Chem 32 Ch2

• A gravimetric method of analysis is based on the measurement of the weight of the analyte or of a compound of known composition that contains the analyte.
• Gravimetry is still used today because it is accurate and precise, an absolute method, and the apparatus required is relatively inexpensive.
• Types of Gravimetry:

  1.       Volatilization Methods

  2.       Extraction Methods

  3.       Electrogravimetry

  4.       Precipitation Methods
• In Volatilization Methods, analysis is based upon the mass of the volatilized substance (direct) or upon mass of the non-volatile residue (indirect).
• Extraction Methods Examples:

  1.       Extraction from Solid

  -          Soxhlet Extraction

  -          Goldfisch Extraction

  2.       Extraction from Liquid

  -          Liquid-Liquid Extraction
• Indirect methods involve removing impurities from a solution containing the desired component so that only the pure component remains as a precipitate. The precipitated material can be filtered off, washed with water, dried, weighed, and then dissolved again if necessary.
• In Electrogravimetry, the mass of the product deposited provides a measure of the analyte concentration. It is also useful in the analysis of metal cations.
• Criteria for successful precipitation:

  -          The desired substance must be completely precipitated

  -          The weighed form of the product should be of known composition

  -          The product should be “pure”, stable and easily filtered
• Steps common to all Gravimetric Methods:

  1.       Preparation of the Solution

  2.       Precipitation

  3.       Digestion

  4.       Filtration/Washing

  5.       Drying and/or Ignition

  6.       Weighing

  7.       Calculation
• Phases of Crystallization:

  A.      Nucleation

  B.      Particle Growth

  
• Characteristics of a Desirable Precipitate:

  -          Can be readily filtered and washed free of contaminants

  -          Low solubility

  -          Unreactive with constituents of the atmosphere

  -          Of known composition after it is ignited
• Types of Precipitates:

  1.       Colloidal Particles

  2. Crystalline Precipitate
• Classification:

  
• Explain:

  
• Factors Affecting the Particle Size:

  -          Solubility of precipitate

  -          Temperature

  -          Reactant

  -          Rate of Addition and Mixing of Reagents
• Von Weimarn Ratio/Equation

         Q = concentration of species at any instant

         S = equilibrium solubility of the substance which precipitates

         Q – S = supersaturation

                RS = relative supersaturation

  
• A colloidal solid is generally encountered when it has such a low solubility that S in RS remains negligible relative to Q hence RS remains high throughout precipitate formation and a colloidal suspension result.
• If Q > S, then the precipitation process will proceed until all reactants are consumed; if Q < S, no precipitation occurs because there is insufficient excess reagent present to overcome the solubility of the precipitate.
• Optimum conditions for precipitation:

  1.       Conduct precipitation from hot solutions (affects S)

  2.       Dilute solutions (affects Q)

  3.       Slow addition of precipitating agent with good stirring

  4.       Control of pH (affects S)

  5.       Digestion of precipitates

  
• Rules on Selectivity of Ion Adsorption

  1.       Solubility Effect (Paneth-Fajans-Hahn Rule) – lowest solubility will be adsorbed

  2.       Concentration Effect – greater concentration will be adsorbed

  3.       Common-Ion Effect – precipitate adsorb ion common to it

  4.       Ion Charge Effect – multi-charged ion will be adsorbed

  5.       Ion Size Effect – ion that is nearly the same size as the lattice ion will be adsorbed
• Why do colloidal suspensions not settle?

  -          The double layer keeps the colloidal precipitate particles from coming into contact with each other for further coagulation.

  -          They are too stable because the colloids are either all (-) or all (+) and thus repel one another

  
• Two ways to increase the probability of coagulation:

  1.       By heating and stirring

  2.       Increasing the electrolyte concentration of the solution
• Contamination in Precipitate Formation

  1.       Coprecipitation – inclusion, occlusion, and surface impurities

  2.       Post-precipitation – insoluble compound precipitates after the analytical precipitate has been formed.
• Coprecipitation

  a.       Inclusion – random incorporation of foreign ions or molecules throughout the crystal.

  b.       Occlusion – impurities are trapped within a growing crystal.

  c.       Surface impurities – impurities are adsorbed onto the surface

   

  
• Alternative Gravimetry Technique:

  1.       Homogenous Precipitation – the precipitating agent is generated slowly by chemical reaction in the analyte solution

  Purpose:

  o   Precipitant appears gradually throughout

  o   Keeps relative saturation low

  o   Formation of larger, less contaminated particles
• The Gravimetric Factor