MA NGANESE CHROMIUM-COB A LT- NICKEL. 307 



from the Greek %po>/jLa (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, Chromous 

 oxide, CrO, the salts of which are, however, very unstable, and chromic 

 oxide or chromium sesquiozide, Cr 2 O 8 , and an acid oxide, chromium 

 trioxide, CrO 8 , 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 indi- 

 cated 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 in water. 



Metallic chromium is used in small proportion as an admixture to steel to 

 which it imparts great hardness. 



Potassium dichromate, Potassii dichromas, K 2 Cr 2 O 7 = 292.28 

 (Bichromate or red chr ornate of potash). 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 Cr0 4 . 



2(FeOCr 2 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 CrO 4 ) + 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. 



Sodium dichromate, Na 2 Cr 2 O 7 .2H 2 O (Bichromate of soda), is manufac- 

 tured by a process analogous to that used for potassium dichromate. The 

 crystallized compound resembles the potassium salt, but dissolves in less than 

 its own weight of water. The crystals being deliquescent, a granulated anhy- 

 drous salt which is but slightly hygroscopic, is also manufactured, and has 

 largely replaced the use of potassium dichromate. 



