Gravimetry 2

1
Gravimetry 2

• The nature of a good precipitate

2
Gravimetry 2

• The nature of a good precipitate
• the precipitate forms quantitatively
• the compound formed is very insoluble
• it is easily separated from its solution, usually
  by filtration
• the precipitate is of high purity, ie does not
  trap impurities
• the mass of the precipitate is much greater than
  that of the species to be determined

3
Gravimetry
4
Gravimetry 2

• The nature of a good precipitate contd
• solution conditions are selected to make a
  crystalline material
• hot solution often helps to grow large pure
• crystals (small crystals dissolve and material
  grows on the larger particles)
• slow release of a reagent is also useful
• after filtration washing must be done
  carefully
• appropriate drying is needed for success

5
Gravimetry 2

• The Gravimetric Factor
• the mass of the precipitate is much greater than
  that of the species determined
• Example Sodium analysis
• Product - NaZn(UO2)3 (CH3COO)9.6H2O
• Molar Mass of product 1539
• Molar Mass - Na 22.99
• The Gravimetric Factor 10/669
• The product mass provides a magnifying effect,
• but of course we want the element mass

6
Gravimetry 2

• Gravimetry
• An ideal other techniques try to emulate
• A slow technique largely outmoded
• Difficult to apply to trace analysis
• Considered too expensive in most industrial
  laboratories