ANALYTICAL CHEMISTRY

• Gravimetric Methods of Analysis


  Methods of analysis where the analyte is converted to a product of known composition which is then weighed
• Two Major Types of Gravimetric Methods of Analysis

  • Precipitation methods
  • Volatilization Methods
• Precipitation Methods


  1. Weighing the sample

  2. Moisture determination

  3. Conversion of the analyte to a sparingly soluble salt (precipitate)

  4. Filtration of the precipitate

  5. Washing the precipitate

  6. Drying or ignition of the precipitate (conversion to a form suitable for weighing)

  7. Weighing of the converted form of the precipitate
• Precipitate


  An aggregate of atoms, ions or molecules greater than 10–4 cm
• Colloidal particles


  Electrically charged particles 10–7 to 10–4 cm in diameter that resist combination to form larger particles
• Ideal gravimetric precipitating agent


  • Reacts specifically or selectively with the analyte
  • Gives a product that is readily filtered and washed free of contaminants
  • Gives a product of sufficiently low solubility so that no significant loss of the analyte occurs during filtration and washing
  • Gives a product that is unreactive with the constituents of the atmosphere
  • Gives a product of known composition after it is dried, or if necessary ignited
• Relative supersaturation


  The ratio of the concentration of a species at any instant (Q) to its equilibrium solubility (S)
• Relative supersaturation is high

  Precipitate tends to be colloidal
• Relative supersaturation is low


  A crystalline solid is more likely to form
• Mechanism of Precipitate Formation

  1. Nucleation - a few ions, atoms or molecules come together to form a stable solid

  2. Particle Growth - further precipitation involves competition between additional nucleation and growth on existing nuclei
• When precipitation occurs at high relative supersaturation, nucleation is the major mechanism and a large number of small particles are formed
• When precipitation occurs at low relative supersaturation, particle growth tends to predominate and deposition of solid on existing particles occurs to the exclusion of further nucleation, resulting in a crystalline suspension
• Conditions for Analytical Precipitation

  • Use dilute solutions (low Q)
  • Add reagents slowly with effective stirring (low Q)
  • Precipitate from hot solution (high S since precipitates have higher solubility at higher temperature)
  • For precipitates whose ions come from weak acids, employ as low a pH as possible for complete precipitation (low Q)
  • Use complexing agents (high S)
• Coagulation/Agglomeration/Flocculation


  The process of converting a colloidal suspension into a filterable solid
• Paneth-Fajans-Hahn Rule


  The ions that are preferentially adsorbed on the surface of a crystal lattice are those which are common to the lattice and are in excess
• Colloidal particles are charged

  They are stabilized by an electric double layer consisting of a primary adsorbed layer and a counter-ion layer
• Coagulation of a colloidal suspension

  Can be brought about by heating and/or increasing electrolyte concentration, which decreases the thickness of the double layer and allows the particles to approach and coagulate
• Peptization


  The process by which a coagulated colloid reverts to its original dispersed state, usually by washing away the electrolyte that caused the coagulation
• Kinetic energy


  Energy of motion
• Barrier to close approach

  Double layer
• Electrolyte concentration


  Increased to shrink counter-ion layer and allow particles to approach
• Peptization


  Process by which a coagulated colloid reverts to its original dispersed state
• Peptization of Colloids


  Washing removes electrolyte, increasing volume of counter-ion layer, reestablishing repulsive forces and detaching particles
• Digestion


  Heating a precipitate in the solution it was formed in, to lose weakly bound water and form a denser mass
• Crystalline precipitates


  • Generally more easily filtered and purified than coagulated colloids
  • Size and filterability can be controlled
• Digestion of crystalline precipitates

  Dissolution and recrystallization at elevated temperature yields a purer, more filterable product
• Methods of Improving Particle Size and Filterability

  1. Minimizing Q by using dilute solutions and slow addition

  2. Maximizing S by precipitating from hot solution or adjusting pH
• Coprecipitation


  Removal of otherwise soluble compounds from solution during precipitate formation
• Types of Coprecipitation


  • Surface adsorption
  • Mixed-crystal formation
  • Occlusion
  • Mechanical entrapment
• Methods for Minimizing Adsorbed Impurities on Colloids

  1. Digestion

  2. Washing with volatile electrolyte

  3. Reprecipitation
• At low supersaturation


  Occlusion and mechanical entrapment are minimized, digestion is helpful
• Surface adsorption and mixed crystal formation


  Are equilibrium processes, occlusion and mechanical entrapment arise from kinetics
• Coprecipitation error


  Impurities may cause negative or positive errors in analysis
• Positive errors always result if contaminant is not a compound of the ion being determined
• When contaminant does not contain the ion being determined, either positive or negative errors may be observed
• Precipitation from homogeneous solution

  Precipitation agent is generated slowly, keeping relative supersaturation low
• Homogeneously formed precipitates are better suited to analysis than solids formed by direct addition
• Drying and ignition of precipitates


  Heating removes solvent and decomposes solids to form a compound of known composition
• Factors affecting particle size

  • Colloidal: high relative supersaturation, nucleation
  • Crystalline: low relative supersaturation, particle growth
• Units of measurement


  • Mass: kilogram, kg
  • Length: meter, m
  • Time: second, s
  • Temperature: Kelvin, K
  • Amount of substance: mole, mol
  • Electric current: ampere, A
  • Luminous intensity: candela, cd