In most cases, the analyte in solution is first converted to a solid by precipitation with an appropriate reagent. The precipitate can then be collected by filtration, washed to remove impurities, 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. This approach has been used to determine the atomic weights of many chemical elements.
In other cases, it may be easier to remove the analyte by vaporization. The analyte may be collected—perhaps in a cryogenic trap or on some absorbent material such as activated carbon—and measured directly. Alternatively, the sample may be weighed before and after it is dried; the difference between the two masses gives the mass of analyte lost. This approach has been especially useful in determining the water content of complex materials such as foodstuffs.
A general procedure for gravimetric analysis is outlined below.
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, a process known as "peptization" may occur during washing. In this case, part of the precipitate reverts to the colloidal form. (For example, AgCl(colloidal) converts reversibly to AgCl(s).) This results in the loss of part of the precipitate because the colloidal form may pass through the filter. Peptization can be reduced with careful technique and washing with a solution of appropriate pH and ionic strength.
A simple example of gravimetric analysis is the measurement of solids suspended in a water sample. A known volume of the suspension is filtered and the collected solids are weighed.
A chunk of ore is treated with concentrated nitric acid and potassium chlorate to convert all of the sulfur content to sulfate (SO42-). The nitrate and chlorate are removed by treating the solution with concentrated hydrochloric acid (HCl). The sulfate is precipitated with barium ions (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 the calculation of an unknown. Usually, the methods also do not require expensive equipment. In light of its high degree of accuracy, when gravimetric analysis is performed correctly, it can also be used to calibrate other instruments in lieu of reference standards.
Gravimetric analysis usually provides for the analysis of only 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, one finds that the former is both faster and allows for the 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. (For example, a colloid may be formed during precipitation gravimetry.) By comparison, hardy methods such as spectrophotometry provide much more efficient analyses.
New World Encyclopedia writers and editors rewrote and completed the Wikipedia article in accordance with New World Encyclopedia standards. This article abides by terms of the Creative Commons CC-by-sa 3.0 License (CC-by-sa), which may be used and disseminated with proper attribution. Credit is due under the terms of this license that can reference both the New World Encyclopedia contributors and the selfless volunteer contributors of the Wikimedia Foundation. To cite this article click here for a list of acceptable citing formats.The history of earlier contributions by wikipedians is accessible to researchers here:
Note: Some restrictions may apply to use of individual images which are separately licensed.