MANGANESE-CHROMIUMCOBALT-NICKEL. 309 



to the dichromate the solution turns yellow, and upon evaporation a salt of 

 the composition, K 2 CrO 4 , is obtained. The reason for this is the fact that, 

 although the dichromate dissociates in the main into 2K' and Cr.,0/' ions, it 

 also dissociates to a slight extent, thus : 



K 2 Cr 2 7 + H 2 O ^ 2K- + 2H- + 2CrO 4 ". 

 As alkali is added, the hydrogen ions are neutralized, thus : 

 2K- + 2(OH)' + 2H- = 2K- + 2H 2 O. 



To keep up the equilibrium, more H* ions and CrO/' ions are formed from the 

 dichromate, the H* ions react with more alkali, etc., until by this process the 

 dichromate is practically all converted into chromate. This change is usually 

 represented by the simple equation : 



K. 2 Cr 2 7 -f- 2KOH =r 21^010, + H 2 O. 



Many chromates, for example, those of barium, lead, silver, mercury, etc., are 

 insoluble in water and are obtained by precipitation. The ionic reaction in 

 the case of barium will serve to illustrate the other cases : 



2K- -f Cr0 4 " + Ba- * + 2C1' = BaCiO 4 + 2K> + 2C1'. 



The same precipitates result when a solution of a dichromate is used, because 

 it contains some CrO/' ions, and as fast as these are removed by precipitation, 

 others are produced to take their place in the system. But the precipitation 

 of the metal as chromate is not complete, as so.me dichromate of the metal re- 

 mains in solution, because of the acid that is liberated in the reaction. The 

 essential change is represented by the simple equation : 



K 2 Cr 2 O 7 -f 2BaCl 2 + H 2 O = 2BaCrO 4 + 2KC1 + 2HC1. 



This reaction is analogous to that between barium chloride and potassium 

 bisulphate : 



KHS0 4 + BaCl 2 = BaSO 4 + KC1 + HC1, 



with the difference that barium sulphate is so difficultly soluble, even in acids, 

 that precipitation is practically complete, whereas the chromates are more 

 easily soluble in acids, and precipitation therefore is only partial. 



Chromic oxide, Cr 2 O 3 (Sesquioxide of chromium), is obtained by 

 heating potassium dichromate with sulphur, when potassium sulphate 

 and chromic oxide are formed : 



K 3 Cr 2 O 7 -1- S = K 2 S0 4 + Cr 2 O 3 . 



By washing the heated mass with water, the chromic oxide is left 

 as a green powder, which is used as a green color, especially in the manu- 

 facture of painted glass and porcelain. Prepared by this method at 

 high temperature the oxide is insoluble in acids, but when obtained 

 in the form of its hydroxide by precipitation it is soluble in acids 

 forming the chromic salts. It is, therefore, a basic oxide. 



