MANGANESE CHROMIUM-COBALT NICKEL. 177 



5. Manganese compounds fused with borax on a platinum wire 

 give a violet color to the* borax bead. 



6. Traces of manganese may be detected by boiling with dilute 

 nitric acid and red lead, when the solution acquires a reddish-purple 

 color due to the formation of permanganic acid. 



Chromium, Or = 52. Found in nature almost exclusively as 

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

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

 from the Greek XP^O- (chroma), color, was given to this metal on 

 account of the beautiful colors of its different compounds, none of 

 which is colorless. Chromium forms two basic oxides, CrO and 

 Cr 2 O 3 , and an acid oxide, CrO 3 , the combinations and reactions of 

 which have to be studied separately. While chromium is closely 

 allied to aluminum and iron on one side, it also shows a resemblance 

 to sulphur, as indicated by the trioxide, CrO 3 , and the acid, H 2 CrO 4 , 

 which are analogous to SO 3 and H 2 SO 4 . Moreover, the barium and 

 lead salts of chromic and sulphuric acids are both insoluble. 



Potassium dichromate, Potassii bichromas, K 2 Cr 2 O 7 = 294 

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

 most important of all chromium compounds, and is the source from 

 which they are obtained. 



Potassium dichromate is manufactured on a large scale by expos- 

 ing a mixture of the finely ground chrome-iron ore with potassium 

 carbonate and calcium hydroxide to the heat of an oxidizing flame 

 in a reverberatory furnace, when both constituents of the ore become 

 oxidized, ferric oxide and chromic acid being formed, the latter 

 combining with the potassium, forming normal potassium chromate, 

 K 2 CrO 4 . 



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



By treating the furnaced mass with water a yellow solution of 

 potassium chromate is obtained, which, upon the addition of sul- 

 phuric acid, is decomposed into potassium dichromate and potassium 

 sulphate : 



2(K 2 Cr0 4 ) + H 2 S0 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. 



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