4U-4 ELEMENTARY CHEMICAL MICROSCOPY 



DETECTION. 



A. In simple salts we may obtain the following colors and 

 reactions: 



a. Soluble chromates are yellow, bichromates red, their solutions 

 yellow. Solutions of chromium salts where chromium acts as a 

 base, when heated in acid solution, are green. 



b. Chromium yields with ammonium hydroxide a bluish or 

 greyish green or greyish lavender hydroxide. In the presence 

 of ammonium salts, especially ammonium chloride, this hydrox- 

 ide is partially soluble with the formation of the compound 

 CrCl 3 • 4 NH 3 . Boiling drives off the ammonia and chromium 

 is completely precipitated as Cr(OH) 3 . 



c. Silver nitrate gives in solutions weakly acid with nitric acid 

 a characteristic deep red chromate with both chromates and 

 bichromates (see Silver, Method B, page 380). In neutral solu- 

 tions silver nitrate gives a precipitate with chromates some- 

 what more readily than with bichromates, but the difference 

 is too slight to be of any practical use in differentiating between 

 the salts. 



d. Alkali chromates added to neutral solutions of manganous 

 salts give a characteristic manganous chromate, but alkali bi- 

 chromates give no such reaction (see Manganese, Method B, 

 page 407). 



B. By Conversion into Cesium Chrome Alum. 



To a drop of the solution to be tested add ammonium 

 hydroxide. Should a reddish liquid result, boil. Decant the 

 solution from the bluish or greenish precipitate. Wash the pre- 

 cipitate once or twice. Add a tiny drop of water and then very 

 carefully the least possible amount of dilute sulphuric acid which 

 will just dissolve the precipitate. Evaporate carefully nearly to 

 dryness and add a tiny drop of water. Finally introduce near 

 the center of the drop a fragment of cesium sulphate. Cesium 

 chrome alum will almost immediately separate in characteristic 

 alum crystals, the octahedron and dodecahedron predominating 

 (isometric). These crystals have a faint bluish tint by trans- 

 mitted light. The peculiar purple color of chrome alum will 

 not be seen unless they attain a relatively large size and reflec- 



