Scientific and technological disciplines rely on measurements of physical and chemical properties. Such measurements are central to, for example, analytical chemistry, that branch of chemistry concerned with determining the identity of a substance (qualitative analysis) or with calculating the
amount of a substance whose identity is known (quantitative analysis). This article includes a description of the techniques and measurements that are most often used by scientists, engineers, and laboratory technicians to identify a substance, separate it into its components, remove impurities, or determine a specific chemical or physical property. Various methods of analysis are discussed, including classical wet techniques such as precipitations and titrations and instrumental methods
such as chromatography, mass spectrometry, spectroscopy, and electroanalysis. Many methods of analysis and measurement involve the interaction of radiation with matter. Accordingly, the sources,
interactions, detection, and measurement of various types of ionizing radiations are discussed in some detail, as are significant applications of radiation detection and measurement in science and industry. The behaviour of materials--e.g., certain metals, ceramics, and plastics--under various conditions is an important factor in determining their suitability for specific applications. Measuring the behaviours and characteristics of materials is the concern of materials testing. Several test methods, along with the properties that they measure, are discussed here. (see also Index: qualitative chemical analysis, quantitative chemical analysis)
Qualitative Chemical Analysis, branch of chemistry that deals with the identification of elements or
grouping of elements present in a sample. The techniques employed in qualitative analysis vary in complexity, depending on the nature of the sample. In some cases it is necessary only to verify the presence of certain elements or groups for which specific tests applicable directly to the sample (e.g., flame tests, spot tests) may be available. More often the sample is a complex mixture, and a systematic analysis must be made in order that all the constituents may be identified. It is customary to classify the methods into two classes: qualitative inorganic analysis and qualitative organic analysis.
The classical procedure for the complete systematic analysis of an inorganic sample consists of several parts. First, a preliminary dry test may be performed, which may consist of heating the sample to detect the presence of such constituents as carbon (marked by the appearance of smoke or char) or water (marked by the appearance of moisture) or introducing the sample into a flame and noting the colour produced. Certain elements may be identified by means of their characteristic flame colours. After preliminary tests have been performed, the sample is commonly dissolved in water for later determination of anionic constituents (i.e., negatively charged elements or groupings of elements)
and cationic constituents (i.e., positively charged elements or groupings of elements). The procedure followed is based on the principle of treating the solution with a succession of reagents so that each reagent separates a group of constituents. The groups are then treated successively with reagents that divide a large group into subgroups or separate the constituents singly. When a constituent has been separated it is further examined to confirm its presence and to establish the amount present (quantitative analysis). Portions of the material are dissolved separately, and different procedures are used for each to detect the cationic and anionic constituents. A typical analytical scheme for the
separation of the cations into groups is summarized in the table. The analysis for anions is more difficult and less systematic than that for cations.
The organic nature of a compound is generally indicated by its behaviour on being heated in air; solids usually melt, then burn with either a smoky or nonsmoky flame, in some instances leaving a black residue of carbon. The elements usually present in these compounds are carbon, hydrogen,
oxygen, nitrogen, sulfur, and, occasionally, phosphorus, halogens, and some metals. Specific tests are available for each of the individual elements.
Quantitative Chemical Analysis, branch of chemistry that deals with the determination of the amount or percentage of one or more constituents of a sample. A variety of methods is employed for quantitative analyses, which for convenience may be broadly classified as chemical or physical, depending upon which properties are utilized. Chemical methods depend upon such reactions as
precipitation, neutralization, oxidation, or, in general, the formation of a new compound. The major types of strictly chemical methods are known as gravimetric analysis (q.v.) and volumetric, or titrimetric, analysis (see volumetric analysis). Physical methods involve the measurement of some
physical property such as density, refractive index, absorption or polarization of light, electromotive force, magnetic susceptibility, and numerous others. An analysis will often require a combination of
methods: qualitative for separating desired constituents from a sample and quantitative for measuring the amounts present.
The basic tool in all quantitative analyses is the analytical balance, used for the accurate weighing of samples and precipitates. For usual analytical work the balance should be able to determine differences in mass of 0.1 milligram (about 0.000004 ounce). In microanalyses the balance must be about 1,000 times more sensitive, and, for special work, balances of even higher sensitivity have been constructed.