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BICHROMATES AND CHROMATES

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Originally appearing in Volume V03, Page 913 of the 1911 Encyclopedia Britannica.
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BICHROMATES AND CHROMATES . See also:

Chromium trioxide dissolves readily in See also:water, and the See also:solution is supposed to contain chromic See also:acid, H2CrO4; the salts of this acid are known as the chromates. In addition to these normal salts, others exist, namely bichromates, trichromates, &c., which may be regarded as combinations of one molecular proportion of the normal See also:salt with one or more molecular proportions of chromium trioxide. The See also:series will thus possess the following See also:general formulae: M,CrO4 M2Cr2O7 1VI2Cr3Olo &c. (M =one See also:atom of a normal chromate bichromate trichromate monovalent See also:metal.) Chromates.—The alkaline chromates are usually obtained by See also:fusion of a chromium See also:compound with an alkaline carbonate and an oxidizing See also:agent, such for example as See also:potassium nitrate or chlorate. The native chrome-ironstone (Cr2O3.FeO) may be used in this way as a source of such compounds, being fused in a reverberatory See also:furnace, along with soda-ash and See also:lime, the oxidizing agent in this See also:case being atmospheric See also:oxygen. They may also be prepared by oxidizing chromium salts (in alkaline solution) with hydrogerr peroxide, See also:chlorine, See also:bleaching See also:powder, potassium permanganate and See also:manganese dioxide. The See also:majority of the chromates are yellow in See also:colour, and many of them are isomorphous with the corresponding sulphates. The alkaline chromates are soluble in water, those of most other metals being insoluble. By the addition of See also:mineral acids, they are converted rapidly into bichromates. They are easily reduced in acid solution by sulphuretted See also:hydrogen, and also by See also:sulphur dioxide to chromium salts. The chromates are See also:stable towards See also:heat; they are poisonous, and may be recognized by the yellow precipitates they give with soluble See also:barium and See also:lead salts.

Potassium chromate, K2CrO4, may be prepared by neutralizing a solution of potassium hichromate with potassium carbonate or with See also:

caustic potash. It crystallizes in yellow rhombic prisms, and is readily soluble in water, the solution having a See also:bitter See also:taste and an alkaline reaction. When heated in a current of sulphuretted hydrogen, or See also:carbon bisulphide, it yields a mixture of chromium sesquioxide and sulphide. When heated with sulphur it yields chromium sesquioxide. See also:Sodium chromate, Na2CrO4•1OH2O, forms See also:pale yellow crystals isomorphous with hydrated scdium sulphate, Na2SO4.10H2O. I t is deliquescent, and melts at 23° C. (M. See also:Berthelot). By evaporation of its aqueous solution at temperatures above 30° C. it may be obtained in the anhydrous See also:condition. Lead chromate, PbCrO4, occurs native as the mineral crocoisite, and may be obtained as an amorphous pale yellow solid by precipitating a soluble lead salt by an alkaline chromate. It is used as a pigment under the name "chrome yellow." When digested for some See also:time with a caustic See also:alkali it is converted into a basic salt, PbCrO4•PbO, a pigment known as " chrome red." It melts readily, and on cooling resolidifies to a See also:brown See also:mass, which at moderately high temperatures gives off oxygen and leaves a See also:residue of a basic leac salt; for this See also:reason fused lead chromate is sometimes made use of in the See also:analysis of organic compounds. See also:Silver chromate, Ag2CrO4, is a dark red amorphous powder obtained when silver nitrate is precipitated by an alkaline chromate.

It is decomposed by the addition of caustic alkalis, forming silver See also:

oxide and an alkaline chromate. Bichromates.—The bichromates are usually of a red or reddish-brown colour, those of the alkali metals being readily soluble in water. They are readily decomposed by heat, leaving a residue of the normal chromate and chromium sesquioxide, and liberating oxygen; ammonium bichromate, however, is completely decomposed into chromium sesquioxide, water and See also:nitrogen. Sulphuretted hydrogen and sulphur dioxide reduce them in acid solution to the condition of chromium salts. Potassium bichromate, K2Cr207, is obtained by fusing chrome ironstone with soda ash and lime (see above), the See also:calcium chromate formed in the See also:process being decomposed by a hot solution of potassium sulphate. After the calcium sulphate has settled, the potassium chromate solution is converted into bichromate by the See also:action of sulphuric acid, and the salt is allowed to crystallize. It forms large triclinic prisms of specific gravity 2.6—2.7, which are moderately soluble in See also:cold water and readily soluble in hot water. The solution is strongly acid in reaction and is very poisonous. Potassium bichromate finds extensive application in organic See also:chemistry as an oxidizing agent, being used for this purpose in dilute sulphuric acid solution, K,Cr2O7 +4H2SO4= K2SO4+Cr2(SO4) 3+4H2O+30. On the addition of concentrated sulphuric acid to a cold saturated solution of the salt, red crystals of chromium trioxide, CrO3, See also:separate (see CHROMIUM), whilst when warmed with concentrated hydrochloric acid and a little water, potassium chlorochromate is produced. When heated with See also:phosphorus trichloride in a sealed See also:tube to 16o° C., potassium chlorochromate, phosphorus oxychloride, potassium chloride, and a complex chromium oxide (possibly Cr30e) are produced (A. See also:Michaelis, Jour. See also:peak.

Chem., 1871, ii. 4, p. 452). Potassium bichromate finds application in See also:

photography, in See also:calico-See also:printing, and in the preparation of bichromate cells. Sodium bichromate, Na2Cr2Oi.2H20, may be obtained by the addition of the requisite quantity of chromium trioxide to a solution of sodium chromate. It crystallizes in See also:hyacinth-red prisms, which are very hygroscopic and melt at 320° C. Trichromates.—The trichromates are obtained by the addition of nitric acid (of specific gravity about 1.2) to solutions of the bichromates. They See also:form rhombic crystals of a red or brown red colour and are readily decomposed by warm water, with formation of the bichromate. Perchromic Acid.—By the addition of hydrogen peroxide to a solution of chromic acid, a See also:fine See also:blue coloration due to a perchromic acid is produced which is readily absorbed by shaking out with See also:ether. The following formulae have been assigned to the com- p. 364), and CrOd.3H20 (T. Fairley, Chem.

See also:

News, 1876, 33, p. 237). The more See also:recent investigations of H. G. Byers and E. E. See also:Reed (Amer. Chem. Jour., 1904, 32, p. 503) show that if metallic potassium be added to an ethereal solution of the blue compound at -20° C., hydrogen is liberated and a See also:purple See also:black precipitate of the per-chromate, of See also:composition KCrO4 or K2Ct20a, is produced; this compound is very unstable, and readily decomposes into oxygen and potassium bichromate. Similar sodium, ammonium, See also:lithium, See also:magnesium, calcium, barium and See also:zinc salts have been obtained. It is shown that the blue solution most probably contains the acid of composition, H2Cr20a, whilst in the presence of an excess of hydrogen peroxide more highly oxidized products probably exist.

End of Article: BICHROMATES AND CHROMATES

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