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. Procedure: 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). Example: 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. Solution: 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 |