is the IUPAC name of the potash alum?

(a) Aluminium potassium sulphate dodecahydrate

(b) Alumio potassium sulphate dodecahydrate

(c) Potassium aluminium sulphite dodecahydrate

(d) Potassium alumino sulphite dodecahydrate

Answer: (a)

5. Potash alum is a

(a) Simple salt     

(b) Complex salt

(c) Acid salt        

(d) Double salt

Answer: (d)

6. Potash alum is used in the purification of water because

(a)It kills the microorganisms

(b)It precipitates the colloidal matter

(c)It removes the hardness of water

(d)It catalyses the removal of impurities

Answer:(b)

7. Which one of the following is not a use of potash alum? 

(a)As a styptic in arresting bleeding 

(b)As a pesticide 

(c)As a mordant in dyeing 

(d)As coagulant for colloidal clay in water

Answer: (b)

8. The molar mass of the potash alum is

(a)474.3884 g/mol

(b)305.3884 g/mol

(c)400.3884 g/mol

(d)200.3884 g/mol

Answer: (a)

9. What quantity of potash-alum is required to prepare 500ml solution having strength 1.5F?

(a)711 gm

(b)355.5 gm

(c)35.55 gm

(d)71.1 gm

Answer: (b)

10. Potash alum is used as

(a)Water softener

(b)Mordant

(c)Stop bleeding

(d)Drying agent

Answer: (a)

Conclusion

Potash alum, also known as potassium sulphate dodecahydrate, is a chemical compound that is widely used. It is double salt that is commonly used in medicine and water treatment. In medicines, potassium alum is primarily used as an astringent (or styptic) and antiseptic. Styptic pencils are rods made of potassium alum that is used topically to reduce bleeding in minor cuts and abrasions, nosebleeds, and haemorrhoids and relieve pain from stings and bites. After shaving, potassium alum blocks are rubbed over wet skin.

Potash alum is made by dissolving an equimolar mixture of hydrated aluminium sulphate and potassium sulphate in a small amount of water containing a little sulphuric acid and then crystallising the resulting solution, which produces octahedral crystals of potash alum.