Molybdenum

Molybdenum, chemical element, symb. Mo, atomic number 42, at. weight 96.0; stands in the 6th group of the periodic system. Natural compounds of Mo: molybdenite MoS₂ and yellow lead ore PbMoO₄. Mo is obtained from MoS₂ by roasting and subsequent reduction of MoO₃ in an electric furnace with carbon. Mo is a gray-steel colored powder, sp. gr. 9.0, melts in a vacuum at about 2,500°; molybdenum is chemically slightly active, in its compounds it exhibits a valence from 2 to 6, easily forms complex compounds: it does not change in air, upon heating it combines with oxygen, carbon, and iron. It dissolves in concentrated nitric and sulfuric acids, it does not dissolve in hydrochloric acid. Solutions of alkalis do not act on Mo. Four simple oxides of Mo are known: MoO, Mo₂O₃, MoO₂, MoO₃ and a whole series of complex ones, for example Mo₂O₅, Mo₃O₈, Mo₆O₁₂. The lower oxides have almost no basic properties, they do not react with acids; the higher one—molybdenum anhydride MoO₃—is weakly acidic and forms molybdic acid H₂MoO₄. Its acidic ammonium salt in a nitric acid solution is a reagent for the PO₄ ion. Molybdenum is used for the manufacture of filaments for incandescent electric light bulbs, for the production of molybdenum steel and alloys; the alloy "stellite" contains 50% Co, 20% Mo, 20% Cr and 10% W and serves for the manufacture of high-speed cutting tools with high hardness; an alloy of 35% Fe, 60% Cr and 5% Mo does not change even from boiling aqua regia. Molybdenum compounds are often used as a catalyst. In histological technique, molybdenum is used in the form of the ammonium salt (NH₄)₂MoO₄ or in the form of phosphomolybdic acid, H₃PO₄·10MoO₃. Both form dark blue lakes (see) with hematoxylin (see), which give intense staining, especially of the nervous system and connective tissue. For staining the nervous system, molybdate-hematein is used, mainly according to Hansen; 10 cm³ of a 1% solution of the ammonium salt of Mo is mixed with 10 cm³ of a 1/4% aqueous solution of hematoxylin and 0.006 parts of potassium permanganate are added; after a short time, a dark, intensely staining solution is obtained. Stain for 3-10 minutes. Overstained sections are differentiated in a highly diluted solution of borax (0.5), potassium ferrocyanide (0.5), and water (100.0); thorough washing in water, alcohol, mounting in xylene-dammar. Nerve cells and fibers, as well as glial inclusions, are of an intense blue-violet color. The stain lasts for decades. Fixation of pieces in chromium salts. The following solution also stains well: ammonium salt of Mo 2.0, Aquae destill. 100.0, hematoxylin 0.5 (the latter is preliminarily dissolved in 10 cm³ of warm water and poured into the solution of the molybdenum salt). 3 cm³ of a 1% solution of potassium permanganate is added. Further processing is the same. Less commonly used is Held's molybdic hematoxylin for staining neuroglia. Phosphomolybdic acid is also included as a component in various other staining differential mixtures (for example, in the Schmorl method for staining bones, in the Kossa method for detecting lime). See also Mallory method.