Microchemical Analysis

a method of analytical chemistry for studying small samples (10^-2-10^-3 g) of various substances (smaller samples, down to 10^-6 g, are studied in ultramicrochemical analysis). Microchemical analysis is used in the semiconductor industry; in metallurgy and mineralogy; in forensic, biochemical, and clinical studies; in organic chemistry (for the analysis of synthesized and natural products), and in radiochemistry.

Experimental techniques and methodology, as well as the experience of the researcher, are of crucial significance in microchemical analysis. In microchemical analysis, researchers work with small volumes of a solution of normal concentration; therefore, generally accepted identification reactions and methods for chemical determination of components are used. However, preference is given to methods based on more sensitive and specific reactions, which make possible identification not only of the major components in a small sample but also of the impurities. Sufficiently precise results are obtained in microchemical analysis using relatively simple apparatus.

The most common technique in qualitative microchemical analysis is the conduct of reactions on filler paper (spot test analysis), which is used in studying both inorganic and organic substances. The ring-oven method, which makes possible identification of separate components in a narrow, defined zone on the paper and their separation and identification in a mixture, offers additional possibilities. Methods of partition and thinlayer chromatography are also used.

Another trend in qualitative microchemical analysis is crystal microscopy. In addition to special analytical methods, simpler methods are also used (for example, colored reaction products may be obtained in a drop of solution on a procelain plate, or precipitates characteristic of certain elements may be produced in capillary tubes).

By analogy with macrochemical analysis, quantitative microchemical analysis of organic and inorganic substances may be gravimetric, titrimetric, or photometric. For organic substances, quantitative microchemical analysis is used to determine the content of individual elements (elemental analysis) or of functional groups (functional analysis), as well as the molecular weight.

Gravimetric determination in microchemical analysis of organic substances is usually conducted using a microbalance with a sensitivity of 10^~6 g. In addition to gravimetric determination, gas chromatography is widely used in organic microchemical analysis.

Titrimetric methods in microchemical analysis occupy a leading position because of their simplicity and high accuracy. Microburettes that can measure a volume as small as 10^-3milliliter, and also vessels of small volume, are used for titration. Electrochemical titration methods, especially coulometric methods, are preferred.

Photometric microidentification, including methods for recording equivalence end points in titration with a colored indicator, has acquired considerable practical significance.

The main trend in the present development of microchemical analysis is the transition to predominant use of physicochemical methods. Combinations of microchemical-analysis techniques with special physical methods of microanalysis are used in the study of small samples of complex composition.