JNTU B.Pharmacy I PHARMACEUTICAL ANALYSIS Previous Question Paper (October-2020 - R17)

A primary standard is a highly pure, stable, and non-hygroscopic substance that can be weighed directly to prepare a solution of known concentration. It must have a high molar mass to minimize weighing errors, be readily available, and react stoichiometrically with the analyte. Examples include anhydrous sodium carbonate (Na2CO3) for acid-base titrations, potassium hydrogen phthalate (KHP) for base standardization, and potassium dichromate (K2Cr2O7) for redox titrations.

A secondary standard is a substance whose concentration cannot be determined by direct weighing because it is not sufficiently pure, is hygroscopic, or is unstable in air. Its concentration must be determined by titration against a primary standard. Examples include sodium hydroxide (NaOH), which absorbs moisture and CO2 from the atmosphere, and hydrochloric acid (HCl), which is volatile and not available in a pure, stable form.

To prepare 100 mL of 0.1M Oxalic acid solution, first calculate the required mass. The molar mass of oxalic acid dihydrate (H2C2O4·2H2O) is 126.07 g/mol. The mass required for 100 mL of 0.1M solution is calculated as:

Standardization procedure:

1. Accurately weigh 1.2607 g of pure oxalic acid dihydrate and dissolve it in distilled water in a 100 mL volumetric flask, making up the volume to the mark.
2. To standardize, pipette 20 mL of the prepared oxalic acid solution into a conical flask and add a few drops of phenolphthalein indicator.
3. Titrate the solution against a previously prepared 0.1M sodium hydroxide (NaOH) solution from a burette until a permanent faint pink color appears.
4. Record the volume of NaOH used and calculate the exact molarity of the oxalic acid using the equation: M1V1/n1 = M2V2/n2, where n1 and n2 are the stoichiometric coefficients from the balanced reaction (1:2 ratio).