CHROMIUM, MOLYBDENUM, TUNGSTEN) URANIUM, ETC. 279 



chromium compounds. It is converted into yellow pigments by means 

 of double decomposition with salts of lead, barium, and zinc. When 

 solutions of the salts of these metals are mixed with potassium 

 dichroraate (in dyeing generally mixed with soda, in order to obtain 

 normal salts), they are precipitated as insoluble normal salts ; for 

 example, 2BaCl 2 + K 2 Cr 2 7 + H 2 O == 2BaCrO 4 + 2KC1 + 2HC1. It 

 follows from this that these salts are insoluble in dilute acids, but 

 the precipitation is not complete (as it would be with the normal salt). 

 The barium and zinc salts are of a lemon yellow colour ; the lead salt 

 has a still more intense colour passing into orange. Yellow cotton 

 prints are dyed with this pigment. The silver salt, Ag 2 CrO 4 , is of a 

 bright red colour. 



When potassium dichromate is mixed with potassium hydroxide 



gravure, photo-lithography, pigment printing, &c. Under the action of light this gelatin 

 is oxidised, and the chromic anhydride deoxidised into chromic oxide, which unites with 

 the gelatin and forms a compound insoluble in warm water, whilst where the light has not 

 acted, the gelatin remains soluble, its properties being unaffected by the presence of 

 chromic acid or potassium dichromate. 



5 Ammonium and sodium dichromates are now also prepared on a large scale. The 

 sodium salts may be prepared in exactly the same manner as those of potassium. The 

 normal salt combines with ten equivalents of water, like Glauber's salt, with which it is 

 isomorphous. Its solution above 80 deposits the anhydrous salt. Sodium dichromate 

 crystals contain Na2Cr 2 O 7 ,2H 2 O. The ammonium salts of chromic acid are obtained 

 by saturating the anhydride itself with ammonia. The dichromate is obtained by 

 saturating one part of the anhydride with ammonia, and then adding a second part of 

 anhydride and evaporating under the receiver of an air-pump. On ignition, the normal 

 and acid salts leave chromic oxide. Potassium ammonium chromate, NH 4 KCrO 4 , is 

 obtained in yellow needles from a solution of potassium dichromate in aqueous ammonia ; 

 it not only loses ammonia and becomes converted into potassium dichromate when 

 ignited, but also by degrees at the ordinary temperature. This shows the feeble energy 

 of chromic acid, and its tendency to form stable dichromates. Magnesium chromate is 

 soluble in water, as also is the strontium salt. The calcium salt is also somewhat soluble, 

 but the barium salt is almost insoluble. The isomorphism with sulphuric acid is shown 

 in the chromates by the fact that the magnesium and ammonium salts form double salts 

 containing six equivalents of water, which are perfectly isomorphous with the corre- 

 ponding sulphates. The magnesium salt crystallises in large crystals containing seven 

 equivalents of water. The beryllium, cerium, and cobalt salts are insoluble in water. 

 Chromic acid dissolves manganous carbonate, but on evaporation the solution deposits 

 manganese dioxide, formed at the expense of the oxygen of the chromic acid. Chromio 

 acid also oxidises ferrous oxide, and ferric oxide is soluble in chromic acid. 



One of the chromates most used by the dyer is the insoluble yellow lead chromate, 

 PbCrO 4 (Chapter XVIII., Note 46), which is precipitated on mixing solutions of 

 PbX 2 with soluble chromates. It easily forms a basic salt, having the composition 

 PbO,PbCrO 4) as a crystalline powder, obtained by fusing the normal salt with nitre and 

 then rapidly washing in water. The same substance is obtained, although impure and 

 in small quantity, by treating lead chromate with neutral potassium chromate, especially 

 on boiling the mixture ; and this gives the possibility of attaining, by means of these 

 materials, various tints of lead chromate, from yellow to red, passing through different 

 orange shades. The decomposition which takes place (incompletely) in this case ia 

 as follows: 2PbCrQ 4 + K 3 CrO^= PbCrO 4 ,PbO + K 2 Cr 2 O 7 that is, potassium dichromate 

 is formed in solution. 



