analytical chemistry(redirected from Hyphenated separation techniques)
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analytical chemistry:see under chemistrychemistry,
branch of science concerned with the properties, composition, and structure of substances and the changes they undergo when they combine or react under specified conditions.
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the science of the methods for studying the composition of material. It consists of two basic divisions: qualitative and quantitative analysis.
Qualitative analysis consists of methods for establishing the qualitative chemical composition of a substance—that is, the identification of atoms, ions, and molecules that enter into the composition of the substance being analyzed. The most important characteristics of all methods of qualitative analysis are specificity and sensitivity. Specificity characterizes the ability to detect the presence of an unknown element in the presence of other elements—for example, iron in the presence of nickel, manganese, chromium, vanadium, or silicon. Sensitivity is defined as the smallest quantity of an element that can be detected by a given method. Sensitivity of modern methods is expressed in magnitudes of the order of 1 mg (one millionth part of a gram).
Quantitative analysis consists of methods for determining the quantitative composition of materials—that is, the quantitative amounts of the chemical elements or of certain compounds in the analyzed substance. Along with specificity and sensitivity, the most important characteristic of every method of quantitative analysis is accuracy. The accuracy of an analysis is expressed in a percentage of error, which in most cases must not exceed 1–2 percent. Sensitivity in quantitative analysis is expressed in percentages.
Many modern methods have extremely high sensitivity. Thus, by radioactivity analysis the presence of copper in silica may be determined with an accuracy up to 2 x 10–8percent.
Because of its specific features, the analysis of organic substances is generally treated separately.
Industrial analysis consists of methods for qualitative and quantitative, organic and inorganic, analyses in their application to a specific definite material and occupies a special place in analytical chemistry. Industrial analysis includes analytical control of the production processes, raw materials, finished products, water, air, emission gases, and so on. The demand is great for rapid methods of industrial analysis requiring 10–15 minutes for an individual analysis.
The determination of the suitability of a given product for satisfying man’s needs has as ancient a history as the production itself. Originally the object of such determinations was to find out the reasons the properties found in products did not correspond to those desired or needed. This was applied to such food products as bread, beer, and wine, which were tested for taste, odor, and color; these organoleptic methods of testing are used in the modern food industry. Raw materials and products of ancient metallurgy—including ores, metals, and alloys used for the manufacture of tools for production (copper, bronze, and iron) or for adornment and barter (gold and silver) were tested for density and mechanical properties by means of trial melts. A number of similar methods for testing special alloys is used to this day in assay analysis. The satisfactory qualities of dyes, ceramic objects, soaps, leathers, textiles, glass, and medicinal preparations were determined. In the process of such analyses, individual metals (gold, silver, copper, tin, iron), alkalis, and acids began to be distinguished from each other.
The methods of analytical chemistry have had exceptional significance in establishing the fundamental laws of chemistry and in refining our concepts about chemical elements.
In the alchemical period of chemistry, which was marked by the development of experimental work, there was an increase in the number of metals, acids, and alkalis that could be identified; the concept of a salt and of sulfur as a combustible substance emerged; many instruments for chemical research were invented; and substances being studied and used were weighed (14th–16th centuries).
The principal significance of the alchemical period for the future of analytical chemistry lay in the discovery of purely chemical methods of distinguishing individual substances. Thus, in the 13th century it was discovered that aquafortis (nitric acid) dissolves silver, but not gold, and aqua regia (a mixture of nitric and hydrochloric acids) dissolves even gold. Alchemists initiated chemical determination; until that time substances had been distinguished by their physical properties.
In the period of iatrochemistry (16th—17th centuries) the importance of chemical methods of research increased even more, especially “wet” methods of qualitative analysis of substances carried into solution. Thus, silver and hydrochloric acid were identified by their reaction of forming a precipitate in a nitric acid medium. Reactions forming colored products were also used—for example, that of iron and tannic acid.
The scientific approach to chemical analysis was inaugurated by the English scientist R. Boyle (17th century), who separated chemistry from alchemy and medicine and, on the basis of the chemical atomic point of view, introduced the concept of the chemical element as an indivisible component of various substances. According to Boyle, the goal of chemistry is the study of these elements and the means by which they unite to form chemical compounds and mixtures. Boyle called the decomposition of substances into elements “analysis.” The whole period of alchemy and iatrochemistry was to a significant degree a period of synthetic chemistry. Many inorganic and a few organic compounds were produced. But since synthesis was closely related to analysis, the main thrust of chemical development at that time was actually analysis. New substances were obtained in the process of more refined decomposition of natural products.
Thus, chemistry developed mainly as analytical chemistry almost until the middle of the 19th century. The efforts of chemists were directed toward developing methods of qualitative determination of various first principles (elements) and toward establishing the quantitative laws of their interaction.
The identification of gases, which had formerly been considered one substance, was of great significance in chemical analysis. This research was initiated by the 17th century Dutch scientist Van Helmont, who discovered carbon dioxide. The greatest progress in these studies was achieved by J. Priestley, K. W. Scheele, and A. L. Lavoisier (18th century). Experimental chemistry acquired a firm foundation in the law of conservation of matter in chemical reactions, established by Lavoisier in 1789. This law was stated even earlier in more general form by M. V. Lomonosov (1758). The Swedish scientist T. A. Bergman used the law of conservation of matter for chemical analysis. It is Bergman who must be precisely credited with the formulation of a systematic scheme of qualitative analysis in which the substances to be analyzed are dissolved, divided into groups by precipitation with reagents, and then further broken down to the point where it is possible to determine each element separately. Bergman proposed hydrogen sulfide and alkalis which are used to this day as basic group reagents. He also systematized “dry” qualitative analysis by heating substances to form beads with formations of various colors.
Further improvement of systematic qualitative analysis was effected by the French chemists L. Vauquelin and L. J. Thénard, by the German chemists G. Rose and K. R. Fresenius, and by the Russian chemist N. A. Menshutkin. From 1920 to 1930 the Soviet chemist N. A. Tananaev, on the basis of the considerably broadened collection of chemical reactions, proposed the drop method of qualitative analysis, which obviates the need for a systematic pattern of analysis, division into groups, and the use of hydrogen sulfide.
Quantitative analysis was originally based on precipitation reactions of the given elements in the form of slightly soluble compounds whose mass was then determined. This gravimetric method of analysis has also been significantly improved since Bergman’s time, as a result of the perfection of balance and weighing technique and the use of different reagents—in particular, organic ones—which form the most insoluble compounds. In the first quarter of the 19th century the French scientist J. L. Gay-Lussac proposed a volumetric method of quantitative analysis, in which the volume of the solutions of reacting substances is measured. This method, called the titration or titrimetric method, is to this day the principal method of quantitative analysis. It has been considerably broadened by the increased number of applicable chemical reactions (precipitation, neutralization, complexing, oxidation-reduction) and by the large number of useful indicators (substances that indicate by a color change the end point of a reaction between solutions) and other means of indication—such as electric conductivity or refractive index.
Lavoisier first analyzed organic substances containing carbon and hydrogen as basic elements by combustion and determined the combustion products—carbon dioxide and water. It was later improved by J. L. Gay-Lussac, L. J. Thénard, and J. Liebig. In 1911 the Austrian chemist F. Pregl worked out a technique for microanalysis of organic compounds which required only a few milligrams of the starting material. In view of the complex molecular structure of organic substances, their large size (polymers), and their highly marked isomerism, organic analysis includes not only elemental analysis—the determination of the relative quantities of single elements in the molecule —but also functional analysis—the determination of the nature and quantity of individual characteristic atomic groupings in the molecule. Functional analysis is based on the characteristic chemical reactions and physical properties of the compounds studied.
By virtue of its specific nature, analysis of organic substances developed along a path separate from inorganic analysis and was not included in the curriculum of analytical chemistry until almost the middle of the 20th century. Analysis of organic substances was regarded as part of organic chemistry. But later, with the emergence of new, mainly physical, methods of analysis and with the wide application of organic reagents in inorganic analysis, both these branches of analytical chemistry began to converge and now constitute a single, common scientific and curricu-lar discipline.
Analytical chemistry as a science includes the theory of chemical reactions and of chemical properties of matter; as such, it coincided with general chemistry in its first period of development. However, in the second half of the 19th century, when the “wet” method—that is analysis primarily in aqueous solution—occupied a dominant position, the object of analytical chemistry became the study of only those reactions that give an analytically valuable, characteristic product: an insoluble or colored compound arising in the course of a rapid reaction. In 1894 the German scientist W. Ostwald first set forth the scientific foundations of analytical chemistry as the theory of the chemical equilibrium of ionic reactions in aqueous solutions. This theory, complemented by the results of all the subsequent development of ionic theory, has become the basis of analytical chemistry.
At the turn of the century, the Russian chemists M. A. H’inskii and L. A. Chugaev initiated the use in inorganic analysis of organic reagents, characterized by great specificity and sensitivity.
Research has shown that for every inorganic ion there is a characteristic chemical reaction with an organic compound that contains a definite functional group (a so-called functional-analytic group). In the 1920’s the role of instrumental methods in analytical chemistry began to increase. This again turned analysis to the study of the physical properties of substances—not to those macroscopic properties with which analysis operated in the period before scientific chemistry, but to atomic and molecular properties. Modern analytical chemistry extensively uses atomic and molecular emission and absorption spectra (visible, ultraviolet, infrared, X-ray, radio-frequency, and gamma-spectra); mass spectrometry of isotopes; the electrochemical properties of ions and molecules; adsorption properties; and so on. The use of analytical methods based on these properties is equally successful in inorganic and organic analysis. These methods significantly increase the possibility of deciphering the composition and structure of chemical compounds and of determining these qualitatively and quantitatively. The sensitivity of determination may reach 10–12 to 10–15 percent of impurities; a very small quantity of the material is required for analysis; they may be used often for a so-called nondestructive control (one not accompanied by destruction of the test sample substance); and they may serve as the basis of automation of processes of industrial analysis.
At the same time wide use of these instrumental methods poses new problems for analytical chemistry as a science and requires generalization of analytical methods, not only on the basis of theory of chemical reactions, but also on the basis of physical theory of the structure of atoms and molecules.
Analytical chemistry, fulfilling an important role in the progress of chemical science, also has enormous significance in the control of industrial processes and in agriculture. The development of analytical chemistry in the USSR is closely tied to industrialization of the country and subsequent general progress. Many institutions of higher learning have departments of analytical chemistry that prepare highly qualified analytical chemists. Soviet scientists are working out the theoretical foundations of analytical chemistry and new methods for solving scientific and practical problems. Concurrently with the development of such fields as the atomic industry, electronics, the production of semiconductors, rare metals, and space chemistry, a need arose for subtle and ultrafine methods for controlling the purity of materials, where in many cases the content of impurities must not exceed one atom in 1–10 million atoms of the finished product. All these problems are being successfully solved by the present-day analytical chemists. The old methods of chemical control of production are also being improved.
The development of analytical chemistry as a special branch of chemistry has also led to the publication of special analytical journals in all industrially developed countries. Two such journals are published in the USSR: Zavodskaia laboratoriia (Plant Laboratory, since 1932) and Zhurnal analiticheskoi khimii (Journal of Analytical Chemistry, since 1946). There are also specialized international journals on separate divisions of analytical chemistry—for example, journals on chromatography and electroanalytical chemistry. Specialists in analytical chemistry are trained in special departments of universities, in chemical and technological institutes and technicums, and in trade-technical colleges.
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Alekseev, V. N. Kolichestvennyi analiz, 4th ed. Moscow, 1972.
Lialikov, Iu. S. Fiziko-khimicheskie melody analiza, 4th ed. Moscow, 1964.
Ewing, G. D. Instrumental’nye metody khimicheskogo analiza. Moscow, 1960. (Translated from English.)
Lur’e, Iu. Iu. Spravochnik po analiticheskoi khimii, 4th ed. Moscow, 1971.
IU. A. KLIACHKO