anachem midterms

• Gravimetric methods of analysis are based on measurement of mass
• Two major types of Gravimetric Methods

  • Precipitation Methods
  • Volatilization Methods
• Example of precipitation method usage:

  The determination of Calcium in natural waters
• In volatilization methods the analyte or its decomposition products are volatilized at a suitable temperature. The volatile product is then collected, weighed, or alternatively, the mass of the product is determined indirectly from the loss in mass of the sample
• In precipitation method, analyte is converted to a sparingly soluble precipitate and the said precipitate is then filtered,washed free of impurities, and converted to a product of known composition by suitable heat treatment then the product is weighed.
• Example of volatilization method usage:

  The determination of Sodium Hydrogen Carbonate content of antacid tablets
• In gravimetric methods of analysis, the ideal precipitating reagent would react with the analyte to give a product that is:

  • Readily filtered and washed free of contaminants
  • Of sufficiently low solubility so that no significant loss of the solid occurs during filtration and washing
  • Unreactive with constituents of the atmosphere
  • Of known composition after it is dried or, if necessary, ignited.
• Precipitates made up of large particles are generally desirable in gravimetric work because large particles are easy to filter and wash free of impurities.
• Two Types of Particles:

  • Colloidal Suspension
  • Crystalline Suspension
• show no tendency to settle from solution, nor are they easily filtered.

  Colloidal Particles
• A colloid is a solid made up of particles having diameters that are

  less than 10-4 cm
• These particles tend to settle spontaneously and are readily filtered

  Crystalline particles
• FACTORS THAT AFFECTS PARTICLE SIZE

  Precipitate solubility

  Temperature

  Reactant concentrations

  Rate at which reactants are mixed
• The particle size is related to relative supersaturation wherein:

  • Q is the concentration of the solute at any instant and S is its equilibrium solubility.
  • When (Q - S)/S is large, the precipitate tends to be colloidal; when (Q - S)/S is small, a crystalline solid is more likely

  
• FORMATION OF PRECIPITATES

  • Nucleation
  • Particle Growth
• process in which a minimum number of atoms, ions, or molecules join together to produce a stable solid.


  nucleation
• believed to increase enormously with increasing relative supersaturation.

  rate of nucleation
• Further precipitation then involves a competition between additional nucleation and growth on existing nuclei


  Particle growth
• only moderately enhanced by high relative supersaturations.

  rate of particle growth