Gravimetric analysis is a collection of quantitative analysis laboratory techniques based on the measurement of an analyte's mass.
One example of a gravimetric analysis technique can be used to determine the amount of an ion in a solution by dissolving a known amount of a compound containing the ion in a solvent to separate the ion from its compound. The ion is then precipitated or evaporated out of solution and weighed. This form of gravimetric analysis is called precipitation gravimetry.
Another form of gravimetric analysis is volatization gravimetry. In this technique, compounds in a mixture are separated by heating them to chemically decompose the specimen. Volatile compounds are vaporized and lost (or collected), leading to a measurable reduction on the mass of the solid or liquid sample.
Precipitation Gravimetric Analysis Example
In order for gravimetric analysis to be useful, certain conditions must be met:
- The ion of interest must fully precipitate from solution.
- The precipitate must be a pure compound.
- It must be possible to filter the precipitate.
Of course, there is error in such an analysis! Perhaps not all of the ion will precipitate. They may be impurities collected during filtration. Some sample may be lost during the filtration process, either because it passes through the filter or else is not recovered from the filtration medium.
As an example, silver, lead, or mercury may be used to determine chlorine because these metals for insoluble chloride. Sodium, on the other hand, forms a chloride that dissolves in water rather than precipitates.
Steps of Gravimetric Analysis
Careful measurements are necessary for this type of analysis. It's important to drive away any water that may be attracted to a compound.
- Place an unknown in a weigh bottle that has its lid cracked open. Dry the bottle and sample in an oven to remove water. Cool the sample in a desiccator.
- Indirectly weigh a mass of the unknown in a beaker.
- Dissolve the unknown to produce a solution.
- Add a precipitating agent to the solution. You may wish to heat the solution, as this increases the particle size of the precipitate, reducing loss during filtration. Heating the solution is called digestion.
- Use vacuum filtration to filter the solution.
- Dry and weigh the collected precipitate.
- Use stoichiometry based on the balanced chemical equation to find the mass of the ion of interest. Determine the mass percent of the analyte by dividing the mass of analyte by mass of unknown.
For example, using silver to find an unknown chloride, a calculation might be:
Mass of dry unknown chloride: 0.0984
Mass of AgCl precipitate: 0.2290
Since one mole of AgCl contains one mole of Cl- ions:
(0.2290 g AgCl)/(143.323 g/mol) = 1.598 x 10-3 mol AgCl
(1.598 x 10-3)x(35.453 g/mol Cl) = 0.0566 g Cl (0.566 g Cl)/(0.0984 g sample) x 100% = 57.57% Cl in unknown sample
Note lead would have been another option for the analysis. However, if lead had been used, the calculation would have needed to account for the fact one mole of PbCl2 contains two moles of chloride. Also note, error would have been greater using lead because lead is not completely insoluble. A small quantity of chloride would have remained in solution instead of precipitating.