Introduction
Gravimetric analysis is a quantitative analytical technique used to determine the amount of an analyte by precipitation and weighing. It is one of the most accurate and precise methods in analytical chemistry, widely used for pharmaceutical, environmental, and industrial applications.
Principle of Gravimetric Analysis
The principle of gravimetric analysis is based on the conversion of an analyte into an insoluble precipitate, which is then filtered, dried, and weighed. The amount of the analyte is calculated from the mass of the precipitate using stoichiometric relationships.
Basic Steps in Gravimetric Analysis
- Precipitation: The analyte is reacted with a precipitating agent to form an insoluble compound.
- Digestion: The precipitate is allowed to settle and grow to improve purity.
- Filtration: The precipitate is separated from the solution using filtration.
- Washing: Impurities are removed from the precipitate by washing with suitable solvents.
- Drying or Ignition: The precipitate is dried or ignited to a constant weight.
- Weighing: The dry precipitate is weighed, and the analyte concentration is calculated.
Steps Involved in Gravimetric Analysis
1. Precipitation
- A precipitating agent is added to form an insoluble compound with the analyte.
- The reaction should be selective and complete, ensuring that only the desired compound precipitates.
- Example: Chloride ions (Cl−) are precipitated as silver chloride (AgCl) using silver nitrate (AgNO3).
2. Digestion (Ostwald Ripening)
- The precipitate is left undisturbed in the hot solution for some time.
- This allows the small particles to recrystallize, improving purity and filterability.
3. Filtration
- The precipitate is separated from the solution using filter paper or a Gooch crucible.
- Vacuum filtration speeds up the process.
4. Washing
- The precipitate is washed with a suitable solvent (e.g., distilled water, dilute acids, or alcohol).
- This removes any adsorbed impurities.
5. Drying or Ignition
- The precipitate is dried in an oven at a specific temperature or ignited in a muffle furnace to convert it into a stable form.
- Example: Calcium oxalate (CaC2O4) is ignited to form calcium oxide (CaO).
6. Weighing and Calculation
- The final mass of the dried precipitate is measured using an analytical balance.
- The amount of analyte is calculated using stoichiometric relationships.
Types of Gravimetric Analysis
- Precipitation Gravimetry: The analyte is precipitated and weighed (e.g., AgCl for chloride determination).
- Volatilization Gravimetry: Analyte is converted into a gas and weighed indirectly (e.g., water content determination).
- Electrogravimetry: The analyte is deposited on an electrode and weighed (e.g., metal plating analysis).
- Thermogravimetric Analysis (TGA): Weight change is measured as temperature changes.
Advantages of Gravimetric Analysis
- Highly accurate and precise
- Does not require expensive instruments
- Applicable to a wide range of elements and compounds
Limitations
- Time-consuming process
- Requires highly pure reagents and careful handling
- Interference from co-precipitation and adsorption
Conclusion
Gravimetric analysis is a classical, reliable, and highly precise method for quantitative chemical analysis. It involves precipitation, filtration, drying, and weighing to determine the analyte concentration. Despite its time-consuming nature, it remains a gold standard in analytical chemistry.