Gravimetric analysis

Gravimetric analysis can be used to determine the concentration of an unknown chloride solution or the percentage by mass of an unknown chloride salt.

A common method is to add an excess of acidified silver nitrate to a solution of the unknown salt to form a silver chloride precipitate. After heating and leaving to cool, the solution and precipitate is filtered, washed and left to dry. Measuring the mass of the silver chloride can be used to determine the original mass of chloride ions in the solution. This video from SpaceyScience demonstrates the technique. 

Questions you can ask your students:

• Why does the precipitate change colour after heating and exposure to light?
• What are the advantages of filtering under vacuum rather than simply using gravity?
• What might lead to the mass of precipitate being greater than it ‘should’ be?
• Why are the washings tested with hydrochloric acid to see if a precipitate forms?
• What reactants might you use for the gravimetric analysis of an unknown alkali metal carbonate?

It’s important when working with folded filter paper in a funnel that students do not fill the funnel above the height of the filter paper as some of the solution will simply escape down the funnel without passing through the filter paper. There are two key errors that can contribute to the recorded mass of the precipitate being too large. Excess water through inadequate drying will increase the mass of the iron, as well as any unreacted silver. Testing the washings with hydrochloric acid to see if a precipitate forms determines whether unreacted silver is still being flushed from the precipitate. Once the washings no longer form a precipitate, it’s a safe bet to conclude that all excess silver ions have been removed. In the case of unknown carbonates, adding calcium chloride to the unknown solution will precipitate calcium carbonate and the reaction method can proceed as above.

Another common gravimetric scenario is determining the water of crystallisation of a known sulfate compound, such as magnesium or copper(II) sulfate. The following video from Malmesbury Education highlights one such technique.

A final gravimetric technique, which is more for context and interest, is thermogravimetric analysis, where the mass of a known sample is monitored as the temperature rises. This can be used to observe the thermal decomposition of a solid or changes to the water of crystallisation with regards to temperature.