380 ELEMENTARY CHEMICAL MICROSCOPY 



B. By Means of Ammonium Bichromate. 



Acidify the test drop with nitric acid. Add a fragment 

 of the reagent at the center. Allow to stand a few seconds. 



Dark red triclinic pleochroic crystals of the formula Ag 2 Cr 2 7 

 appear in the form of thin plates, having a rectangular or more 

 or less symmetrical coffin-like outline. Aggregates of irregular 

 broken scales are also abundant. 



Insufficiently acidified drops or those which are very concen- 

 trated yield as the first crop of crystals, tiny rods or needles so 

 dark colored as to appear black; after a time there will generally 

 separate in addition to these rods, the characteristic plates and 

 scales mentioned above. 



Cold solutions of lead yield only a bright yellow amorphous 

 precipitate. But from hot solutions, thin but long and slender 

 monoclinic prisms are formed, not however of lead bichromate 

 but having the composition PbCr0 4 . Lead chromate is soluble 

 in sodium hydroxide solutions. 



Mercurous salts yield with ammonium bichromate, in solutions 

 acidified with nitric acid, a number of different compounds (see 

 Mercury) varying in composition and appearance according to 

 the conditions which obtain. There is, however, little danger of 

 confusing these salts with the silver bichromate, since they all 

 appear as dark red crosses and bundles of irregular outline. These 

 compounds may, however, seriously interfere with the recognition 

 of silver if the latter is present only in traces. Mercurous chro- 

 mate is insoluble in sodium hydroxide, a distinction from lead. 1 



Bismuth salts yield irregular crystallites, small prisms and hexa- 

 gonal grains which are yellowish, orange or reddish brown in color. 

 The salt formed is probably bismuthyl bichromate (BiO^CroO?- 



Silver bichromate can be recrystallized from hot water, but 

 better results follow the use of dilute nitric acid or of ammonium 

 hydroxide. From hot nitric acid very beautiful preparations can 

 be obtained. According to some investigators the crystals which 

 separate on cooling from a hot neutral aqueous solution of the 

 bichromate precipitate are not silver bichromate, but normal 

 silver chromate, AgoCr04. 



1 If, however, only a minute quantity of sodium or potassium hydroxide is used, 

 a red basic chromate of lead results. 



