In pharmaceutical analysis, the estimation of sodium chloride (NaCl) can be done through precipitation titrations.
Precipitation titrations are a type of volumetric analysis where a precipitating agent is added to the sample to cause the formation of a solid precipitate.
The endpoint of the titration is reached when all of the analyte has reacted to form the precipitate, and this can be detected using an appropriate indicator or by observing a change in some physical property.
Here are the steps involved in estimating sodium chloride using a precipitation titration:
Reagents and Equipment:
Sample: The pharmaceutical sample containing sodium chloride.
Standard Solution: A standardized solution of a suitable precipitating agent. For the estimation of sodium chloride, a silver nitrate (AgNO3) solution is commonly used as a titrant.
Indicator: Potassium chromate (K₂CrO₄), which forms a reddish-brown precipitate (Ag₂CrO₄) at the endpoint.
Glassware: Burettes, pipettes, beakers, and Erlenmeyer flasks for accurate measurements.
Stirring rod: To facilitate mixing.
Procedure:
1. Preparation of the Sample:
Weigh a known amount of the pharmaceutical sample containing sodium chloride.
Dissolve it in distilled water in a beaker.
2. Add Indicator:
Add a few drops of potassium chromate indicator to the solution.
The solution will turn yellow due to the presence of chromate ions.
3. Titration:
Titrate the sample solution with standardized AgNO₃ solution using a burette.
Silver ions (Ag⁺) react with chloride ions (Cl⁻) to form a white precipitate of silver chloride (AgCl).
Reaction:
AgNO₃ + NaCl → AgCl + NaNO₃
Continue adding AgNO₃ until the endpoint is reached, which is indicated by a color change from yellow to reddish-brown due to the formation of silver chromate (Ag₂CrO₄).
4. Note the Volume:
Record the volume of AgNO₃ solution used to reach the endpoint.
5. Calculations:
Use the following formula to calculate the amount of NaCl in the sample:
Amount of NaCl (g) = (Volume of AgNO3 solution (mL) × Normality of AgNO3 solution × Molecular weight of NaCl) / 1000
6. Express Results:
Report the result as the percentage of sodium chloride in the pharmaceutical sample.