Candidates must be familiar with the technique of gravimetric analysis, including use of:

• an accurate electronic balance, including the tare function → a weighing boat can be placed on a balance and it can be tared before material is added

• a weighing boat

  

• weighing by difference:

  — first the weighing boat is weighed

  — then approximately the mass of substance needed is added and accurately recorded

  — then the mass is transferred and the “empty” (with residue) weighing boat is weighed

  — the accurate mass is then the mass of substance and the boat $-$ mass of the residue and the boat

• the term ‘weighing accurately approximately’ → this involves weighing out approximately the prescribed/known mass, e.g. $2 \space mg$, and accurately recording to two decimal places the final mass, e.g. $2.00 \space mg$

• heating to constant mass

  — heating a substance

  — allowing to cool in a desiccator to prevent absorption of water

  — weighing

  — repeating the steps of heating, cooling and weighing until no further changes in mass are observed

→ Some reasons why the results obtained from this experimental method might vary include impurities being present in the substance, the substance not being fully dry or not completely heated to a constant mass, or some water being reabsorbed between heating to a constant mass and taking the measurements (the substance might not have been fully cooled in the desiccator).

Gravimetric analysis is used to determine the mass of an element or compound in a substance. This is a more accurate method than volumetric analysis.

→ The substance is converted into another substance of known chemical composition, which can be readily isolated and purified.

→ The conversion can occur either through precipitation or volatilisation.

In precipitation conversion the substance undergoes a precipitation reaction. The precipitate is separated from the filtrate and the filtrate tested to ensure the reaction has gone to completion. The precipitate is washed, dried to constant mass and then weighed.

In volatilisation conversion the substance is heated and any volatile products (often water) are evaporated. The substance is heated to constant mass and the final mass recorded.