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Gravimetric analysis describes a set of methods in analytical chemistry for the quantitative determination of an analyte based on the mass of a solid. A simple example is the measurement of solids suspended in a water sample: A known volume of water is filtered, and the collected solids are weighed.
In most cases, the analyte must first be converted to a solid by precipitation with an appropriate reagent. The precipitate can then be collected by filtration, washed, dried to remove traces of moisture from the solution, and weighed. The amount of analyte in the original sample can then be calculated from the mass of the precipitate and its chemical composition.
In other cases, it may be easier to remove the analyte by vaporization. The analyte might be collected -- perhaps in a cryogenic trap or on some absorbent material such as activated carbon -- and measured directly. Or, the sample can be weighed before and after it is dried; the difference between the two masses gives the mass of analyte lost. This is especially useful in determining the water content of complex materials such as foodstuffs.
Additional recommended knowledge
Washing and Filtering
The precipitate is often washed to remove impurities adsorbed onto the surface of the particles. Washing may be done with a solution of the precipitating agent, to avoid redissolving a slightly soluble salt. With many precipitates, peptization occurs during washing. Here part of the precipitate reverts to the colloidal form (e.g. AgCl(colloidal) <-> AgCl(s).) This results in the loss of part of the precipitate because the colloidal form may pass through on filtration. Peptization can be reduced with careful technique and washing with a solution of appropriate pH and ionic strength.
A chunk of ore is treated with concentrated nitric acid and potassium chlorate to convert all of the sulfur to sulfate (SO42-). The nitrate and chlorate are removed by treating the solution with concentrated HCl. The sulfate is precipitated with barium (Ba2+) and weighed as BaSO4.
Gravimetric analysis, if methods are followed carefully, provides for exceedingly precise analysis. In fact, gravimetric analysis was used to determine the atomic masses of many elements to six figure accuracy. Gravimetry provides very little room for instrumental error and does not require a series of standards for calculation of an unknown. Methods also do not require often expensive equipment. Gravimetric analysis, due to its high degree of accuracy, when performed correctly, can also be used to calibrate other instruments in lieu of reference standards.
Gravimetric analysis usually only provides for the analysis of a single element, or a limited group of elements, at a time. Comparing modern dynamic flash combustion coupled with gas chromatography with traditional combustion analysis will show that the former is both faster and allows for simultaneous determination of multiple elements while traditional determination allowed only for the determination of carbon and hydrogen. Methods are often convoluted and a slight mis-step in a procedure can often mean disaster for the analysis (colloid formation in precipitation gravimetry, for example). Compare this with hardy methods such as spectrophotometry and one will find that analysis by these methods is much more efficient.
|This article is licensed under the GNU Free Documentation License. It uses material from the Wikipedia article "Gravimetric_analysis". A list of authors is available in Wikipedia.