A RAPID VOLUMETRIC METHOD FOR THE DETERMI- 

 NATION OF ARSENIC IN ARSENATES 



By James M. Bell 



The rapid and exact method of Mohr for the determination of 

 arsenioiis acid depends on the reaction : 



AS2O3 + 2H2O + 2I,^As,0, + 4Hr 



This reaction is reversible, going toward the right when one of the re- 

 action products is removed as it is formed. Two methods have been 

 proposed for the removal of hydrogen iodide, both based upon the 

 reduction of the hydrogen-ion concentration. In the first of these 

 methods, the solution is kept neutral or slightly alkaline by means of 

 a large excess of sodium bicarbonate, added just previously to the 

 titration with iodine solution. This method gives satisfactory results. 

 The evolution of carbon dioxide may be avoided by the use of di- 

 sodium phosphate instead of sodium bicarbonate, as was proposed by 

 Washburn. 1 Advantage is here taken of the slight hydrogen-ion con- 

 centration in solution containing both disodium phosphate and mono- 

 sodium phosphate. This method also gives excellent results. 



A volumetric method of equal accuracy for arsenates was sought 

 in connection with a study of the arsenates, the results of which will 

 appear in a later paper. The volumetric method of the Association of 

 Official Chemists 2 depends on the reversal of the reaction as written 

 above, followed b}' titration with iodine solution. This is effected by 

 treating the arsenate with rather concentrated sulphuric acid and 

 potassium iodide and by removal of one of the constituents on the left 

 of the equation, iodine. Long continued boiling follows until all the 

 iodine has been volatilized. The solution is then neutralized and the 

 arsenious oxide titrated against iodine solution in the presence of 

 sodium bicarbonate. The removal of iodine by long continued boiling, 

 sometimes until fumes of SO 3 are evolved requires careful watching 

 and may also involve the removal by volatilization of arsenious iodide. 

 A yellow deposit of this substance frequently appears around the neck 

 of the flask and it is probable that some of it may have escaped. This 

 loss is indicated by low results obtained with known samples and by 

 the failure to obtain duplicate results. 



1 Jour. Am. Chem. Soc, 30, 31 (1908). 



^ Report of the Committee on Editing Tentative and Official Metliods of Analj'sis 

 (1916), p. 67. 



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