Osmium

OSMIUM (chemical symbol Os), a chemical element, atomic weight 190.9, atomic number 76, a metal of the platinum group. In a finely divided state, metallic osmium oxidizes in the air into a tetroxide (osmic acid), OsO₄. In laboratory practice, the term osmium usually refers to osmium tetroxide, the form in which it is commercially available. This compound consists of colorless crystals that melt at 40°, are volatile, possess a pungent, unpleasant odor, and strongly irritate mucous membranes, which is why it is always sold in sealed glass tubes in precise weights from 0.1 to 1.0 g. Osmium dissolves slowly in water up to 5%, and in a 6% NaCl solution up to 6%. In laboratory practice, 1% and 2% aqueous solutions are used. Osmium is an excellent fixative, instantly killing cells not only when applied as a solution 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, by the precipitation of metallic osmium [see separate table (pp. 133–184), fig. 1]. Fat blackened by 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 olein is present as an impurity; preparations fixed with osmium should be treated with 70–90% ethyl alcohol for final blackening of the fat. Osmiated fat preparations should be stored in glycerin, glycerin-gelatin, or syrup. Osmium is also used to detect the intracellular Golgi apparatus (Kopsch, Kolachev). Osmium penetrates tissues slowly and not deeply; therefore, very small pieces (no more than 3 mm thick) or films are taken for fixation. Osmium is rarely used in a pure solution; it is more often used in mixtures, the most common of which are: Hermann's fluid (1% PtCl₄—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); and Champy's fluid (1% chromic acid—7.0, 3% potassium bichromate—7.0, and 2% OsO₄—4.0). V. Fomin. OSMOSIS, the movement of a solvent through a semipermeable membrane that retains—completely or partially—dissolved substances. Osmosis occurs through a membrane in the direction of the liquid in which the dissolved substances retained by the semipermeable membrane are in a higher concentration and accordingly produce greater osmotic pressure (see). The movement of liquid through porous membranes can also occur under the influence of electrical forces (see Electro-osmosis).