THE ACTION OF AIR ON ALKALIC ARSENITES. 127 



acetic acid and then tested with nitrate of silver, a pure yellow precip- 

 itate is produced without a trace of brown, and the same is the case if 

 the amount of arsenic acidbe increased even to five per cent. Beyond 

 that quantity the brownish red tint becomes perceptible. 



But if to the solution in which the yellow precipitate has been form- 

 ed, very dilute nitric acid be cautiously added, drop by drop, the 

 arsenite of silver being more readily soluble in nitric acid than the 

 arseniate, will dissolve first, and leave the latter with its characteristic 

 color. The smallest excess of nitric acid, however, dissolves the 

 arseniate and destroys the experiment. By adding ammonia and again 

 trying the nitric acid we can almost always succeed in producing the 

 red color. That the nitric acid employed in the process exerts no 

 oxidizing action on the arsenious acid is easily proved by using a pure 

 arsenite, when no trace of brown coloration can be obtained. 



Instead of nitric acid we may employ acetic, in which the arsenite 

 of silver is readily soluble, but the arseniate quite or nearlj^ insoluble. 

 With either acid, however, it is somewhat difficult to detect the pres- 

 ence of the arsenic acid when it does not amount to more than one per 

 cent. When ten per cent, of arseniate is present the brownish red 

 color is produced immediately ; with seven per cent, the color is 

 fainter, and with five per cent, the precipitate is so nearly pure yellow 

 that no safe - conclusions can be drawn as to the presence of arsenic 

 acid from the color alone. 



Sokitions of equal quantities of arsenious acid in potassa, carbonate 

 and bicarbonate of potassa, carbonate of soda, bicarbonate and caustic 

 soda, were prepared twelve months ago, with a view to some experi-, 

 ments on this subject, but have remained untouched until the present 

 time, the last tv/o being unfortunately lost. They were all freely ex- 

 posed to the air, but in neither of the first four cases could any brown 

 coloration be detected on neutralizing with acetic acid, and precipitat- 

 ing by nitrate of silver. In all of them, however, the arseniate was 

 readily detected by the process above described, thus confirming, as 

 might'have been expected, the results of that most accurate analyst, 

 Fresenius. 



While the oxidation of the arsenious acid was thus clearly proved, 

 the amount converted into arsenic acid was found to be exceedingly 

 small, and what is most remarkable is, that the solution in bicarbonate 

 of potassa showed a much larger proportion of arseniate than either 

 of the others, but in no case was the quantity very large, herein differ- 

 ing from Fresenius' experiments. 



The following experiments were made with the view of confirming Fre- 

 senius' statement with regard to the rapid oxidation of arsenite of soda : 



