experiment 3

• Gravimetry is a classical method in analytical chemistry involving the conversion of ionic compounds into their insoluble forms using precipitating agents
• Solid particles formed through gravimetry can be quantified and analyzed using their mass alone
• Gravimetry is considered one of the most accurate analytical methods since the 18th century
• Theodore William Richards used the concept of gravimetry to weigh the atomic masses of 25 elements up to the sixth decimal place, leading to his Nobel Prize award in 1914
• Precipitating agents used in gravimetric analysis must:

  • Be specific towards one analyte
  • Have a very low solubility/ion-product constant
  • Be chemically stable
  • Have a known stoichiometric ratio after drying or ignition
• Analytes often form crystalline precipitates upon reacting with a precipitating agent like Ag+ reacting with Cl- to form AgCl
• Colloidal precipitates are harder to precipitate out and filter, but can be coagulated or agglomerated by adding heat with constant stirring or by adding electrolytes to enhance colloidal adsorption
• In the experiment, iron from an unknown solution is converted into iron (III) oxide by reacting it with ammonia
• The hydrous oxide form of iron may exist as a gelatinous colloid, which can be converted into precipitates using an electrolyte solution and further reprecipitation
• After collecting the precipitate, the hydrous oxide can be ignited in a constant-weighed crucible forming Fe2O3
• The precipitates must be isolated from the solution and collected for ignition to ensure accurate results
• The constant weight of the crucible must be accurately determined as the mass of the precipitate will be obtained by weighing the crucible
• All mass measurements must be done at room temperature
• The experiment is divided into two parts that can be done simultaneously and might consume two laboratory sessions:

  PART 1: Constant weighing of crucibles

  • Heat the crucible with cover until red-hot, cool in a desiccator, and weigh it thrice to determine the "constant mass"

  PART 2: Precipitating iron solution

  • Convert the unknown amount of Fe3+ into Fe2O3 by reacting with ammonia, filtering, washing, dissolving in HCl, reprecipitating, and igniting the product