156 METALS AND THEIR COMBINATIONS. 



Chromium, Cr = 52.4. Found in nature almost exclusive!} 7 as 

 chrome-iron ore, FeO.Cr 2 O 3 , a mineral analogous in compo- 

 sition to magnetic iron ore, FeO.Fe 2 O 3 . The name chromium, 

 from the Greek xp"f* a , (chroma), color, was given to this metal on 

 account of the beautiful colors of its different compounds, none 

 of which is colorless. Chromium forms a basic oxide, Cr 2 O 3 , 

 and an acid oxide, Cr0 3 , the combinations and reactions of which 

 have to be studied separately. 



Potassium dichromate, Potassii bichromas, K 2 Cr 2 O r = 294.8 (Bi- 

 chromate or red chromate of potassium). This salt is by far the most 

 important of all combinations of chromium, and is the source 

 from which they are obtained. 



Potassium dichromate is manufactured on a large scale by 

 exposing a mixture of the finely ground chrome-iron ore with 

 potassium carbonate and calcium hydrate to the heat of an 

 oxidizing flame in a reverberatory furnace, when both con- 

 stituents of the ore become oxidized, ferric oxide and chromic 

 acid being formed, the latter combining with the potassium, 

 forming neutral potassium chromate, 



2(FeOCr 2 O 3 ) + 4K 2 CO 3 + 7O = Fe 2 O 3 + 4CO 2 + 4(K 2 CrO 4 ). 



By treating the heated mass with water, a yellow solution of 

 potassium chromate is obtained, which, upon the addition of 

 sulphuric acid, is decomposed into potassium dichromate and 

 potassium sulphate : 



2(K 2 CrOJ + H 2 SO 4 = K 2 Cr 2 O 7 + K 2 SO 4 + H 2 O. 



The two salts may be separated by crystallization. Potassium 

 dichromate forms large, orange-red, transparent crystals, which 

 are easily soluble in water; heated by itself oxygen is evolved, 

 heated with hydrochloric acid chlorine is liberated, heated with 

 organic matter or reducing agents these are oxidized. 



To explain the constitution of dichromates we have to assume 

 that chromic anhydride, CrO 3 , is capable of forming two acids: 



CrO 3 + H 2 O = H 2 CrO 4 = Chromic acid. 

 2Cr0 3 + H 2 O = H 2 Cr 2 O 7 = Dichromic acid. 



Chromium trioxide, Acidum chromicum, Cr0 3 = 100.4 (Chromic 

 acid. Chromic anhydride), is prepared by adding sulphuric acid to 



