Practical Steps for Precipitation in Chemistry

Study Notes

Precipitation: A step in chemical analysis to form a solid (precipitate) from a solution.
Digestion: Allowing the precipitate to stand in the presence of the solution it came from (mother liquor) to promote coagulation and clumping.
Filtration: Separating the precipitate from the liquid.
Washing: Removing impurities from the precipitate using a suitable wash liquid.
- Washing liquid properties:
  - Shouldn't dissolve the precipitate or hydrolyze it.
  - Shouldn't cause peptization (reverse of coagulation).
  - Shouldn't cause further precipitation of impurities.
Drying or Ignition: Removing water and other volatile substances from the precipitate.
- Drying: Heating below 250°C. Not accompanied by chemical structure changes.
- Ignition: Heating above 250°C. Often accompanied by chemical structure changes.
Weighing: Determining the mass of the dry precipitate.

Filtration separates the precipitate from the solution (mother liquor).
Crystalline precipitates filter easily.
Colloidal precipitates often pass through the filter paper, a problem requiring more careful washing.

Drying: Heating the precipitate to a temperature of less than 250°C to remove water.
Ignition: Heating the precipitate to a temperature above 250°C, often causing a chemical change in the precipitate's structure.

AgCl precipitate uses acidulated water with HNO3 in Volhard's method as the washing liquid.
5% aqueous ammonia is the wash liquid used for calcium oxalate.