Gravimetric

• Gravimetric methods


  Quantitative methods that are based on determining the mass of a pure compound to which the analyte is chemically related
• Precipitation gravimetry


  • The analyte is separated from a solution of the sample as a precipitate and is converted to a compound of known composition that can be weighed
• Volatilization gravimetry


  • The analyte is separated from other constituents of a sample by converting it to a gas of known chemical composition that can be weighed
• Electrogravimetry


  • The analyte is separated by deposition on an electrode by an electrical current
• Precipitation gravimetry


  1. The analyte is converted to a sparingly soluble precipitate

  2. The precipitate is filtered, washed free of impurities

  3. The precipitate is converted to a product of known composition by suitable heat treatment

  4. The precipitate is weighed
• Specific reagents


  React only with a single chemical species
• Selective reagents


  React with a limited number of species
• Ion


  Dissolved, diameter ~10-8 cm
• Colloid


  Suspended, diameter 10-7~10-4 cm
• Particle


  Settled from solution (filterable), diameter >10-4 cm
• Relative supersaturation (RSS)


  (Q - S)/S, where Q is the concentration of the solute at any instant and S is the concentration solute at equilibrium
• Relative supersaturation (RSS)


  • Large RSS - Nucleation dominates, smaller particles (colloidal)
  • Small RSS - Particle growth dominates, larger particles (crystalline)
• Nucleation


  The initial formation process in which a minimum number of atoms, ions, or molecules join together to give a stable solid
• Particle growth


  The subsequent growth after nucleation
• Coagulation


  The process of agglomerating individual colloidal particles into a filterable, amorphous mass that will settle out of solution
• Adsorption


  A process in which a substance (gas, liquid, or solid) is held on the surface of a solid
• Absorption


  A process in which a substance is taken into the pores of a solid
• Colloidal suspensions are stable because all of the particles of the colloid are either positively or negatively charged and thus repel one another</b>
• Coagulation of a colloidal suspension can often be brought about by a short period of heating, particularly if accompanied by stirring
• An even more effective way to coagulate a colloid is to increase the electrolyte concentration of the solution
• Peptization


  The process by which a coagulated colloid reverts to its original dispersed colloidal suspension state
• Coagulation


  The process by which a colloid is made to precipitate out of suspension
• Coagulation of a colloid

  1. Increase the electrolyte concentration of the solution

  2. Adds a suitable ionic compound to the colloidal suspension

  3. Increases the concentration of counter-ions in the vicinity of each particle

  4. Decreases the volume of solution containing sufficient counter-ions to balance the charge of the primary adsorption layer

  5. Shrinks the counter-ion layer around the particles

  6. Allows the particles to approach one another more closely and agglomerate
• Peptization of Colloids


  1. Washing the coagulated colloid removes some of the electrolyte responsible for coagulation

  2. Repulsive forces and original colloidal state are reestablished
• Digestion


  1. Heating a coagulated colloid in the solution from which it was formed (the mother liquor) and allowing it to stand

  2. Weakly bound water is lost from the precipitate

  3. Results in a denser mass that is easier to filter
• Crystalline precipitates are generally more easily filtered and purified than coagulated colloids
• Improving particle size of crystalline solids

  1. Minimizing Q (using dilute solutions, adding precipitating reagent slowly with good mixing)

  2. Maximizing S (precipitating from hot solution or by adjusting the pH)
• Digestion improves the purity and filterability of both colloidal and crystalline precipitates
• Coprecipitation


  A process in which normally soluble compounds are carried out of solution by a precipitate, resulting in impurities within the desired precipitates
• Types of Coprecipitation


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


  The impurity is chemically or physically adsorbed onto the surface of precipitates
• Mixed-Crystal Formation


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


  A type of coprecipitation in which a compound (foreign ions in the counter-ion layer) is physically trapped within a precipitate during rapid precipitate formation
• Mechanical Entrapment


  A type of coprecipitation in which coprecipitated physically trap a pocket of solution within a precipitate during rapid precipitate formation
• Mixed-crystal formation may occur in both colloidal and crystalline precipitates, but occlusion and mechanical entrapment are confined to crystalline precipitates
• Homogeneous Precipitation


  1. A process in which a precipitate is formed by slow generation of a precipitating reagent homogeneously throughout a solution

  2. Solids formed are generally purer and larger in size
• Homogeneous Precipitation Examples


  • Using urea to generate OH- as precipitant for Al(III), Fe(III)
  • Using NH2SO3H to generate SO4^2- as precipitant
• Heating a gravimetric precipitate removes the solvent and any volatile species carried down with the precipitate
• Some precipitates are ignited to decompose the solid and form a compound of known composition (the weighing form)
• Gravimetric Factor


  An algebraic expression that converts grams of a compound into grams of a single element, calculated as the ratio of the formula weight of the substance being sought to that of the substance weighed