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qualitative analysis[′kwäl·ə‚tād·iv ə‚nal·ə·səs]
the set of chemical, physicochemical, and physical methods used to discover and identify the elements, radicals, ions, and compounds in a substance or mixture of substances under analysis.
Qualitative analysis forms one of the basic divisions of analytical chemistry. The most important features of qualitative analysis techniques are (1) specificity (selectivity), that is, the possibility of discovering an unknown element in the presence of another, and (2) sensitivity, determined by the smallest quantity of an element that can be found in a drop of solution (0.01–0.03 milliliter) by the given method; a sensitivity of 1 μg has been attained by modern techniques.
Classical qualitative analysis of inorganic substances is carried out by either the “dry” method or by the “wet” method. The dry method involves testing for flame coloration with a gas burner and the formation of colored beads (vitreous fusions) when heating the powder from the substance tested (usually a salt or metal oxide) with a small quantity of sodium tetraborate or phosphoric salt (NaNH4HPO4·4H2O). Wet analysis (in solutions) is effected by macro-, semimicro-, micro-, and ultra-microtechniques. In macroanalysis the quantity of analyzed substance is greater than 100 mg and the volume of solution is greater than 5 ml; in ultramicroanalysis the measurements are less than 0.1 mg and less than 0.05 ml, respectively.
Since the qualitative analysis of inorganic compounds in aqueous solutions is based on ionic reactions, it is divided into the analysis of cations and the analysis of anions. Cations are generally separated into five groups according to the solubility of their sulfur salts. Anions are most often grouped according to the different solubilities of their barium or silver salts. If the analyzed substance is found to contain ions that can be determined by selective reagents, then analysis is carried out using the fractional method.
Along with the classical chemical methods, physical and physicochemical (that is, instrumental) techniques are widely used in qualitative analysis that are based on the study of the optical, electric, magnetic, thermal, catalytic, adsorptive, and other properties of the analyzed substances. These methods have a number of advantages over the chemical techniques, since in many cases they make it possible both to eliminate the preliminary chemical separation of the analyzed sample into its component parts and to record the results of the analysis continuously and automatically. Furthermore, a considerably smaller quantity of analyzed sample is required when using physical and physicochemical methods to determine small amounts of admixtures. These methods include spectral analysis, luminescent analysis, mass spectroscopy, polarography, chromatography, activation analysis, and kinetic analysis. The qualitative analysis of organic compounds is conducted by element analysis and functional analysis and by determining the basic physicochemical properties of the analyzed substances.
V. V. KRASNOSHCHEKOV