gravimetric analysis

• Gravimetric Analysis


  A group of analytical methods in which the amount of analyte is determined by the measurement of the mass of a pure substance containing the analyte
• Gravimetric Analysis is an ABSOLUTE method
• Steps Involved In Gravimetric Analysis

  1. Preparation of the solution

  2. Precipitation

  3. Digestion

  4. Filtration

  5. Washing

  6. Drying/Ignition

  7. Weighing

  8. Calculation
• Preparation of the Sample

  • Preliminary separation to separate potential interferences before precipitating analyte
  • Adjustment of solution condition (pH/temp/vol/concentration of test substance)
• Precipitation


  • The precipitating reagent is added at a concentration that favors the formation of a "good" precipitate
  • A large excess of precipitant should be avoided because this increases chances of adsorption on precipitate
  • No new precipitate should be formed after addition of drop of precipitating agent
• Digestion


  • Let precipitate stand in contact with the solution from which it was precipitated usually at high temperature
  • Small particles tend to dissolve and precipitate on the surfaces of the larger crystals
• Filtration


  • Precipitate is separated from its mother solution
  • Filtration media used: filter papers, filter pulps, filter mats, crucible fitted with porous plate (Sintered glass filters)
• Washing


  • Co-precipitated impurities, especially those on surface, removed by washing the precipitate
• Drying or Ignition


  • To remove solvent and wash electrolytes, it is heated at 110 to 120°C for 1 to 2 hrs
• Weighing


  • After the precipitate is allowed to cool, it is weighed (in the crucible) using a properly calibrated analytical balance instrument
• Types Of Gravimetric Analysis

  • Precipitation
  • Volatilization
• Example for Precipitation


  • Calcium can be determined gravimetrically by precipitation of calcium oxalate and ignition of the oxalate ion to calcium oxide
• Example of volatilization


  • Water can be separated from most inorganic compounds by ignition, the evolved water can then be absorbed
• Advantages of Gravimetric Analysis


  • Accurate and precise
  • Avoids problems with temperature fluctuations, calibration errors, and other problems associated with volumetric analysis
• Disadvantages of Gravimetric Analysis

  • Proper lab technique is critical
  • Time consuming
  • Coprecipitation is a major drawback