338 ELEMENTARY CHEMICAL MICROSCOPY 



arsenic acid, thus seriously interfering with the test. Solution 

 of the precipitated arsenate in ammonium hydroxide and draw- 

 ing off will usually effect a partial separation at least, and yield 

 a more satisfactory test, but on the other hand the rendering of 

 the drop alkaline may lead to the separation of arsenates which 

 are soluble in acids but insoluble in alkaline solution. 



Arsenic acid applied as indicated may yield with calcium salts 

 a separation of the compound NH 4 CaAsO4 6 H 2 0, ortho- 

 rhombic, isomorphous with the corresponding phosphate; the 

 crystals appear as large envelope-like crystallites with more or 

 less ragged edges. If the solution be dilute hemimorphic forms 

 identical with those of ammonium magnesium phosphate are 

 seen, but generally of a larger size. Strontium yields minute 

 stars and crystalline grains; barium a dense amorphous pre- 

 cipitate. 



Members of the magnesium group yield colorless crystalline 

 double ammonium arsenates isomorphous with their double 

 ammonium phosphates. Good crystalline compounds will be 

 obtained with the alkaline earths and with the magnesium group 

 only when considerable ammonium hydroxide has been added 

 to the reagent or when the test drop is distinctly ammoniacal; 

 under these circumstances the detection of silver as arsenate may 

 be masked. 



Although silver arsenate is of little value as an identity test 

 for silver it is of considerable use in detecting arsenates. 



Precautions. 



The arsenic acid may be added directly to the test drop to 

 either neutral or to weak nitric acid solutions, but the best and 

 most uniform results seem to follow the procedure suggested 

 above. 



The amount of ammonium hydroxide added to the reagent 

 drop must never be sufficient to neutralize all the arsenic acid 

 and give rise to an alkaline solution. 



Note. 



It is of theoretical interest to consider in connection with the 

 arsenic acid test for silver, the behavior of compounds of the 



