Gravimetric analysis is any procedure in which the masses of substances are measured in order to determine the composition of a sample.
Three main types:
•Determining the degree of hydration of a substance by heating a sample to drive off water.
•Determining the molecular formula of a gas by measuring the masses of reactants and products in a gaseous reaction.
•Determining the composition of a sample by precipitating a component of the sample.
1. Weigh the sample.
2. Heat the sample in an oven at 110 degrees Celcius (to drive off water).
3. Cool in a dessicator (allows further drying out).
4. Reweigh. (Repeat steps 2-4 until the weight is constant).
A sample of canned food was heated to determine its water content. Using the results below, determine the original % water present in the food.
Initial mass of food 500g
After heating 136g
After reheating 125g
After reheating 125g.
Final weight is when there is no more mass loss.
Work out mass of water lost (original mass minus final mass).
Mass of water = 500-125 = 375g.
•% water in sample by mass = Mass of water x 100
initial mass of sample
% Water loss= 375/500 x100= 75%.
Gravimetric analysis often involves the formation of a precipitate. It is the precipitate that is weighed and dried.
A precipitate is an insoluble substance- solid suspended in a liquid.
Precipitation equations are written as an ionic equations- the spectator ions are removed.
Eg. Ag+(aq) + Cl-(aq) --> AgCl(s)
If a precipitate is to be formed as part of a gravimetric analysis it must have certain properties:
•It must be insoluble
•It must be the only precipitate that forms.
•It must not decompose during the process
•It must have a known formula.
Some typical precipitates used:
•AgCl for analysis of Cl- in foods
•Fe2O3 for analysis of Fe in ores
•BaSO4 for analysis of S in fertilisers
•Mg2P2O7 for analysis of P in washing powders.
Dessicator used to dry any solids.
Diagram courtesy of
E. Generalic, http://glossary.periodni.com/glossary.php?en=desiccator
Vacuum flask used to collect precipitates.
Diagram courtesy of E.Generalic, http://glossary.periodni.com/glossary.php?en=vacuum+filtration