Reagent, Chemical

Reagent, Chemical 

any of a group of chemical preparations (substances) used in the laboratory for such purposes as analysis and research, for example, in the study of the preparation, properties, and transformations of various compounds. Chemical reagents are, in most cases, individual substances, although certain mixtures, such as petroleum ether, are also considered reagents. Sometimes the term “reagent” is applied to special-purpose solutions of rather complex composition, for example, the reagent used in the Nessler method of detecting ammonia. Chemical reagents are available in such varying degrees of purity as extremely pure, chemically pure, analytically pure, pure, reagent grade, and technical grade, which is packaged in small containers. Many chemical reagents are specially prepared for laboratory use, but purified chemical products are also available for industrial purposes. The purity of reagents in the USSR is controlled by the All-Union State Standard and by Technical Specifications.

Chemical reagents are also categorized by composition as, for example, inorganic reagents, organic reagents, and reagents containing radioisotopes. The intended use of reagents dictates their classification as analytical reagents, chemical indicators, or organic solvents. The importance and practical value of analytical reagents depend mainly on their sensitivity and selectivity. The sensitivity of a reagent is the smallest quantity or smallest concentration of a substance (ion) that can be detected or quantitatively determined by the addition of the reagent. For example, a magnesium ion at a concentration of 1.2 milligrams per liter still yields a noticeable precipitate after the addition of dibasic sodium phosphate and ammonium chloride solutions. Reagents with even greater sensitivity are also available. The term “specific” is assigned to reagents that yield a characteristic reaction with the ion or substance being analyzed under known conditions irrespective of the presence of other ions. Very few specific reagents are known; one example is starch, which is used to detect iodine.

Analytical chemistry uses mainly selective and group reagents. A selective reagent reacts with a small number of ions; a group reagent is used for the simultaneous isolation of several ions. Selective analytical reagents are primarily complex organic compounds capable of forming characteristic chelate compounds with metal ions. Organic reagents that figure prominently in inorganic analysis include 8-hydroxyquinoline, diphenylthiocarbazone (dithizone), alpha-benzoin oxime, l-nitroso-2-naphthol, dimethylglyoxime, trihydroxyfluorones, complexone III, certain hydroxyazo compounds, dithiocarba-mates, diethyldithiophosphate, diantipyrylmethane, and other pyrazolone derivatives. Many reagents are used in organic functional analysis. These reagents, which include phenylhy-drazine, 2,4-dinitrophenylhydrazine, semicarbazide, and thio-semicarbazide, are used in the qualitative and quantitative determination of aldehydes and ketones.

Since many chemical reagents are toxic, flammable, and explosive, handling requires the observance of safety precautions.

REFERENCES

Khimicheskie reaktivy i preparaty. Editor in chief, V. I. Kuznetsov. Moscow-Leningrad, 1953. (Handbook.)
Perrin, D. Organicheskie analiticheskie reagenty. Moscow, 1967. (Translated from English.)
Busev, A. I. Sintez novykh organicheskikh reagentov dlia neorganicheskogo analiza. Moscow, 1972.

A. I. BUSEV