JAWAHARLAL NEHRU TECHNOLOGICAL UNIVERSITY HYDERABAD B.Pharmacy II Year PHARMACEUTICAL ANALYSIS-I Previous Question Paper (March-2021 - R17)

Sodium thiosulphate is a secondary standard and must be standardized against a primary standard like potassium iodate or potassium dichromate. It is typically prepared as a 0.1 N solution by dissolving the calculated amount of sodium thiosulphate pentahydrate in freshly boiled and cooled distilled water to prevent microbial degradation and remove dissolved carbon dioxide.

Preparation of 0.1 N Sodium Thiosulphate:

1. Calculate the amount required: The equivalent weight of Na₂S₂O₃·5H₂O is 248.18 g/mol. For 1000 mL of 0.1 N solution, weigh approximately 24.82 g of the salt.
2. Dissolve the salt in 1000 mL of freshly boiled and cooled distilled water.
3. Add 0.1 g of sodium carbonate as a stabilizer to maintain a slightly alkaline pH, which prevents the growth of sulfur-oxidizing bacteria and inhibits decomposition.
4. Store in a cool, dark place in an amber-colored glass bottle.

Standardization using Potassium Iodate:

The standardization is based on the reaction between potassium iodate and excess potassium iodide in an acidic medium to liberate iodine, which is then titrated with the sodium thiosulphate solution.

1. Accurately weigh about 0.15 g of dried potassium iodate and dissolve it in 50 mL of water in a conical flask.
2. Add 2 g of potassium iodide and 10 mL of 2 M hydrochloric acid.
3. Titrate the liberated iodine immediately with the sodium thiosulphate solution until the solution becomes pale yellow.
4. Add 2 mL of starch indicator; the solution turns blue.
5. Continue the titration dropwise until the blue color disappears, indicating the endpoint.
6. Calculate the exact normality using the formula: N₁V₁ = N₂V₂, where N₁ and V₁ are the normality and volume of sodium thiosulphate, and N₂ and V₂ are the normality and volume of potassium iodate.

Potassium permanganate (KMnO₄) is a powerful oxidizing agent used in permanganometry. It is not a primary standard because it is difficult to obtain in a high state of purity and often contains traces of manganese dioxide (MnO₂), which catalyzes its own decomposition. Therefore, a solution of approximately 2M concentration is prepared and subsequently standardized against a primary standard like sodium oxalate.

Preparation of 2M Potassium Permanganate Solution:

1. Calculate the required amount of KMnO₄. The molar mass of KMnO₄ is approximately 158.03 g/mol. To prepare 1000 mL of a 2M solution, 316.06 grams of KMnO₄ are required.
2. Dissolve the calculated amount of KMnO₄ in distilled water.
3. Heat the solution to boiling and maintain it for about 15-30 minutes to oxidize any organic impurities present in the water.
4. Allow the solution to cool and stand for 24-48 hours.
5. Filter the solution through a sintered glass crucible or glass wool to remove the precipitated MnO₂, as it acts as an autocatalyst for further decomposition.
6. Store the solution in a clean, amber-colored glass bottle to protect it from light.

Standardization of Potassium Permanganate Solution:

The solution is standardized against primary standard sodium oxalate (Na₂C₂O₄) in an acidic medium (using H₂SO₄).

1. Weigh accurately a known amount of dried sodium oxalate and dissolve it in dilute sulfuric acid.
2. Heat the solution to approximately 70-80°C to initiate the reaction.
3. Titrate the hot sodium oxalate solution with the prepared KMnO₄ solution from a burette until a faint permanent pink color appears, indicating the endpoint.
4. The reaction proceeds according to the equation: 2KMnO₄ + 5Na₂C₂O₄ + 8H₂SO₄ → K₂SO₄ + 2MnSO₄ + 5Na₂SO₄ + 10CO₂ + 8H₂O.
5. Calculate the exact molarity of the KMnO₄ solution using the stoichiometry of the reaction, where 2 moles of KMnO₄ react with 5 moles of sodium oxalate.