MANGANESE AND CHROMIUM 531 



Potassium Chromate K 2 CrO 4 . Powdered chromite is 

 roasted with potash and lime: 



4FeO,Cr 2 O 3 + 8K 2 CO 3 + 70 2 -> 2Fe 2 3 + 8K 2 CrO 4 + 8C0 2 f 



and the potassium chromate dissolved out of the residue. It is 

 a yellow, soluble salt (the ion Cr0 4 = is yellow), with which in- 

 soluble chromates of other metals, such as lead chromate PbCr0 4 , 

 can be precipitated. 



The Dichromates. When a solution of potassium sulphate 

 is mixed with an equivalent amount of sulphuric acid, potassium 

 bisulphate is obtainable by evaporation : K 2 S0 4 + H 2 S04 

 2KHS0 4 . The dry acid salt, when heated, loses water (p. 271), 

 giving the pyrosulphate (or disulphate) : 2KHS0 4 <= K 2 S 2 7 +H 2 0, 

 but the latter, when redissolved, returns to the condition of acid 

 sulphate. The second action is instantly reversed in presence of 

 water. Now, when an acid is added to a chromate we should 

 expect the chromic acid H 2 Cr04, thus liberated, to interact, giving 

 an acid chromate (say, KHCr0 4 ). No acid chromates are known, 

 however, and instead of them, pyrochromates or dichromates 

 are produced, with elimination of water. In other words, the 

 second of the above actions is not appreciably reversible in pres- 

 ence of water when chromates are in question: 



2K 2 CrO 4 + H 2 S0 4 . - K 2 Cr 2 7 + H 2 O + K 2 SO 4 . (1) 



In terms of the ionic hypothesis, S 2 07 = is unstable in water, and 

 interacts with the OH~ ion it contains, giving water and sulphate- 

 ion, while Cr 2 C>7 = is stable in water and is formed from the inter" 

 action of water and chromate-ion : 



+ 20H- fc> H 2 O + 2S0 4 =, 

 Cr 2 7 = + 20H- => H 2 + 2Cr0 4 = . (2) 



