Bichromates_And_Chromates

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:—

M2CrO4 | M2Cr2O7 | M2Cr3O10 &c. | (M = one atom of a  
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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 hydrogen 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 acids, they are converted rapidly into bichromates. They are easily reduced in acid solution by sulphuretted hydrogen, and also by sulphur dioxide to chromium salts. The chromates are stable towards heat; they are poisonous, and may be recognized by the yellow precipitates they give with soluble barium and lead salts.

Potassium chromate, K2CrO4, may be prepared by neutralizing a solution of potassium bichromate 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 it yields chromium sesquioxide. Sodium chromate, Na2CrO4·10H2O, forms pale yellow crystals isomorphous with hydrated sodium sulphate, Na2SO4·10H2O. It 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 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 time with a caustic 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 brown mass, which at moderately high temperatures gives off oxygen and leaves a residue of a basic lead 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 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 nitrogen. Sulphuretted hydrogen and sulphur dioxide reduce them in acid solution to the condition of chromium salts.

Potassium bichromate, K2Cr2O7, 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 2.6-2.7, 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, K2Cr2O7 \+ 4H2SO4 = KaSO4 \+ Cr2(SO4)3 \+ 4H2O + 3O. On the addition of concentrated sulphuric acid to a cold saturated solution of the salt, red crystals of chromium trioxide, CrO3, separate (see Chromium), whilst when warmed with concentrated hydrochloric acid and a little water, potassium chlorochromate is produced. When heated with phosphorus trichloride in a sealed tube to 160° C., potassium chlorochromate, phosphorus oxychloride, potassium chloride, and a complex chromium oxide (possibly Cr3O6) are produced (A. Michaelis, Jour. prak. Chem., 1871, ii. 4, p. 452). Potassium bichromate finds application in photography, in calico-printing and in the preparation of bichromate cells. Sodium bichromate, Na2Cr2O7·2H2O, may be obtained by the addition of the requisite quantity of chromium trioxide to a solution of sodium chromate. It crystallizes in 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 form rhombic crystals of a red or brown 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 compound:—H2O2·CrO3 (H. Moissan, Comptes rendus, 1883, 97, p. 96); H2O2·2HCrO4 (M. Berthelot, Comptes rendus, 1889, 108, p. 25); Cr2O7·xH2O (L.C.A. Barreswil, Ann. chim. et phys., 1847 [3], 20, p. 364), and CrO6·3H2O (T. Fairley, Chem. News, 1876, 33, p. 237). The more recent investigations of H.G. Byers and E.E. 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 purple black precipitate of the perchromate, of composition KCrO4 or K2Cl2O8, is produced; this compound is very unstable, and readily decomposes into oxygen and potassium bichromate. Similar sodium, ammonium, lithium, magnesium, calcium, barium and zinc salts have been obtained. It is shown that the blue solution most probably contains the acid of composition, H2Cr2O8, whilst in the presence of an excess of hydrogen peroxide more highly oxidized products probably exist.

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