LESSON 3 - PRECIPITATIONS (ANACHEM LAB)

• Categories of chemical analysis

  • Classical analysis
  • Instrumental analysis
• Classical analysis


  • Uses no mechanical or electronic instruments other than a balance
• Instrumental analysis


  • Involves the use of an instrument, other than a balance, to perform the analysis
• Types of quantitative analyses

  • Gravimetric analysis
  • Volumetric analysis
• Gravimetric analysis


  Relies on a critical mass measurement
• Volumetric analysis


  Involves the measurement of the volume of a solution whose concentration is known and applied to determine the concentration of the analyte
• Precipitation reaction


  If two soluble compounds containing ions that form an insoluble solid are mixed in solution, the insoluble compound may or may not separate from the solution depending upon the concentration of the ions
• Precipitate


  A solid that forms from a solution
• Precipitant


  A chemical that causes a solid to form in a liquid solution
• Methods to recover a precipitate

  • Filtration
  • Centrifugation
  • Decantation
• Crystalline precipitates


  • Well-defined / regularly shaped crystals whose shapes are a function of the composition of the salt
  • Examples: calcium oxalate, alkaline earth sulfates
• Gelatinous precipitates


  • Flocculated colloids; jelly-like, hydrous masses
  • Trap a great deal of water and many impurities
  • Very difficult to work with
  • The cation & anion react to form a soluble colloid and then coagulate to filterable-sized particles
  • Examples: metal hydroxides such as ferric hydroxide
• Curdy precipitates


  • Aggregates of small porous particles
  • The cation & anion react to form a soluble colloid and then coagulate to filterable-sized particles
  • When reasonable in size, they are filtered rapidly, easily and completely
  • Examples: silver halides compounds
• Example of Crystalline

  • calcium oxalate
  • alkaline earth sulfates
• Example of Gelatinous

  • Metal Hydroxides
• Example of Curdy

  • silver halides compounds
• Paper or Cellulose Filter

  • Particle retention level can go down to 2.5
• Glass Microfibre Filter

  • Particle retention level goes down to the submicron range
• Membrane Filter

  • Particle retention level can go down as low as 0.02