Gravimetric Methods of Analysis
Gravimetric analysis is based on the measurement of the weight of the analyte or of a compound of known composition that contains the analyte. There are two general types of gravimetric methods: precipitation and volatilization.
Gravimetric analysis of a substance by precipitation involves isolation of an ion in a solution as a sparingly soluble precipitate that either has a known composition or can be converted to a product of known composition.
The precipitate is filtered and washed free of contaminants, dried or ignited and weighed. The amount of the original ion can be determined from the mass and known composition of the precipitate.
For a successful determinations, the precipitate must
– have a sufficiently low solubility so that losses from dissolving are negligible
-be of known composition after drying and igniting
– be easily filtered
– be readily washed free of impurities
– not react in the atmosphere
– have a high purity. It difficult to obtain a product which is pure , but careful precipitation and sufficient washing helps reduce the level of impurity.
Unwanted precipitation may occur with the desired product. This is called co-precipitation. It cannot be avoided but can be minimized by slow precipitation and thorough washing.
The precipitate should consist of crystals large enough to be easily washed and filtered. Large crystals also have smaller surface areas for surface adsorption of foreign materials.
Vacuum filtration is used to increase the filtration rate. A Buchner or suction funnel is fitted to a suction flask with an adaptor. A sheet of filter paper of a suitable size just to cover all the holes in the funnel is placed in the funnel and moistened with a little of the solvent to be used in the filtration.
The filter flask is then connected to a vacuum source. The mixture to be filter is poured onto the filter paper and the vacuum rapidly pulls the liquid through the funnel. The filtrate is collected in the suction flask. The suction flask is made of thick glass to prevent breakage when a vacuum is applied.
Drying to a constant mass
The filter paper with the product is transferred to a weighed crucible and the filter paper is ignited. The sample is dried to a constant, reproducible mass. This is especially important in gravimetric analysis because solids may absorb water from the air and increase in weight. The sample is usually dried in a low temperature oven. The oven is electrically heated and can maintain a constant temperature. The sample is allowed to cool then weighed. It is heated again cooled and weighed a second time. The process is repeated until a constant mass is found. It is important to allow the sample to return to room temperature before weighing to prevent convection currents around the balance which can lead to a steady increase in mass while the sample is on the balance. This can severely disrupt the accuracy of the method. A calculation should be able to help you see how this works.
The treatment of a 0.800g sample of impure potassium chloride with excess aqueous silver nitrate resulted in precipitation of 1.460g of silver chloride. Calculate the percentage of potassium chloride in the sample.
Number of Moles in 1.46g of AgCl = 1.460/ 143.5
= 0.01 mol
Since 1 mole of AgCl is formed from 1 mole of Cl- ions
Ag+ (aq) + Cl- = AgCl
And 1 mole of Cl- ions is formed from mole of KCl
KCl = K+ + Cl-
The number of moles of KCl reacted = 0.01moles
The mass of 0.01 moles of KCl = 0.01 x 74.5 = 0.745g
Thus the % KCl in the sample = 0.745/0.800 = 93.13%
Gravimetric analysis of a substance by volatilization involves heating an analyte or its decomposition products into the gaseous form. The gaseous products can be collected and weighed or the mass of the products can be determined indirectly from the loss in mass of the sample.
Volatization is commonly used to determine the amount of water and carbon dioxide in a sample. Heating the sample or collecting the water vapour in a solid desiccant can determine water. The amount of water can be determined from the increase in the mass of the desiccant. Alternatively, the amount of water can also be determined indirectly from the mass loss from the sample as a result of heating.
This method assumes that water is the only component volatilized.
Carbon dioxide can be determined from decomposition of carbonates. The mass of carbon dioxide is determined from the increase in weight of a solid absorbent.
A 2.5g sample of impure calcium carbonate was decomposed with excess hydrochloric acid. The liberated carbon dioxide collected in an absorbent was found to weigh 0.88g. Calculate the percentage of calcium carbonate in the sample.
Number of moles in 0.88g of CO2 = 0.88/44
CaCO3 + 2HCl = CaCl2 + CO2 + H2O
1 mole of CO2 is formed from 1 mole of CaCO3, therefore 0.02moles of CaCO3 was used in the reaction
Mass of CaCO3 used = 0.02 x 100
% CaCO3 in sample = 2×100/ 2.5