CHAPTER 4

• Gravimetric analysis


  • One of the most accurate and precise methods of macro-quantitative analysis
  • The analyte is selectively converted into an insoluble form (precipitate)
  • The separated precipitate is then dried or ignited, possibly to another form and is accurately weighed
  • From the weight of the precipitate and knowledge of its chemical composition, we can calculate the weight of the analyte in the desired form
• Properties of the ideal precipitate in gravimetric analysis

  • Sufficiently insoluble (the precipitate is of such low solubility that losses from dissolution are negligible)
  • Easily filterable (crystals of large particle size so as not to pass through the filtering system)
  • Very pure (less possibility that the precipitates carry some of the other constituents of the solution with them)
  • Should possess a known composition (known chemical structure)
• Steps of Gravimetric Analysis

  1. Precipitation

  2. Digestion

  3. Filtration

  4. Washing

  5. Drying or Ignition

  6. Weighing

  7. Calculations
• Supersaturation


  When a solution contains a lot of dissolved ions more than it can accommodate, it is in an unstable state and this favours rapid nucleation to form a large number of small particles
• Types of impurities in precipitates


  • Occlusion
  • Inclusion (Isomorphous Replacement)
  • Surface Adsorption
  • Post Precipitation
• Peptization


  The process of forming stable colloids by using an electrolyte to break up a precipitate and disperse it into the colloid
• Gravimetric factor


  The ratio (nMwanalyte/mMwppt) that relates the moles of the product finally weighed to the moles of analyte
• Gravimetric analysis applied to different samples

  1. Orthophosphate (PO4^3-) determined by weighing as ammonium phosphomolybdate, (NH4)PO4.12MoO3

  2. Manganese content in an ore determined by converting the manganese to Mn3O4 and weighing it

  3. Piperazine content in an impure commercial material determined by precipitating and weighing the diacetate