Quantitative Analysis


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quantitative analysis

[′kwän·ə·tād·iv ə′nal·ə·səs]
(analytical chemistry)
The analysis of a gas, liquid, or solid sample or mixture to determine the precise percentage composition of the sample in terms of elements, radicals, or compounds.

Quantitative Analysis

 

the aggregate of chemical, physicochemical, and physical methods of determining the quantitative ratios of constituents in the substance being analyzed. Together with qualitative analysis, quantitative analysis is one of the major branches of analytical chemistry. Depending on the amount of substance taken for analysis, there are macroanalysis, semimicroanalysis, microanalysis, and ultramicroanalysis. A macroanalysis uses a sample usually greater than 100 mg and a solution volume greater than 10ml; an ultramicroanalysis uses 1–10−1 mg and 10−3−10−6ml, respectively.

Depending on the object being studied, inorganic quantitative analysis and organic quantitative analysis are distinguished, with the latter in turn divided into elemental analysis, functional group analysis, and molecular analysis. Elemental analysis makes it possible to determine the compositions of elements (ions), and functional group analysis, the content of functional (reactive) atoms or groups in the object being analyzed. Molecular quantitative analysis entails analysis of individual chemical compounds characterized by a definite molecular mass. Phase diagram analysis is of considerable importance and constitutes an aggregate of methods for the separation and analysis of individual structural (phase) components of heterogeneous systems.

In addition to specificity and sensitivity, an important feature of quantitative analysis is accuracy, that is, the value of the relative error in the determination. Accuracy and sensitivity are expressed in percent.

The classical chemical methods of quantitative analysis include gravimetric analysis, which is based on the accurate measurement of the weight of the substance, and volumetric analysis. The latter includes titrimetric volumetric analysis, comprising methods of measuring the volume of the reagent solution expended in the reaction with the substance under analysis, and gas volumetric analysis, comprising methods of measuring the volume of the gaseous products being analyzed.

In addition to the classical chemical methods, physical and physicochemical (instrumental) methods of quantitative analysis are widespread; they are based on the measurement of optical, electrical, adsorption, catalytic, and other characteristics of the substances that are dependent on their amount (concentration). These methods are usually divided into the following groups: electrochemical methods (conductometry, polarography, potentiometry); spectral, or optical, methods (emission and absorption spectral analysis, photometric analysis, colorimetric analysis, turbidimetric analysis, luminescence analysis); chromatographic methods (liquid, gas, and gas-liquid chromatography); X-ray methods (absorption and emission X-ray analysis, X-ray phase analysis); radiometric methods (activation analysis); and massspectrometric methods.

These methods, which are inferior to chemical methods in accuracy, are significantly superior in sensitivity, selectivity, and the speed at which the analysis is performed. The accuracy of the chemical methods of quantitative analysis is usually within the limits of 0.005–0.1 percent; the error by instrumental methods is 5–10 percent and sometimes significantly greater. The sensitivity of some methods of quantitative analysis is given in Table 1.

As a rule, microscopic amounts of a substance are required in physical and physicochemical methods of quantitative analysis. The analysis can often be conducted without destroying the

Table 1. Sensitivity of some methods of quantitative analysis
(in percent)
Volumetrie10−1
Gravimetric10−2
Emission spectral10−4
Absorption x-ray spectral10−4
Mass-spectrometric10−4
Coulometric10−5
Luminescence10−6-10−5
Photometric colorimetric10−7-10−4
Polarographic10−8-10−6
Activation10−9-10−8

sample; continuous and automatic recording of the results is also sometimes possible. These methods are employed for analyzing high-purity substances, evaluating product yields, and studying the properties and structures of various substances and compounds.

V. V. KRASNOSHCHEKOV

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