GRAVIMETRIC ANALYSIS

• Gravimetric


  Based on the measurement of mass
• Types of gravimetric methods

  • Precipitation methods
  • Volatilization methods
• Precipitation methods

  • Analyte is converted to a sparingly soluble precipitate
  • Precipitate is filtered, washed free of impurities, and converted to a product of known composition by suitable heat treatment
  • The product is weighed
• Volatilization methods


  • Analyte or its decomposition products are volatilized at a suitable temperature
  • The volatile product is then collected and weighed, or the mass of the product is determined indirectly from the loss in mass of the sample
• Properties of an ideal precipitating reagent


  • Readily filtered and washed free of contaminants
  • Sufficiently low solubility so that no significant loss of the solid occurs during filtration and washing
  • Unreactive with constituents of the atmosphere
  • Of known composition after it is dried or, if necessary, ignited
• Particle size and filterability of precipitates

  • Large particles are generally desirable as they are easy to filter and wash free of impurities
  • Colloidal particles show no tendency to settle from solution and are not easily filtered
  • Particles of a crystalline suspension tend to settle spontaneously and are readily filtered
• Factors affecting particle size

  • Precipitate solubility
  • Temperature
  • Reactant concentrations
  • Rate at which reactants are mixed
• Relative supersaturation

  • Q is the concentration of the solute at any instant and S is its equilibrium solubility
  • When (Q - S)/S is large, the precipitate tends to be colloidal
  • When (Q - S)/S is small, a crystalline solid is more likely
• Formation of precipitates

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

  2. Particle growth: further precipitation then involves a competition between additional nucleation and growth on existing nuclei
• Controlling particle size for crystalline precipitates

  • Elevate the temperature to increase the solubility of the precipitate, dilute solutions, and slow addition of the precipitating agent with good stirring
  • Larger particles can also be obtained by pH control, provided the solubility of the precipitate depends on pH
  • Digestion of crystalline precipitates (without stirring) for some time after formation frequently yields a purer, more filterable product
• Controlling particle size for colloidal precipitates

  • Coagulate, or agglomerate, the individual particles to give a filterable, amorphous mass that will settle out of solution
  • Adsorption is a process in which a substance is held on the surface of a solid
  • 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 is a process by which a coagulated colloid returns to its dispersed state
• Coprecipitation


  A 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
• Surface adsorption


  • A common source of coprecipitation that is likely to cause significant contamination of precipitates with large specific surface areas, coagulated colloids
  • Coagulation of a colloid does not significantly decrease the amount of adsorption
  • The net effect is the carrying down of an otherwise soluble compound as a surface contaminant
• Minimizing adsorbed impurities on colloids

  • Digestion: a process in which a precipitate is heated without stirring for an hour or more
  • Reprecipitation: a drastic but effective way to minimize the effects of adsorption
• Mixed-crystal formation

  • One of the ions in the crystal lattice of a solid is replaced by an ion of another element
  • The extent of mixed-crystal contamination is governed by the law of mass action and increases as the ratio of contaminant to analyte concentration increases
• Occlusion and mechanical entrapment

  • Occlusion is 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
  • Both are at a minimum when the rate of precipitate formation is low, under conditions of low supersaturation
  • Digestion is often remarkably helpful in reducing these types of coprecipitation
• Homogeneous precipitation


  • A process in which a precipitate is formed by slow generation of a precipitating reagent homogeneously throughout a solution
  • Homogeneously formed precipitates, both colloidal and crystalline, are better suited for analysis than a solid formed by direct addition of a precipitating reagent
• Drying and ignition of precipitates

  • After filtration, a gravimetric precipitate is heated until its mass becomes constant
  • Some precipitates are also ignited to decompose the solid and form a compound of known composition, called the weighing form
• Calculating results from gravimetric data


  The results are generally computed from the mass of sample and the mass of a product of known composition
• Calculating results from gravimetric data

  • Determination of Calcium in Natural Waters
  • Determination of Sodium Hydrogen Carbonate content of antacid tablets
• Calculating results from gravimetric data - example 1

  1. Step 1: Determine the mass of CaO

  2. Step 2: Calculate the number of moles of Ca in the sample

  3. Step 3: Calculate the concentration of Ca in the water per 100 mL
• Calculating results from gravimetric data - example 2

  1. Step 1: Calculate the number of moles of Fe2O3

  2. Step 2: Calculate the number of moles of Fe present

  3. Step 3: Calculate the %Fe present

  4. Step 4: Calculate the %Fe3O4
• Applications of gravimetric methods


  • Inorganic anions and cations
  • Organic substances
  • Do not require calibration or standardization
• Inorganic precipitating agents


  • Form slightly soluble salts or hydrous oxides with the analyte
  • Not very selective
• Organic precipitating agents

  • Coordination compounds: form slightly soluble nonionic products
  • Ionic compounds: form products in which the bonding between the inorganic species and the reagent is largely ionic
• Coordination compound organic precipitating agents

  • Contain at least 2 functional groups, each capable of bonding with a cation by donating a pair of electrons
  • Typically form five- or six-membered rings
  • Examples: 8-hydroxyquinoline, dimethylglyoxime, sodium tetraphenyl borate
• 1. Hydroxyquinoline


  • The solubilities of its metal chelates vary widely from cation to cation and are pH-dependent
• Dimethylglyoxime


  • An organic precipitating agent of unparalleled specificity
  • Has an exasperating tendency to creep up the sides of the container as it is filtered and washed
• Sodium tetraphenylborate


  • An important example of an organic precipitating reagent that forms salt-like precipitates
• Volatilization methods

  • Determination of water
  • Determination of carbon dioxide
  • Determination of sulfides and sulfates