Gravimetric

• What are the properties of an ideal precipitating reagent?

  1. Readily filtered and washed
  2. Low solubility to prevent loss
  3. Unreactive with atmosphere
  4. Known composition after drying or ignition
• Why are large particles desirable in gravimetric work?

  They are easy to filter and wash
• What are the steps involved in volatilization methods?

  • Volatilize analyte or its decomposition products
  • Collect and weigh the volatile product
  • Alternatively, determine mass from loss in sample
• What is a colloid?

  A solid with particles less than $10^{-4} \text{ cm}$
• What are gravimetric methods of analysis based on?

  Measurement of mass
• What are the two major types of gravimetric methods?

  Precipitation methods and volatilization methods
• What are gravimetric methods of analysis based on?

  Measurement of mass
• What are the steps involved in precipitation methods?

  • Convert analyte to a sparingly soluble precipitate
  • Filter and wash the precipitate
  • Convert to a product of known composition
  • Weigh the product
• What are the two major types of gravimetric methods?

  Precipitation methods and volatilization methods
• What are the steps involved in volatilization methods?

  • Volatilize analyte or its decomposition products
  • Collect and weigh the volatile product
  • Alternatively, determine mass from loss in sample
• What are the steps involved in precipitation methods?

  • Convert analyte to a sparingly soluble precipitate
  • Filter and wash the precipitate
  • Heat to convert to a known composition
  • Weigh the final product
• How do particles in a crystalline suspension behave?

  They tend to settle spontaneously
• What factors affect particle size in precipitates?

  • Precipitate solubility
  • Temperature
  • Reactant concentrations
  • Rate of mixing reactants
• What are the properties of an ideal precipitating reagent?

  1. Readily filtered and washed
  2. Low solubility to prevent loss
  3. Unreactive with atmosphere
  4. Known composition after drying or ignition
• What does the term relative supersaturation refer to?

  It relates to solute concentration and equilibrium solubility
• What happens when \((Q - S)/S\) is large?

  Precipitate tends to be colloidal
• Why are large particles desirable in gravimetric work?

  They are easy to filter and wash
• What is nucleation?

  Formation of a stable solid from atoms or ions
• How does relative supersaturation affect nucleation rate?

  It increases with higher relative supersaturation
• What are the steps involved in controlling particle size for crystalline precipitates?

  • Elevate temperature to increase solubility
  • Use dilute solutions
  • Slow addition of precipitating agent with stirring
  • Control pH if it affects solubility
• What methods can be used to control particle size of colloidal precipitates?

  • Coagulate or agglomerate particles
  • Use adsorption to minimize impurities
  • Heat with stirring to promote coagulation
  • Increase electrolyte concentration
• What is a colloid?

  A solid with particles less than $10^{-4} \text{ cm}$
• What is peptization?

  Return of a coagulated colloid to dispersed state
• What is coprecipitation?

  • Process where soluble compounds are carried out by a precipitate
  • Can lead to contamination of precipitates
• How do particles in a crystalline suspension behave?

  They tend to settle spontaneously
• What is surface adsorption in coprecipitation?

  Adsorption causing contamination of precipitates
• What factors affect particle size in precipitates?

  • Precipitate solubility
  • Temperature
  • Reactant concentrations
  • Rate of reactant mixing
• What are methods to minimize adsorbed impurities on colloids?

  • Digestion: heating without stirring
  • Reprecipitation: double precipitation method
• What is mixed-crystal formation?

  • Replacement of one ion in a crystal lattice
  • Governed by the law of mass action
• What does the term relative supersaturation refer to?

  It relates to solute concentration and equilibrium solubility
• What are occlusion and mechanical entrapment?

  • Occlusion: trapping of compounds during rapid growth
  • Mechanical entrapment: crystals lying close during growth
• How can occlusion and mechanical entrapment be minimized?

  • Low rate of precipitate formation
  • Digestion after formation
• What happens when (Q - S)/S is large?

  Precipitate tends to be colloidal
• What is nucleation?

  Joining of atoms to produce a stable solid
• What is homogeneous precipitation?

  Formation of precipitate by slow generation of reagent
• How does relative supersaturation affect nucleation?

  Increases the rate of nucleation
• What happens after filtration of precipitates?

  • Precipitate is heated until mass is constant
  • Some are ignited to form a known composition
• What is involved in particle growth after nucleation?

  • Competition between nucleation and growth
  • Growth is moderately enhanced by supersaturation
• How can crystalline precipitates be controlled?

  • Elevate temperature to increase solubility
  • Dilute solutions
  • Slow addition of precipitating agent
  • Control pH if necessary
• How are results computed from gravimetric analysis?

  • Based on mass of sample and product
  • Direct calculation from experimental data