128 THE ACTION OF AIR ON ALKALIC ARSENITES. 



Equal quantities of arsenious acid were dissolved in potassa, soda, 

 and their bicarbouates, the latter being used in excess. After free 

 exposure to the air for three weeks it was found that all the solutions 

 gave a pure canary yellow precipitate with nitrate of silver, but that 

 in the case of the soda solution the presence of an appreciable quan- 

 tity of arseniate could be detected by using nitric or acetic acid 

 in the manner above mentioned. The potassa solution gave an exceed- 

 ingly faint, almost imperceptible, trace of arseniate ; and the same 

 was the case with the bicarbonate solutions. Portions of these liquids 

 which had been kept in close stoppered bottles gave of course no indi- 

 cation of arseniate. From comparative experiments the quantity of 

 arseniate in the soda solution was estimated at between two and three 

 per cent., a result which differs most unaccountably from that obtained 

 by Fresenius. 



From these experiments it would appear that, contrary to Mohr's 

 statements, oxidation does take place in solutions of alkalic arsenites 

 exposed to the air, but that the amount of oxidation is in most cases 

 so small as not to interfere with the employment of such solutions in 

 accurate volumetric experiments, unless under such abnormal and un- 

 knov/n conditions as apparently occuredin Fresenius' experiments. 



It would seem therefore advisable in preparing solutions for such 

 purposes to adopt Mohr's suggestion, viz., to acidulate the arsenite of 

 soda solution with sulphuric acid, and then to dilute to one litre (or 

 other measure), and before using the solution to supersaturate with 

 carbonate of soda. All oxidation would thus be prevented, as free 

 arsenious acid in solution does not pass into arsenic acid. A solution 

 which had been very frequently exposed to the air for the last six 

 years, showed no trace of the latter. 



From experiments made since the above was written it would appear 

 that the use of acetic acid in this mode of testing is far preferable to 

 nitric acid, the least excess of which dissolves the arseniate. If a 

 considerable excess of acetic acid, containing about twenty per cent., 

 be added to the precipitate, the arsenite dissolves, and the arseniate 

 separates as a light floculent matter on the surface, exhibiting very 

 distinctly the brown color. In this manner less than one per cent, 

 can be readily detected. Experiments made on some of Fowler's 

 solution, which had been kept two months, discovered slight traces 

 of arseniate. It would be interesting to ascertain under what circum- 

 stances Fresenius' experiments were made ; in the above, in which so 

 very little oxidation took place, the solutions in potash and soda were 

 made as neutral as possible. 



