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Encyclopedia Britannica - Main :: BER-BLA |
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BICHROMATES AND CHROMATES . Chromium trioxide dissolves readily in water, and the solution is supposed to contain chromic 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 salt with one or more molecular proportions of chromium trioxide. The series will thus possess the following general formulae:M,CrO4 M2Cr2O7 1VI2Cr3Olo &c. (M =one atom of a normal chromate bichromate trichromate monovalent metal.) Chromates.The alkaline chromates are usually obtained by fusion of a chromium compound with an alkaline carbonate and an oxidizing agent, such for example as 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 furnace, along with soda-ash and lime, the oxidizing agent in this case being atmospheric oxygen. They may also be prepared by oxidizing chromium salts (in alkaline solution) with hydrogerr peroxide, chlorine, bleaching powder, potassium permanganate and manganese dioxide. The majority of the chromates are yellow in 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 mineral
sulphur
Potassium chromate, K2CrO4, may be prepared by neutralizing a solution of potassium hichromate with potassium carbonate or with caustic potash. It crystallizes in yellow rhombic prisms, and is readily soluble in water, the solution having a bitter taste and an alkaline reaction. When heated in a current of sulphuretted hydrogen, or carbon bisulphide, it yields a mixture of chromium sesquioxide and sulphide. When heated with sulphur
pale yellow crystals isomorphous with hydrated scdium sulphate, Na2SO4.10H2O. I t is deliquescent, and melts at 23 C. (M. Berthelot). By evaporation of its aqueous solution at temperatures above 30 C. it may be obtained in the anhydrous condition. Lead chromate, PbCrO4, occurs native as the mineral
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 time with a caustic alkali it is converted into a basic salt, PbCrO4PbO, a pigment known as " chrome red." It melts readily, and on cooling resolidifies to a brown mass, which at moderately high temperatures gives off oxygen and leaves a residue of a basic leac salt; for this reason fused lead chromate is sometimes made use of in the analysis of organic compounds. 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 oxide
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 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 calcium chromate formed in the 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 action of sulphuric acid, and the salt is allowed to crystallize. It forms large triclinic prisms of specific gravity 2.62.7, which are moderately soluble in 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 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, separate
oxide
peak
Trichromates.The trichromates are obtained by the addition of nitric acid (of specific gravity about 1.2) to solutions of the bichromates. They 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 fine blue coloration due to a perchromic acid is produced which is readily absorbed by shaking out with ether. The following formulae have been assigned to the com- p. 364), and CrOd.3H20 (T. Fairley, Chem. News, 1876, 33, p. 237). The more recent
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