Module 10: Gravimetry

• Gravimetry


  are quantitative methods that are based on determining the mass of a pure compound to which the analyte is chemically related.
• Gavimetry


  analysis method  that is based on mass
• Gravimetry

   Advantages: It provides a little room for instrumental error and does not require a series of  standards for calculation of an unknown:
• Gravimetry

  Disadvantage: It usually provides only for the analysis of a single element, or a limited group  of elements at a time.
• Gravimetry


  Principles: Solution reaction between analytes and reagents to give sparingly soluble  products; filtration, drying or ignition of precipitates; electrolytic deposition of metals;  weighing. Precipitation method is the most common.  
• Gravimetry

  Relative precision 0.1–1%.
• Gravimetry


  Disadvantages: Requires careful and time-consuming procedures with scrupulously clean  apparatus and very accurate weighing. Coprecipitation cannot always be avoided.
• Gravimetry


  Advantages: does not required series of standard and inexpensive.
• Steps in Gravimetric Analysis

  1. Preparation of sample
  2. Precipitation
  3. Digestion
  4. Filtration & Washing
  5. Drying or ignition
  6. Weighing
  7. Calculation
• Drying or Ignition

  removing solvent (water); ignite in 110-120°C for 1-2 hours (60-120 minutes)
• Precipitation gravimetry


  the analyte is converted to a sparingly soluble precipitate.
• Properties of precipitate in Precipitation gravimetry

  • easy to filter
  • free of contaminants
  • insoluble
  • concentration to pure compounds
  • large particles
• Mechanism of Precipitate Formation

  • Nucleation - a process in which a minimum number of atoms, ions,  or molecules join together to give a stable solid.
  • Further precipitation then is governed by the competition between  additional nucleation and growth of existing nuclei (particle growth).
• Colloidal Precipitates

  Coagulation of Colloids - Coagulation can be hastened by heating, by stirring, and by  adding an electrolyte to the medium.

  
  

  Adsorption - ions are retained on the surface of a solid.

  
  

  Absorption - retention of a substance within the pores of a solid.

  
  

  Reprecipitation - effective way to minimize the effects of adsorption. In this process, the filtered solid is redissolved and reprecipitated.
• Peptization


  is the process by which a coagulated colloid reverts to its original dispersed state.
• Coprecipitation


  the process when otherwise soluble compounds are removed from solution during precipitate formation.
• Post-precipitation


  happens after the experiment.
• Isomorphic Inclusion


  substitution of the precipitate lattice with impurity ions of  similar crystallinity .
• Non-isomorphic inclusion


  solid solution of the impurity within the precipitate.
• Homogeneous precipitation


  a process in which a precipitate is formed by slow  generation of a precipitating reagent homogeneously throughout a solution.
• Volatilization gravimetry


  the analyte is separated from other constituents of  a sample by converting it to a gas of known chemical composition.
• Electrogravimetry


  the analyte is separated by deposition on an electrode by  an electrical current.
• Thermogravimetric


  method of thermal analysis in which changes in physical  and chemical properties are measured as a function of increasing temperature or  as a function of time.
• Precipitation gravimetry


  type of gravimetric analysis where the analyte is separated from a solution of the sample as a precipitate.
• Colloidal suspension


  whose tiny particles are invisible to the naked eye (10-7 to 10-4 cm in diameter).
• Crystalline  suspension


  temporary dispersion of such particles in the liquid phase.
• Tyndall effect


  the phenomenon where particles of colloidal dimensions scatter visible radiation.
• Supersaturated solution


  unstable solution that contains a higher solute  concentration than a saturated solution.
• Surface adsorption


  a common source of coprecipitation and is likely to cause significant contamination of precipitates with large specific surface areas.
• Mixed-crystal formation


  a type of coprecipitation in which a contaminant ion replaces an ion in the lattice of a crystal.
• Occlusion 


  type of coprecipitation in which a compound is trapped within a pocket formed during rapid crystal growth.
• Mechanical entrapment 


  it occurs when crystals lie close together during growth.