MICROCHEMICAL REACTIONS OF THE COMMON ELEMENTS 349 



Monodinic. Primary ammonium arsenate; pri- 

 mary sodium arsenate. 

 Tridinic. 



DETECTION. 



A. Through the Formation of Arsine and its Reaction with a 

 Crystal of Silver Nitrate. 



Use the distilling tube, Fig. 133, page 248, as a generator, 

 as indicated in Figs. 138 and 139. 



FIG. 139. 



Apparatus for the Detection of Arsenic. 



Fit the side tube with a plug of soft wood P. Introduce two or 

 three fragments of arsenic-free zinc Z, and through a pipette 

 dilute hydrochloric acid A (the acid will not flow into the lower 

 part of the tube until the plug P is loosened). Insert a loose 

 plug of absorbent cotton C which has been soaked in lead acetate 

 and dried. The plug P is next withdrawn. The acid is allowed 

 to flow upon the pure zinc; a tiny drop of water 5 is introduced 

 into the side tube and the plug reinserted. This drop makes a 

 tight seal and prevents loss of gas. The tube is now tipped 

 downward and a tube drawn down to a capillary and containing 

 loosely a tiny crystal S of silver nitrate and one L of lead acetate 

 is attached by means of a short piece of rubber tube R. From 

 time to time the crystal S is examined to see if it changes color. 

 If after some minutes S remains clear and colorless remove P, 

 insert the material to be tested by means of a bit of drawn-out 



