6. Gravimetric Methods

• Gravimetric methods


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


  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


  Analyte is separated from other constituents of a sample by converting it to a gas of known chemical composition. The mass of the gas serves as a measure of the analyte concentration
• Electrogravimetry


  Analyte is separated by deposition on an electrode by an electrical current. The mass of this product provides a measure of the analyte concentration
• Gravimetric titrimetry

  Mass of a reagent of known concentration required to react completely with the analyte provides the information needed to determine the analyte concentration
• Atomic mass spectrometry


  Uses a mass spectrometer to separate the gaseous ions formed from the elements making up a sample of matter. The concentration of the resulting ions is then determined by measuring the electrical current produced when they fall on the surface of an ion detector
• Precipitation Gravimetry

  General steps:

  1. Conversion of analyte to soluble precipitate

  2. Filtration of precipitate

  3. Washing of precipitate to free it from impurities

  4. Conversion of precipitate to a product of known composition by suitable heat treatment

  5. Weighing of precipitate

  6. Computation of the amount of analyte
• Determination of Calcium in Water by the Association of Official Analytical Chemists

  An excess of oxalic acid, H2C2O4, is added to an aqueous solution of the sample. Ammonia is then added, which neutralizes the acid and causes essentially all of the calcium in the sample to precipitate as calcium oxalate.

  2. The precipitate is filtered using a weighed filtering crucible.

  3. The precipitate is washed.

  4. The precipitate is dried and ignited.

  5. After cooling, the crucible and precipitate are weighed.

  6. The mass of calcium oxide is determined by subtracting the known mass of the crucible. The calcium content of the sample is computed.
• Properties of precipitating reagents and precipitate

  • It should react specifically or at least selectively with the analyte

  2. The product should be:

  A. Easily filtered and washed free of contaminants

  B. Of sufficiently low solubility that no significant loss of the analyte occurs during filtration and washing

  C. Unreactive with constituents of the atmosphere

  D. Of known chemical composition after it is dried or if necessary, ignited
• Particle size and filterability of precipitates

  • - Precipitates consisting of large particles are generally desirable for gravimetric work because these particles are easy to filter and wash free of impurities and are usually purer than are precipitates made up of fine particles
  • Colloidal suspensions: solid particles with diameters that are less than 10-4 cm; show no tendency to settle from solution and are difficult to filter
  • Crystalline suspensions: particles with dimensions on the order of tenths of a millimeter or greater; tend to settle spontaneously and are easily filtered
• Factors that determine the particle size of precipitates

  • - Precipitate solubility
  • Temperature
  • Reactant concentrations
  • Rate at which reactants are mixed
• Mechanism of precipitate formation

  Nucleation: a process in which a minimum number of atoms, ions, or molecules join together to give a stable solid

  2. Particle Growth: growth of existing nuclei
• If nucleation predominates, a precipitate containing a large number of small particles results, and if growth predominates, a smaller number of larger particles is produced
• At high relative supersaturation, nucleation is the major precipitation mechanism, and a large number of small particles is formed
• At low relative supersaturations, the rate of particle growth tends to predominate, and deposition of solid on existing particles occurs rather than further nucleation. Low relative supersaturation produces crystalline suspensions
• Experimental control of particle size

  • - Elevated temperatures to increase the solubility of the precipitate
  • Dilute solutions (to minimize Q)
  • Slow addition of the precipitating agent with good stirring (to minimize the concentration of the solute (Q) at any given instant)
  • If the solubility of the precipitate depends on pH, larger particles can also be produced by controlling pH (use of either Sodium hydroxide or Hydrochloric acid)
• Coagulation of Colloids

  • Colloidal suspensions are stable because all of the particles of the colloid are either positively or negatively charged and thus repel one another
  • The process by which ions are retained on the surface of a solid is known as adsorption. The adsorption of ions on an ionic solid originates from the normal bonding forces that are responsible for crystal growth
  • Colloidal suspensions can often be coagulated by heating, stirring, and/or the addition of an electrolyte
• Colloidal precipitates have a primary adsorption layer consisting mainly of adsorbed ions, and a counter-ion layer of excess opposite charge ions in the surrounding solution. This electric double layer imparts stability to the colloidal suspension
• Primary adsorption layer

  Positively charged layer on colloidal particle
• Counter-ion layer


  Layer of solution with excess anions that balances the charge on the surface of the particle
• Electric double layer


  Primarily adsorbed ions and counter-ion layer that imparts stability to the colloidal suspension
• Peptization


  1. Process by which a coagulated colloid reverts to its original dispersed state

  2. Occurs during washing of precipitates to remove contaminants

  3. To avoid, wash with a solution containing a volatile electrolyte
• Digestion


  Heating a precipitate in the solution from which it was formed and allowing it to stand
• Colloids


  • Best precipitated from hot, stirred solutions containing sufficient electrolyte to ensure coagulation
• Methods of improving particle size and filterability

  1. Use dilute solutions

  2. Add precipitating agent slowly with good mixing

  3. Precipitate from hot solution

  4. Adjust pH of precipitating medium

  5. Digest without stirring
• Coprecipitation


  Process in which normally soluble compounds are carried out of solution by a precipitate
• Types of coprecipitation

  • Surface adsorption
  • Mixed-crystal formation
  • Occlusion
  • Mechanical entrapment
• Regardless of the method of treatment, a coagulated colloid is always contaminated to some degree, even after extensive washing
• Reprecipitation

  Filtered solid is redissolved and reprecipitated to minimize effects of adsorption
• The first precipitate usually carries down only a fraction of the contaminant present in the original solvent
• Methods for homogeneous generation of precipitating agents

  • Urea
  • Trimethyl phosphate
  • Ethyl oxalate
  • Dimethyl sulfate
  • Trichloroacetic acid
  • Tioacetamide
  • Biacetyl + hydroxylamine
  • 8-Acetoxyquinoline
• Inorganic precipitating agents

  • Ammonia
  • Hydrogen sulfide
  • Ammonium sulfide
  • Ammonium phosphate
  • Sulfuric acid
  • Hexachloroplatinic acid
  • Oxalic acid
  • Ammonium molybdate
• Common inorganic precipitating agents

  • NH3(aq)
  • H2S
  • (NH4)2S
  • (NH4)2HPO4
  • H2SO4
  • H2PtCl6
  • H2C2O4
  • (NH4)2MoO4
  • HCl
  • AgNO3
  • (NH4)2CO3
  • NH4SCN
  • NaHCO3
  • HNO3
  • H5IO6
  • NaCl, Pb(NO3)2
  • BaCl2
  • MgCl2, NH4Cl
• Inorganic precipitating agents


  • They typically form slightly soluble salts or hydrous oxides with the analyte
  • Few inorganic reagents are selective