and Laboratory Methods. 1739 



It is of theoretical interest to consider in connection with the arsenic acid 

 test for silver, the behavior of compounds of the elements analogous to arsenic 

 as shown by their position in the Periodic System. We find, for example, crys- 

 talline salts of silver with phosphorus, as silver phosphate ; with antimony, silver 

 antimonate ; with vanadium, silver vanadate ; with chromium, silver chromates ; 

 with molybdenum, silver molybdates. Of these salts the chromates and vana- 

 dates can be employed for the detection of silver, but the phosphates, antimon- 

 ates, and molybdates cannot be made to yield sufficiently characteristic results. 

 Exercises for Practice. 



Test a neutral solution of AgNOg in the manner suggested above. 



Recrystallize a preparation of AggAsO^ from HNO3. 



Try another preparation with NH4OH. 



Test a mixture of Ag and Pb. Then one of Ag and Hg. 



With Sodium Phosphate. 



Neutral solutions of silver salts yield with this reagent an immediate dense 

 yellow precipitate consisting of tiny grains. In a few seconds there appear stars, 

 crosses and arrow-headed crystallites. Better crystals are obtained by recrys- 

 tallizing from ammonium hydroxide. From this solvent dense, highly refractive 

 yellow tetrahedra and globular masses separate, together with three and four 

 armed crystallites which probably have the composition AggPO^. 



In solution in ammonium hydroxide the salt seems to exist as the tetra-am- 

 monia compound AggPO^ •4NH3 corresponding to that obtained with arsenic, 

 but it is apparently less stable than the latter. 



Silver phosphate can also be recrystallized from acetic acid and from nitric 

 acid; the solubility of this salt in acetic acid being a distinction from the 

 arsenate. 



With Oxalic Acid. 



Oxalic acid produces in neutral solutions an immediate fine granular precipi- 

 tate of silver oxalate, Ag2C204. After a few seconds thin plates appear which 

 take the form of rhombs with very acute angles; hexagons and occasionally 

 imperfect prisms are also seen. The rhombs extinguish parallel to a line 

 bisecting the acute angles. 



If, however, to the test drop there is first added a trace of nitric acid and 

 then the drop of the reagent allowed to flow in, neat prisms are obtained. These 

 prisms polarize strongly and show vivid colors between crossed nicols. 



Often preparations of silver oxalate are obtained which bear a striking 

 resemblance to crystals of cadmium oxalate. 



Tartrates of Silver. 



Primary sodium tartrate added to test drops just acidified with nitric acid 

 precipitates clusters and single prisms with obliquely truncated ends. Twins are 

 frequent. (The preparations obtained resemble closely those of the double 

 oxalate of potassium and glucinum, Fig. 65.) 



