Gravimetric Analysis

• Gravimetric Analysis

  An analytical technique used to determine the mass of an element or compound in a substance which involves accurately weighing substances
• What are the two main weighing techniques used?

  Tared weighing and weighing by difference
• Describe tared weighing

  Put weighing boat on balance and press tare zero button

  Weigh out required mass of substance
• Describe weighing by difference

  Place beaker on balance and record mass (don't zero!)

  Weigh out required mass of substance (record mass of beaker and substance)

  Transfer solid to reaction vessel and then reweigh beaker (record mass of beaker and residue)

  Mass of solid used in reaction = (mass of beaker + substance)-(mass of beaker and residue)
• Precipitation

  A solution of the substance being analysed is converted into an insoluble product (precipitate) by the addition of a precipitating reagent. A precipitating reagent itself must be soluble but will react with the substance being analysed to produce a precipitate

  The precipitate can be separated from the filtrate (solution) by filtration. The filtrate is tested with more of the precipitating reagent to ensure the reaction has gone to completion. The precipitate is washed, dried to constant mass and then weighed.
• Volatilisation

  The substance being heated is done so by a crucible (and lid).

  In volatilisation conversion, the substance is heated and any volatile products (often water) are evaporated.

  After heating, the crucible and lid are transferred to a desiccator.

  Once cooled the crucible and contents are reweighed.

  The substance is heated to constant mass and the final mass is recorded.
• In gravimetric 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.
• "Weigh accurately approximately" means to weigh out as close as possible to a certain mass but ensure that you record the actual mass given on the balance.