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 Ag2Cr207 
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 j 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 PbCr04. 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.^ 
Bismuth salf s 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 (Bi0)2Cr207. 
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, Ag2Cr04. 
If, however, only a minute quantity of sodium or potassium hydroxide is used, 
a red basic chromate of lead results. 
