126 THE ACTION OF AIR ON ALKALIC ARSENITES. 



ON THE ACTION OF AIR ON ALKALIC ARSENITES. 



BY HENRY CROFT, D.C.L., 



PEOPES^OE OS CHEMIiXBT, UNITEESIXT COLLEGE, TOEONTO. 



Read hefore the Canadian Institute, January, 1858. 



It is well known that a solution of arsenite of soda has been em- 

 ployed of late years as an important agent in volumetric analysis, 

 especially in the determination of the value of chloride of lime. 

 Fresenius (Chemical Gazette, No. 300,) has objected to this process, 

 that the alkalic arsenite when in solution is liable to oxidation, and 

 that by exposure to the air a notable quantity of the arseniate is 

 formed, which of course interferes with the accuracy of the volumetric 

 determination. 



He states that a freshly prepared solution of arsenite of soda gives 

 the usual pale yellow precipitate with nitrate of silver, but that with a 

 solution which has been exposed for some time, a precipitate having a 

 brown tinge is obtained, and he affirms that nearly the whole of the 

 arsenious acid is converted into arsenic in the course of three weeks, if 

 the bottle in which the alkalic solution is contained be opened rather 

 frequently. He also detected the presence of arseniate in Fowler's 

 solution, a fact which is of some importance in a medicinal point of 

 view. 



On the other hand, Mohr, (to whom we are indebted for most im- 

 portant improvements in this very elegant branch of chemical analysis,) 

 denies the oxidation of the liquor arsenicalis, and also of the arsenite 

 of soda in presence of excess of carbonate, and by volumetric experi- 

 ments with a solution of the arsenite mixed with starch and a normal 

 solution of iodine m iodide of potassium, he proved that no change 

 had taken place in a solution which had been kept ten months. He 

 states that this latter solution gave, with nitrate of silver, a pure canary 

 yellow precipitate, without any admixture of brown. 



The following experiments, without fully clearing up the difficulty, 

 at least exhibit one of the causes of the discrepancies in the statements 

 of the above mentioned eminent chemists. 



The formation of a pure canary yellow coloured precipitate in a 

 solution of an alkalic arsenite, is no proof that an arseniate is not 

 present. If 99 parts of arsenious acid be mixed with 1 part of arsenic 

 acid, and dissolved in potassa, the solution carefully neutralized with 



