Gravimetric Analysis


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gravimetric analysis

[grav·ə′me·trik ə′nal·ə·səs]
(analytical chemistry)
That branch of quantitative analytical chemistry in which a desired constituent is converted, usually by precipitation or combustion, to a pure compound or element, of definite known composition, and is weighed; in a few cases a compound or element is formed which does not contain the constituent but bears a definite mathematical relationship to it.

Gravimetric Analysis

 

one of the important methods of quantitative chemical analysis; based on the accurate measurement of the weight of a substance. The substance whose weight is to be determined is usually separated from the sample being analyzed in the form of a poorly soluble compound of constant, known chemical composition, since the extraction of a chemically pure element entails great difficulties and is sometimes impossible.

Gravimetric analysis begins by taking an accurately weighed amount of the sample being analyzed and dissolving it. Then, by adding the appropriate reagent, a poorly soluble precipitate of a compound is produced; it contains the element being determined. The precipitate is separated from the solution by filtration, washed, and dried or calcined to constant weight. If the weight of the sample a taken for analysis, the weight of the precipitate b, and its composition are known, the content X of the substance to be found is calculated (generally as a percentage by weight): X = a x F x 100/b, where F is a conversion factor equal to the ratio of the atomic weight of the substance to be determined (or a multiple of the latter) to the molecular weight of the compound of the precipitate. For example, when determining the amount of iron (atomic weight 55.85) separated as its oxide (molecular weight 159.70),

The most important part of gravimetric analysis is the production of an easily filtrable, poorly soluble precipitate—if possible, in the form of large crystals (the loss of material caused by solubility should not exceed .1 mg)—that is free from foreign substances that are not removed by drying or calcination. Gravimetric analysis is characterized by high accuracy: experimental error does not exceed 0.1 percent, and with particularly painstaking work it may be reduced to 0.02–0.03 percent. The drawbacks of gravimetric analysis are the time required and the need to use comparatively large amounts of material for analysis (about 0.5 g). The latter disadvantage can be eliminated by using micro and ultramicro methods.

Gravimetric analysis is used to determine the chemical composition of rocks, minerals, and alloys and for quality control of raw materials and finished products in a number of branches of industry. Among the various kinds of gravimetric analysis are assay and electrochemical analysis.

REFERENCE

Kreshkov, A. P. Osnovy analiticheskoi khimii, 3rd ed., vol. 2. Moscow. 1971.
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