MANGA NESE- CJIR OMIUM- COB A LT- NICK EL . 311 



Tests for chromium. 



a. Of chromates. 



(Use the reagent solution of potassium chromate, K 2 CrO 4 .) 

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

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

 The solution now contains chromium in the basic form. (See explana- 

 tion above.) (Plate II., 4.) The conversion of a chromate to a 

 chromium salt is more readily accomplished by heating the chromic 

 solution with alcohol and hydrochloric acid; the alcohol is partly 

 oxidized, being converted into aldehyde, which has a peculiar but 

 pleasant odor. 



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 CrO 4 + Pb(NO 3 ) 2 = PbCrO 4 + 2KNO 3 . 



3. Barium chloride produces a pale yellow precipitate of barium 

 chromate, BaCrO 4 ; insoluble in sodium hydroxide. 



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

 Ag 2 Cr0 4 (Plate II., 7). ' 



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

 mate, Hg 2 CrO 4 (Plate II., 8). 



6. On pouring a layer of ether upon a solution of hydrogen 

 dioxide, adding a few drops of potassium dichromate solution, a 

 little sulphuric acid, and shaking, the ether assumes a blue color, due 

 to the formation of unstable perchromic acid. A very delicate test. 



b. Of salts of chromium. 

 (Use a 5 per cent, solution of chrome-alum, or chromic chloride, CrCl 3 .) 



7. To the solution add ammonium hydroxide or ammonium sul- 

 phide : in both cases the green hydroxide of chromium, Cr(OH) 3 , is 

 precipitated (Plate II., 5). Compare with aluminum. 



2CrCl 3 + 3(NH 4 ) 2 S + 6H,O = 6NH 4 C1 + 3H 2 S + 2Cr(OH) 3 . 



8. Potassium or sodium hydroxide causes a similar green precipi- 

 tate of chromic hydroxide, which is soluble in an excess of the 

 reagent, but is re-precipitated on boiling for a few minutes. 



Ammonia water causes precipitation of chromic hydroxide, but 

 the precipitate is nearly insoluble in excess of the reagent. 



