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CHROMIUM (symbol Cr. atomic weight 52.1)

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Originally appearing in Volume V06, Page 298 of the 1911 Encyclopedia Britannica.
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CHROMIUM (See also:symbol Cr. atomic See also:weight 52.1) , one of the metallic chemical elements, the name being derived from the See also:fine See also:colour (Gr. xpmµa) of its compounds. It is a member of the See also:sixth See also:group in the periodic See also:classification of the elements, being included in the natural See also:family of elements containing See also:molybdenum, See also:tungsten and See also:uranium. The See also:element is not found in the See also:free See also:state in nature, nor to any large extent in See also:combination, occurring chiefly as chrome-ironstone, Cr2O3•FeO, and occasionally being found as crocoisite, PbCrO4, chrome-ochre, Cr203, and chrome See also:garnet, CaO•Cr2O3.3SiO2, while it is also the cause of the colour in See also:serpentine, chrome-See also:mica and the See also:emerald. It was first investigated in 1789 by L. N. See also:Vauquelin and Macquart, and in 1797 by Vauquelin, who found that the See also:lead in crocoisite was in combination with an See also:acid, which he recognized as the See also:oxide of a new See also:metal, The metal can be obtained by various processes. Thus Sainte Claire Deville prepared it as a very hard substance of See also:steel-See also:grey colour, capable of taking a high See also:polish, by strong ignition of chromic oxide and See also:sugar See also:charcoal in a See also:lime crucible. F. See also:Wohler reduced the sesquioxide by See also:zinc, and obtained a shining See also:green See also:powder of specific gravity 6.81, which tarnished in See also:air and dissolved in hydrochloric acid and warm dilute sulphuric acid, but was unacted upon by concentrated nitric acid. H. See also:Moissan (Comptes rendus, 1893, 116, p. 349; 1894, 119, p.

185) reduces the sesquioxide with See also:

carbon, in an electric See also:furnace; the product so obtained (which contains carbon) is then strongly heated with lime, whereby most of the carbon is removed as See also:calcium See also:carbide, and the See also:remainder by See also:heating the purified product in a crucible lined with the See also:double oxide of calcium and chromium. An easier See also:process is that of H. See also:Goldschmidt (Annalen, 1898, 301, p. 19) in which the oxide is reduced by metallic See also:aluminium; and if care is taken to have excess of the sesquioxide of chromium See also:present, the metal is obtained quite free from aluminium. The metal as obtained in this process is lustrous and takes a polish, does not melt in the oxyhydrogen See also:flame, but liquefies in the electric arc, and is not affected by air at See also:ordinary temperatures. Chromium as prepared by the Goldschmidt process is in a passive See also:condition as regards dilute sulphuric acid and dilute hydrochloric acid at ordinary temperatures; but by heating the metal with the acid it passes into the active condition, the same effect being produced by heating the inactive See also:form with a See also:solution of an alkaline halide. W. Hittorf thinks that two allotropic forms of chromium exist (Zeit. fiir phys. Chem., 1898, 25, p. 729; 1899, 30, p. 481; 1900, 34, p. 385), namely active and inactive chromium; while W.

Ostwald (ibid., 1900, 35, pp. 33, 204) has observed that on dissolving chromium in dilute acids, the See also:

rate of solution as measured by the See also:evolution of See also:gas is not continuous but periodic. It is largely made as ferro-chrome, an alloy containing about 6o-7o% of chromium, by reducing See also:chromite in the electric furnace or by aluminium. Chromium and its salts may be detected by the fact that they give a deep green See also:bead when heated with See also:borax, or that on See also:fusion with See also:sodium carbonate and See also:nitre, a yellow See also:mass of an alkaline chromate is obtained, which, on solution in See also:water and acidification with acetic acid, gives a See also:bright yellow precipitate on the addition of soluble lead salts. Sodium and See also:potassium hydroxide solutions precipitate green chromium hydroxide from solutions of chromic salts; the precipitate is soluble in excess of the See also:cold See also:alkali, but is completely thrown down on boiling the solution. Chromic acid and its salts, the chromates and bichromates, can be detected by the See also:violet coloration which they give on addition of See also:hydrogen peroxide to their dilute acid solution, or by the fact that on See also:distillation with concentrated sulphuric acid and an alkaline chloride, the red vapours of chromium oxychloride are produced. The yellow colour of normal chromates changes to red on the addition of an acid, but goes back again to yellow on making the solution alkaline. Normal chromates on the addition of See also:silver nitrate give a red precipitate of silver chromate, easily soluble in See also:ammonia, and with See also:barium chloride a yellow precipitate of barium chromate, insoluble in acetic acid. Reducing agents, such as sulphurous acid and sulphuretted hydrogen, convert the chromates into chromic salts. Chromium in the form of its salts may be estimated quantitatively by precipitation from boiling solutions with a slight excess of ammonia, and boiling until the free ammonia is nearly all expelled. The precipitate obtained is filtered, well washed with hot water, dried and then ignited until the weight is See also:constant. In the form' of a chromate, it may be determined by precipitation, in acetic acid solution, with lead acetate; the lead chromate precipitate collected on a tared See also:filter See also:paper, well washed, dried at zoo° C. and weighed; or the chromate may be reduced by means of See also:sulphur dioxide to the condition of a chromic See also:salt, the excess of sulphur dioxide expelled by boiling, and the estimation carried out as above.

The atomic weight of chromium has been determined by S. G. Rawson, by the See also:

conversion of pure ammonium bichromate into the trioxide (See also:Journal of Chem. See also:Soc.,1899, 55, p.223), the mean value obtained being 52•o6; and also by C. Meinecke, who estimated the amount of silver, chromium and See also:oxygen in silver chromate, the amount of oxygen in potassium bichromate, and the amount of oxygen and chromium in ammonium bichromate (See also:Ann., 1891, 261, p. 339), the mean value obtained being 51.99. Chromium forms three See also:series of compounds, namely the chromous salts corresponding to CrO, chromous oxide, chromic salts, corresponding to Cr203, chromium sesquioxide, and the chromates corresponding to CrO3, chromium trioxide or chromic anhydride. Chromium sesquioxide is a basic oxide, although like alumina it acts as an acid-forming oxide towards strong bases, forming salts called chromites. Various other oxides of chromium, intermediate in See also:composition between the sesquioxide and trioxide, have been described, namely chromium dioxide, Cr20a•CrO3, and, the oxide CrO3.2Cr203. Chromous oxide, CrO, is unknown in the free state, but in the hydrated condition as CrO•H20 or Cr(OH)2 it may be prepared by precipitating chromous chloride by a solution of potassium hydroxide in air-free water. The precipitate so obtained is a See also:brown amorphous solid which readily oxidizes on exposure, and is decomposed by See also:heat with liberation of hydrogen and formation of the sesquioxide. The sesquioxide, Cr203, occurs native, and can be artificially obtained in several different ways, e.g., by igniting the corresponding hydroxide, or chromiut trioxide, or ammonium bichromate, or by passing the vapours of chromium oxychloride through a red-hot See also:tube, or by ignition of mercurous chromate.

In the amorphous state it is a dull green, almost infusible powder, but as obtained from chromium oxychloride it is deposited in the form of dark green hexagonal crystals of specific gravity 5.2. After ignition it becomes almost insoluble in acids, and on fusion with silicates it See also:

colours them green; consequently it is used as a pigment for colouring glassand See also:china. By the fusion of potassium bichromate with boric acid, and extraction of the melt with water, a See also:residue is See also:left which possesses a fine green colour, and is used as a pigment under the name of Guignet's green. In composition it approximates to Cr203•H20, but it always contains more or less See also:boron trioxide. Several forms of hydrated chromium sesquioxide are known; thus on precipitation of a chromic salt, free from alkali, by ammonia, a See also:light See also:blue precipitate is formed, which after drying over sulphuric acid, has the composition Cr20a•7H_0, and this after being heated to 200° C. in a current of hydrogen leaves a residue of composition CrO.OH or Cr203•H20 which occurs naturally as chrome ochre. Other hydrated oxides such as Cr203.2H2O have also been described. Chromium trioxide, CrO3, is obtained by adding concentrated sulphuric acid to a cold saturated solution of potassium bichromate, when it separates in See also:long red needles; the See also:mother liquor is drained off and the crystals are washed with concentrated nitric acid, the excess of which is removed by means of a current of dry air. It is readily soluble in water, melts at 193° C., and is decomposed at a higher temperature into chromium sesquioxide and oxygen; it is a very powerful oxidizing See also:agent, acting violently on See also:alcohol, converting it into acetaldehyde, and in glacial acetic acid solution converting See also:naphthalene and See also:anthracene into the corresponding See also:quinones. Heated with concentrated hydrochloric acid it liberates See also:chlorine, and with sulphuric acid it liberates oxygen. Gaseous ammonia passed over the oxide reduces it to the sesquioxide with formation of See also:nitrogen and water. Dissolved in hydrochloric acid at -2o°, it yields with solutions of the alkaline chlorides compounds of the type MCI•CrOC13, pointing to pentavalent chromium. For salts of this acid-forming oxide and for perchromic acid see BICHRoMATES.

Thechromites maybe looked upon as salts of chromium sesquioxide with other basic oxides, the most important being chromite (q.v.). Chromous chloride, CrC12, is prepared by reducing chromic chloride in hydrogen; it forms See also:

white silky needles, which dissolve in water giving a deep blue solution, which rapidly absorbs oxygen, forming basic chromic salts, and acts as a very strong reducing agent. The bromide and iodide are formed in a similar manner by heating the metal in gaseous hydrobromic or hydriodic acids. Chromous sulphate, CrSO,•7H2O, isomorphous with ferrous sulphate, results on dissolving the metal in dilute sulphuric acid or, better, by dissolving chromous acetate in dilute sulphuric acid, when it separates in blue crystals on cooling the solution. On pouring a solution of chromous chloride into a saturated solution of sodium acetate, a red crystalline precipitate of chromous acetate is produced; this is much more permanent in air than the other chromous salts and consequently can be used for their preparation. Chromic salts are of a blue or violet colour, and apparently the chloride and bromide exist in a green and violet form. Chromic chloride, CrC13, is obtained in the anhydrous form by igniting a mixture of the sesquioxide and carbon in a current of dry chlorine; it forms violet laminae almost insoluble in water, but dissolves rapidly in presence of a trace of chromous chloride; this See also:action has been regarded as a catalytic action, it being assumed that the insoluble chromic chloride is first reduced by the chromous chloride to the chromous condition and the See also:original chromous chloride converted into soluble chromic chloride, the newly formed chromous chloride then reacting with the insoluble chromic chloride. Solutions of chromic chloride in presence of excess of acid are green in colour. According to A. See also:Werner, four hydrated chromium chlorides exist, namely the green and violet salts, CrCl3.6H2O, a See also:hydrate, CrC13.10H2Oand one CrC13.4H20. The violet form gives a See also:purple solution, and all its chlorine is precipitated by silver nitrate, the aqueous solution containing four ions, probably Cr(OH2)6 and three chlorine ions. The green salt appears to dissociate in aqueous solution into two ions, namely CrC12(OH2)a and one chlorine See also:ion, since practically only one-third of the chlorine is precipitated by silver nitrate solution at o° C.

Two of the six water molecules are easily removed in a desiccator, and the salt formed, CrCl3.4H20, resembles the original salt in properties, only one-third of the chlorine being precipitated by silver nitrate. In accordance with his theory of the constitution of salts Werner formulates the hexahydrate as CrC12•(OH2)a•CL2H2O. Chromic bromide, CrBr3, is prepared in the anhydrous form by the same method as the chloride, and resembles it in its properties. The iodide is unknown. The fluoride, CrF3, results on passing hydrofluoric acid over the heated chloride, and sublimes in needles. The hydrated fluoride, CrF3.9H20, obtained by adding ammonium fluoride to cold chromic sulphate solution, is sparingly soluble in water, and is decomposed by heat. Oxyhalogen derivatives of chromium are known, the oxychloride, CrO2C12, resulting on heating potassium bichromate and See also:

common salt with concentrated sulphuric acid. It distils over as a dark red liquid of boiling point 117° C., and is to be regarded as the acid chloride corresponding to chromic acid, CrO2(OH)2. It dissolves See also:iodine and absorbs chlorine, and is decomposed by water with formation of chromic and hydrochloric acids; it takes See also:fire in contact with sulphur, ammonia, alcohol, &c., and explodes in contact with See also:phosphorus; it also acts as a powerful oxidizing agent. Heated in a closed tube at 18o° C. it loses chlorine and leaves a See also:black residue of trichromyl chloride, Cr308C12, which deliquesces on exposure to air. metal, and further, that the ammonia See also:molecule may be replaced by such other molecular See also:groups as -NO2, &c. Of the types studied the following may be mentioned: the diammine chromium thiocyanates, M[Cr(NH3)s•(SCN)a], the chloraquotetrammine chromic salts, Rh[Cr(NHa)4•H20•Cl], the aquopentammine or roseo-chromium salts,R',[Cr(NH a)6• H20],thechlorpentammine or purpureo-chromium salts, Ri2[Cr(NHa)s•Cl], the nitrito pentammine or xanthochromium salts, R'2[NO2•(NHa)s•Cr], the luteo or hexammine chromium salts, Ria[(NH3)s•Cr], and the rhodochromium salts: where Ri a monovalent acid See also:radical and M = a monovalent basic radical.

For the preparation and properties of these salts and a discussion on their constitution the papers of S. F. Jorgensen and of A. Werner in the Zeitschrift See also:

fur anorganisehe Chemie from 1892 onwards should be consulted. P. See also:Pfeiffer (Berichte, 1904, 37, 4255) has shown that chromium salts of. the type [CrIC2H4(See also:NH2)212X2]X exist in two stereo-isomeric forms, namely, the cis- and trans- forms, the dithiocyan-diethylenediamine-chromium salts being the trans- salts. Their configuration was determined by their relationship to their oxalo-derivatives; the cis-dichloro chloride, [CrC,Ha(NH2)2C12]Cl•H2O, See also:compound with potassium oxalate gave a See also:carmine red crystalline complex• salt, [Cr[C2H4(NH2)2]C2O4][CrC2H4(NH2)2•(C2O1)2]1IH2O, while from the trans-chloride a red complex salt is obtained containing the unaltered trans-dichloro group [CrC2H4(NH2)rC12].

End of Article: CHROMIUM (symbol Cr. atomic weight 52.1)

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