1911 Encyclopædia Britannica/Molybdenum

MOLYBDENUM [symbol, Mo; atomic weight, 96 (O＝16)] a metallic chemical element. The name is derived from Gr. μόλυβδος, lead, and was originally employed to denote many substances containing or resembling lead; ultimately the term was applied to graphite and to molybdenum sulphide. The difference between these two latter substances was first pointed out by Cronstedt, and in 1778 C. Scheele prepared molybdic acid from the sulphide. Molybdenum occurs in nature chiefly as the minerals molybdenite (MoS₂) and wulfenite (PbMoO₄), and more rarely as molybdic ochre (MoO₃) and ilsemannite; it also occurs in many iron ores. The metal may be obtained by heating the trioxide with carbon in the electric furnace (H. Moissan, Comptes rendus, 1893, 116, p. 1225), or by the Goldschmidt method (Rosenheim and Braun, Zeit. anorg. Chem., 1905, p. 311) or by dissociating the tetra- and pentachloride in a graphite crucible with an electric current below 1330° (J. N. Pring and W. Fielding, Jour. Chem. Soc., 1909, 95, p. 1497). It forms a grey coloured powder of specific gravity 9·01; it is malleable, and not as hard as glass. It is rapidly oxidized on heating to a temperature of 500°–600° C., and also when fused with nitre or potassium chlorate. It is soluble in dilute nitric acid, and in concentrated sulphuric acid; in the latter case with the formation of a blue solution which on heating becomes colourless, molybdenum trioxide being formed with the liberation of sulphur dioxide.

Molybdenum combines with oxygen to form many oxides, the most important of which are: the monoxide, MoO.n(H₂O), the sesquioxide, Mo₂O₃, the dioxide, MoO₂, and the trioxide, MoO₃. Molybdenum monoxide, MoO.n(H₂O), is a black powder obtained when the dichloride is boiled with concentrated potash solution. According to W. Muthmann and W. Nagel (Ber., 1898, 31, p. 2009), this oxide does not exist, the reaction leading to the formation of an hydroxide according to the equation: Mo₃Cl₄(OH)₂+4KHO+3H₂O＝3Mo(OH)₃+4KBr+3H. Molybdenum sesquioxide, Mo₂O₃, a black mass insoluble in acids, is formed by heating the corresponding hydroxide in vacuo, or by digesting the trioxide with zinc and hydrochloric acid. Molybdenum dioxide, MoO₂, is formed by heating sodium trimolybdate, Na₂Mo₃O₁₀, to redness in a current of hydrogen (L. Svanberg and H. Struve, Jour. prak. Chem., 1848, 44, p. 301), or by long fusion of a mixture of ammonium molybdate, potassium carbonate, and boron trioxide (W. Muthmann, Ann., 1887, 238, p. 114). It forms quadratic prisms, having a violet reflex and insoluble in boiling hydrochloric acid. Molybdenum trioxide, MoO₃, is prepared by oxidizing the metal or the sulphide by heating them in air, or with nitric acid. It, is a white powder, which turns pale yellow on heating, and melts at a red heat. It sublimes in small rhombic tables or needles, and is slightly soluble in cold water, the solution possessing an acid reaction. Several hydrated forms of the oxide are known, and a colloidal variety may be obtained by the dialysis of a strong hydrochloric acid solution of sodium molybdate. Molybdenum trioxide, like chromium trioxide, is an acidic oxide, and forms salts known as molybdates. The normal molybdates show a tendency to pass into polymolybdates. The molybdates are also capable of combining with other oxides (such as phosphorus and arsenic pentoxides) yielding very complex salts. The ordinary ammonium molybdate, used as a test reagent for phosphates, is a salt of composition (NH₄)₁₀Mo₁₂O₄₁; it has been examined physicochemically by J. Sand and F. Eisenlohr (Abst. J.C.S., 1907, ii. pp. 178, 179). The molybdates may be recognized by the fact that they give a white precipitate on the addition of hydrochloric or nitric acids to their solutions, and that with reducing agents (zinc and sulphuric acid) they give generally a blue coloration which turns to a green and finally to a brown colour.

Molybdenum combines with the halogen elements in varying proportions, forming with chlorine a di-, tri-, tetra- and penta-chloride, and similar compounds with bromine and iodine. Molybdenum dichloride (MoCl₂)3 or Cl₄Mo₃Cl₂ (chlormolybdenum chloride), is prepared together with some tetrachloride) by heating the trichloride in a stream of carbon dioxide (C. W. Blomstrand, Jour. f. prak. Chem., 1857, 71, p. 449; 1861, 82, p. 433). It is a yellow amorphous powder which is soluble in dilute alkalis, the solution on acidification giving an hydroxide, Cl₄Mo₃(OH)₂, which is soluble in nitric acid, and does not give a reaction with silver nitrate. The molecular weight determinations of W. Muthmann and W. Nagel (Ber., 1898, 31, p. 2009) show the salt to possess the composition Mo₃C_l6. Molybdenum trichloride, MoCl₃, is obtained when the pentachloride is heated to a temperature of about 250° C. in a current of hydrogen. It forms red crusts, is insoluble in cold water, but is decomposed by boiling water. It is easily soluble in hot nitric acid. Molybdenum pentachloride, MoCl₅, is obtained when molybdenum is gently heated in dry chlorine (L. P. Liechti and B. Kempe, Ann., 1873, 169, p. 345). It is a dark-coloured crystalline solid which melts at 194° C. and boils at 268° C. It fumes in moist air and deliquesces gradually. It is occasionally used as a chlorine carrier. It is soluble in absolute alcohol and in ether. Molybdenum disulphide, MoS₂, is found as the mineral molybdenite, and may be prepared by heating the trioxide with sulphur or sulphuretted hydrogen. It is a black crystalline powder, resembling graphite in appearance. It is readily oxidized by nitric acid, and when strongly heated in a current of hydrogen is reduced to the metallic condition. Molybdenum trisulphide, MoS₃, is obtained by saturating a solution of an alkaline molybdate with sulphuretted hydrogen and adding a mineral acid. It is a brown powder which on heating in air loses sulphur and leaves a residue of the disulphide. A tetrasulphide, MoS₄, has also been described.

Many varying values have been given for the atomic weight of molybdenum. J. J. Berzelius (Pogg. Ann., 1826, 8, p. 23), by converting lead molybdate into lead nitrate, obtained the value 95·2; while J. B. A. Dumas (Ann., 1860, 113, p. 32), by converting the trioxide into the metal, obtained the value 95·65. K. Seubert and W. Pollard (Zeit. anorg. Chem., 1895, 8, p. 434) using this second method obtained the value 96·28; whilst E. F. Smith and P. Maas (Zeit. anorg. Chem., 1894, 5, p. 280), by heating pure sodium molybdate in hydrochloric acid and estimating the amount of sodium chloride formed, obtained the value 96·087.