JNTU B.Pharmacy II PHARMACEUTICAL ANALYSIS-I Previous Question Paper (March-2022 - R17)

Sodium thiosulphate (Na₂S₂O₃·5H₂O) is not a primary standard because it is hygroscopic and reacts with dissolved CO₂ and microorganisms. It must be prepared as an approximately 0.1 M solution and standardized against a primary standard like potassium iodate (KIO₃) or potassium dichromate (K₂Cr₂O₇).

Preparation: Dissolve approximately 25 g of sodium thiosulphate pentahydrate in 1000 mL of freshly boiled and cooled distilled water. Add 0.1 g of sodium carbonate to stabilize the solution against bacterial decomposition.

Standardization: Weigh accurately about 0.15 g of dried potassium iodate and dissolve in 50 mL of water. Add 2 g of potassium iodide and 10 mL of 2 M sulfuric acid. The reaction produces iodine, which is titrated against the sodium thiosulphate solution using starch as an indicator until the blue color disappears.

Errors are the difference between the measured value and the true value. They are classified into three main types:

1. Determinate (Systematic) Errors: These have a definite cause and can be identified and corrected. They include:
   • Operational errors: Personal errors like loss of sample or improper handling.
   • Methodic errors: Errors due to non-ideal chemical behavior, such as incomplete reactions or solubility of precipitates.
   • Instrumental errors: Faults in equipment, such as uncalibrated glassware or faulty balances.
2. Indeterminate (Random) Errors: These arise from uncontrollable variables in the measurement process. They follow a Gaussian distribution and cannot be eliminated, but their effect can be minimized by taking multiple readings and calculating the mean.
3. Gross Errors: These are caused by human mistakes, such as misreading a burette, incorrect calculations, or sample contamination. They are usually large and require the experiment to be repeated.