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 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 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 it yields chromium sesquioxide . Sodium chromate, Na2CrO4•1OH2O, forms 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 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 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 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, 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 2.6—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, 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 (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 16o° C., potassium chlorochromate, phosphorus oxychloride, potassium chloride, and a complex chromium oxide (possibly Cr30e) are produced (A . Michaelis, Jour. peak .

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

photography, in calico-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 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 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 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 per-chromate, of composition KCrO4 or K2Ct20a, 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, H2Cr20a, whilst in the presence of an excess of hydrogen peroxide more highly oxidized products probably exist .