Methods used in Gravimetric Analysis
· Precipitation
· Filtration
· Washing
· Drying or ignition
· Weighing
Precipitation Method
Precipitation method is a process of weighing an element in form of precipitates which is separated by filtration from solution.
Factors affecting on precipitation method:
· Precipitates must be free from soluble impurities
· Precipitates must be insoluble in solution
· Precipitates must be readily separated from the solution by filtration
· Precipitates must be convertible into a pure compound by ignition or by simple evaporation
Example: Gravimetric analysis of a sulphate or halides
Na2SO4 + BaCl2 → BaSO4 + NaCl
NaCl + AgNO3 → AgCl + NaNO3
Filtration:
Filtration is a process of separating precipitates from the mother liquor. The media employed for filtration are:
· Filter paper
· Gooch crucible
· Porous fritted plates of resistance glace (Pyrex sintered crucible)
· Vitreosil filtering crucibles (Silica)
· Porcelain filtering crucibles (Porcelain)
The choice of filtering medium depends on nature of the precipitates and by cost factor.
Properties of filter paper:
· It should be Ash-less
· Size and diameter of filter paper should be according to the bulk of the precipitates
· Pores of filter must be smaller than the size of the particle of the precipitates.
Example: Bulky precipitates like aluminium hydroxide need a larger filter paper than dense precipitates like barium sulphate.
Washing
Precipitates must be washed with liquid to remove all soluble impurities sticking with the precipitates.
Properties of Ideal washing liquids:
· Having no solvent action on precipitates but must removes all foreign impurities
· Should not form any volatile product with precipitates
· Should easily volatile on ignition
· Should have no dispersive action on the precipitates
· Should not interfere with precipitates
If precipitates get colloid during filtration the solution used for washing must contain electrolytes e. g. ammonium nitrate solution is used for washing iron (III) hydroxide.
If precipitates tends to oxidize during filtration, then solution used for washing must reconvert the oxidize form to its original one e. g. acidified hydrogen sulphide water for copper sulphide.
If precipitates settles rapidly or is gelatinous by nature, then washing is done by decantation. Gelatinous precipitates require more washing than crystalline ones e. g. aluminium hydroxide requires more washing than calcium oxalate.
Drying or Ignition
After washing, precipitates are dried or ignited depending on the nature of precipitates and on the filtering medium.
Precipitates that are dried (below 250°C) or ignited (above 250°C) should be collected on filter paper, porcelain filtering crucibles or silica filtering crucibles. The temperature at which precipitates are dried or ignited can be determined by a study of thermogravimetry.
Thermogravimetry is a technique in which a change in the weight of a substance is recorded as a function of temperature or time. It is used in conjunction with other techniques like differential thermal analysis (DTA), gas chromatography and mass spectrometry.
Crystalline precipitates absorb water or solvent can be easily removed by heating the precipitates e. g. cuprous thiocyanate.
Weighing
Ignited sample is cooled for a few minutes and then kept in a desicator to avoid moisture adsorption and then weighed on a chemical balance.