MANGANESE CHROMIUM COBALT NICKEL. 179 



chromic hydroxide is precipitated as a bluish-green gelatinous sub- 

 stance : 



Cr,(S0 4 ) 8 + 6NH 4 OH == 3[(NH 4 ) 2 SOJ + Cr 2 (OH) 6 . 



By dissolving this hydroxide in the different acids, the various 

 salts, such as chloride, Cr 2 Cl 6 , sulphate, etc., are obtained. Chromic 

 sulphate, similar to aluminum sulphate, combines with potassium or 

 ammonium sulphate and water, forming chrome alum, K 2 SO 4 .Cr 2 

 (SO 4 ) 3 24H 2 O ; it is a purple salt, and is isomorphous with other 

 alums. 



Analytical reactions. 



a. Of chromic acid or chromates. 



(Use potassium chromate, K 2 CrO 4 .) 



1. Hydrogen sulphide added to an acidified solution of a chromate 

 changes the red color into green with precipitation of sulphur. The 

 solution now contains chromium in the basic form. (See explanation 

 above.) (Plate II., 4.) The conversion of chromic acid into oxide 

 is more readily accomplished by heating the chromic solution with 

 alcohol and hydrochloric acid ; the alcohol is partly oxidized, being 

 converted into aldehyde. 



2. Soluble lead salts produce a yellow precipitate of lead chromate 

 (chrome yellow), PbCrO 4 , insoluble in acetic, soluble in hydrochloric 

 acid and in sodium hydroxide (Plate II., 6) : 



K 2 Cr0 4 + Pb(NO 3 ) 2 = PbCrO, + 2KNO 3 . 



3. Barium chloride produces a pale yellow precipitate of barium 

 chromate, BaCrO 4 : 



K 2 Cr0 4 + BaCl 2 =r BaCrO, + 2KC1. 



4. Silver nitrate produces a dark-red precipitate of silver chromate, 

 Ag a CrO 4 (Plate II., 8): 



2AgNO 3 + K 2 CrO 4 = 2KNO 3 + Ag 2 CrO 4 . 



5. Mercurous nitrate produces a red precipitate of mercurous chro- 

 mate, Hg 2 CrO 4 . 



b. Of salts of chromium. 

 (Use chrome-alum or chromic chloride, Cr 2 Cl 6 .) 



6. To chromic chloride or sulphate add ammonium hydroxide or 

 ammonium sulphide : in both cases the green hydroxide of chromium, 

 Cr 2 (OH) 6 , is precipitated (Plate II., 5) : 



Cr 2 Cl 6 + 3[(NH 4 ) 2 S] + 6H 2 O = 6NH 4 C1 + 3H 2 S + Cr 2 (OH) 6 . 



