158 METALS AND THEIR COMBINATIONS. 



chrome alum, K2S0 4 .Cr 2 3S0 4 .24H 2 0; it is a purple salt, and is 

 isomorphous with other alums. 



Analytical reactions. 

 a. Of chromic acid or chromates. 



1. Hydrosulphuric acid 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.) 



2. Soluble lead salts produce a yellow precipitate of lead 

 chromate (chrome-yellow), PbCr0 4 , insoluble in acetic, soluble 

 in hydrochloric acid: 



K 2 CrO 4 + Pb2NO 3 = PbCrO 4 + 2KNO 3 . 



3. Barium chloride produces a pale yellow precipitate of 

 barium chromate, BaCr0 4 : 



K 2 CrO 4 + BaCl 2 = BaCrO 4 -f 2KC1. 



4. Silver nitrate produces a dark red precipitate of silver 

 chromate, Ag 2 Cr0 4 : 



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



5. Mercurous nitrate produces a red precipitate of mercurous 

 chromate, Hg 2 CrO 4 . 



b. Of salts of chromium. 



6. To chromic chloride or sulphate add ammonium hydrate 

 or ammonium sulphide: in both cases the green hydrate of 

 chromium is precipitated : 



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



7. Potassium or sodium hydrate causes a similar green pre- 

 cipitate of chromic hydrate, which is soluble in an excess of 

 the reagent, but is reprecipitated on boiling for a few minutes. 



c. Of chromium in any form. 



8. Compounds of chromium, when mixed with sodium (or 

 potassium) carbonate and nitrate, give, when heated upon plati- 

 num foil, a yellow mass of the alkaline chromate. 



