1738 



Journal of Applied Microscopy 



MICRO-CHEMICAL ANALYSIS. 

 XIX. 



THE COMMON METALS.— SILVER GROUP. 

 ///. Arsenic Acid added to solutions containing Silver precipitates Silver Arsenate. 

 3 AgN03+H3As04=Ag3As04 + 3HN03. 

 Method. — Next to the test drop, which should be of moderate concentration, 

 place a small drop of water into which introduce a little arsenic acid and then a 

 tiny drop of ammonium hydroxide. Stir the reagent drop and cause it to flow 

 into the test drop. A fine granular precipitate is immediately produced ; later,, 

 thin plates and plate-like prisms appear (Fig. 78). The majority of the crys- 

 tals 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. 

 Remarks. — The arsenic acid can 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 pro- 

 cedure 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. 



Silver arsenate is insoluble in acetic acid, 

 soluble in hot nitric acid and easily soluble in 

 ammonium hydroxide. Good preparations 

 can be obtained by recrystallizing from either 

 of these solvents. 

 In case ammonium hydroxide is employed, the colorless solution resulting 

 contains the compound Ag3 AsO^ • 4NH;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 variable composition. 



Although the crystals of silver arsenate are neat, well formed and character- 

 istic, the reaction cannot be considered as a satisfactory one for silver because 

 of the fact that most of the other metals usually associated with silver are also 

 precipitated by arsenic acid, thus seriously interfering with the test. Solution of 

 the precipitated arsenate in ammonium hydroxide and drawing off, will 

 usually affect 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 separa- 

 tion of arsenates which are soluble in acids but insoluble in alkaline solution. 



\pVx.-o.o\ vmm. 



Fig. 78. 



