Gravimetric Analysis

Gravimetric Analysis is analysis based on the determination of mass. It involves precipitating and weighing products to give mainly quantitative but also qualitative results. A precipitate is a solid formed when two solutions are mixed.

Steps of a basic gravimetric analysis experimental design involve:

  1. Weighing the original sample i.e. Fertiliser
  2. Blending the sample or more commonly, dissolving the sample in H2O
  3. Adding solution to form a precipitate.
  4. Filter and wash the precipitate to get rid of residual ions.
  5. Dry and weigh till constant mass is achieved.

Water Content

As the procedure of performing gravimetric analysis often requires the precipitation of solids out of solution, the weight of residual amounts of water may disguise the actual weight of the product. Heating to constant mass is a procedure used to determine the percentage of water in the sample and to find the actual weight of the product. It is carried out by heating the sample at 110°C (to ensure that water is boiled off) until there is no change in mass. If this step is foregone in the experimental design of gravimetric analysis, the weight of the sample would be too high and so will the overall result.

The empirical formula indicates the simplest whole number ratio of elements in a compound. The molecular formula indicates the actual number of atoms in a compounds.

The amount of limiting reagent will determine the amount of product formed. This is done by multiplying the amount of limiting reagent with the mole ratio to your desired product/reactant.

Possible reasons for error for gravimetric analysis

Results too low:

  • Incomplete precipitation
  • Loss of precipitation

Results too high:

  • Sample was not washed
  • Sample was not dried.