MICROCHEMICAL REACTIONS OF SILVER 
383 
e. Repeat the last experiment, adding this time the (NH4)2Cr207 first, and then 
the H2SO4. 
/. Test several different preparations containing mixtures of the Ca group 
and Ag. 
g. Test a mixture of AgNOs and HgNOa. 
h. Make a rather concentrated neutral test drop of AgNOs, add a tiny crystal 
of Na2S04. Study the Ag2S04, which soon separates. Then add to the prepara¬ 
tion a fragment of (NH4)2Cr207. Note well all that takes place. If a selenate is 
at hand, substitute it in a new preparation for the Na2S04. 
C. By Means of Arsenic Acid. 
The reagent is made by introducing into a drop of a 
dilute solution of arsenic acid a tiny drop of dilute ammonium 
hydroxide; stir. 
Apply the reagent by Method I, page 299. 
Silver arsenate, Ag3As04, (hexagonal) in the form of a fine 
granular precipitate is immediately produced; later, crystallites, 
thin plates and plate-like prisms appear. Finally many of the 
crystals which separate have the appearance of hexagonal plates. 
Their color by transmitted light varies from a reddish yellow 
in very thin plates to reddish brown with a tinge of dirty violet 
or even deep black as the thickness of the crystals increases. 
Crystallites bristling with long slender needles also abound. 
Silver arsenate is insoluble in acetic acid, soluble in hot nitric 
acid and easily soluble in ammonium hydroxide. Good prepa¬ 
rations can be obtained by recrystallizing from either of the 
latter solvents. 
In case ammonium hydroxide is employed, the colorless solu¬ 
tion resulting contains the compound Ag3As04 • 4 NH3, as has 
been shown by Widman. This tetra-ammonia salt can be made 
to crystallize in the absence of air in colorless needles, but on 
coming in contact with the oxygen of the air they turn red. It 
follows from this that the crystals obtained by recrystallizing 
silver arsenate from ammonium hydroxide are doubtless of vari¬ 
able composition. 
Although the crystals of silver arsenate are neat, well formed 
and characteristic, the reaction cannot be considered as a satis¬ 
factory one for silver because of the fact that most of the other 
metals usually associated with silver are also precipitated by 
