cont.

• Mass spectrometer


  Used to separate gases formed from the elements making up a complex matter
• Precipitates


  • Large particles are more suitable for gravimetric work because they are easier to filter and give a purer product than fine particles
• Factors that determine the particle size of precipitates

  • Colloidal dispersion
  • Crystalline suspension
• Colloidal dispersion


  Particles visible to the naked eye (10 to 10^-6 cm in diameter), difficult to filter, show no tendency to settle from solution
• Crystalline suspension


  Tend to settle spontaneously, easily filtered
• Relative supersaturation


  Q/S, where Q is the concentration of the solute at any instant and S is the equilibrium solubility
• Mechanism of precipitate formation

  1. Nucleation

  2. Particle growth
• Colloidal precipitates


  Too small to be retained by filters
• Peptization of colloids


  The process by which a coagulated colloid reverts to its original dispersed state
• Crystalline precipitates are generally more easily filtered and purified than pre-coagulated colloids
• Types of coprecipitation


  • Surface adsorption
  • Mixed crystal formation
  • Occlusion
  • Mechanical entrapment
• Surface adsorption


  A process in which normally soluble compounds are carried out of solution by a precipitate
• Mixed crystal formation


  One of the ions in the crystal lattice of a solid is replaced by an ion of another element
• Occlusion


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


  Occurs when crystals lie close together during growth
• Coprecipitation effects are at a minimum when the rate of precipitate formation is low, that is, under conditions of low supersaturation
• Homogeneous precipitation


  A process in which a precipitate is formed by slow generation of a precipitating reagent