MICROCHEMICAL REACTIONS OF THE COMMON ELEMENTS 337 



e. Repeat the last experiment, adding this time the (NH4) 2 Cr 2 O7 first, and then 

 the H 2 S0 4 . 



/. Test several different preparations containing mixtures of the Ca group 

 and Ag. 



g. Test a mixture of AgNO 3 and HgNO 3 . 



h. Make a rather concentrated neutral test drop of AgNO 3 , add a tiny crystal 

 of Na 2 SO 4 . Study the Ag 2 SO 4 , which soon separates. Then add to the prepara- 

 tion a fragment of (NH4) 2 Cr 2 O 7 . Note well all that takes place. If a selenate is 

 at hand, substitute it in a new preparation for the Na 2 SO 4 . 



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 /, page 251. 



Silver arsenate AgsAsC^ (hexagonal) in the form of a fine 

 granular precipitate is immediately produced; later, thin plates 

 and plate-like prisms appear. The majority 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 crystal 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 Ag 3 As0 4 - 4 NH 3 , 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 



