Osmium

OSMIUM (chemical symbol Os), chemical element, atomic weight 190.9, atomic number 76, a metal of the platinum group. In a finely divided state, metallic osmium is oxidized in air to the tetroxide (osmic acid), OsO₄. Under the name of osmium in laboratory practice, osmium tetroxide is usually meant, in which form it is commercially available. This compound forms colorless crystals, melting at 40°, volatile, possessing a pungent, unpleasant odor and strongly irritating the mucous membranes, which is why it is always sold in sealed glass tubes in precise weights of 0.1 to 1.0 g. In water, osmium dissolves slowly up to 5%, and in a 6% sodium chloride solution up to 6%. In laboratory practice, 1% and 2% aqueous solutions are used. Osmium is an excellent fixative that instantly kills cells not only when solutions are applied, but also in the form of vapors. In addition, it is an excellent reagent for fat and fat-like substances, which it blackens and converts into an insoluble state with the formation, according to some authors, of a lower oxide OsO₂, and according to others with the precipitation of metallic osmium [see separate plate (pp. 133-184), Fig. 1]. Fat blackened with osmium can be decolorized by subsequent treatment with hydrogen peroxide, old turpentine that has stood in the light with access to air, and a number of essential oils (cedar, bergamot, clove, etc.). In preparations mounted in Canada balsam, osmiated fat is decolorized and dissolved, while some lipoids remain unchanged. Olein is easily blackened by osmium; palmitic and stearic fats are blackened only if mixed with olein; preparations fixed with osmium should be treated with 70–90% ethyl alcohol for final blackening of the fat. Osmiated fat preparations should be preserved in glycerin, glycerin-gelatin, or syrup. Osmium is also used to detect the intracellular reticular apparatus of Golgi (Kopsch, Kolatchev). Osmium penetrates tissues slowly and not deeply; therefore, very small pieces (no more than 3 mm in thickness) or films are taken for fixation. In pure solution, osmium is used rarely, more often in mixtures, of which the most commonly used are: Hermann's fluid (1% platinum chloride - 15.0, 2% OsO₄ - 4.0, and glacial acetic acid - 1.0); Flemming's fluid (1% chromic acid - 15.0, 2% OsO₄ - 4.0, and glacial acetic acid - 1.0); Champy's fluid (1% chromic acid - 7.0, 3% potassium bichromate - 7.0, and 2% OsO₄ - 4.0). V. Fomin.