I'Jll.j PUBLIC DOCUMENT — No. 31. 129 



arsenic. The presence of such impurities as cuprous and ferrous 

 compounds, sulfuroiis and nitrous acids or other oxidizable sub- 

 stances is a source of error by the iodine titration method. 



Iodine Method for Arsenates. 



The increasing use of lead arsenate as an insecticide resulted 

 in a demand for a rapid volumetric method for the determina- 

 tion of the arsenic acid. The Gooch and Browning process/ as 

 modified by Haywood,^ serves to readily reduce arsenic acid to 

 arsenous, in which form the iodine titration method is applicable. 

 The process in our hands did not at first prove satisfactory, but 

 eventually yielded concordant results after minor changes. As 

 the differences are largely a matter of detail, not involving- 

 principle, only the modified process will be given. 



Transfer 2 grams of finely ground sample, together with 60 

 cubic centimeters of nitric acid (1-3), to a 500 cubic centimeter 

 graduated flask ; bring to boil, cool, make to volume and filter. 

 Pipette 50 or 100 cubic centimeters into a 150 cubic centimeter 

 Jena Griffin beaker, add 10 cubic centimeters of sulfuric acid 

 (2-1), evaporate, heat in an air bath at 150-200° C. to expel 

 last traces of moisture, and then on asbestos board, to the appear- 

 ance of dense white fumes, to insure complete removal of nitric 

 acid. Add a small quantity of water, and when cold, filter 

 through a sugar tube under suction into a 300 cubic centimeter 

 Erlenmeyer flask, and wash to about 150 cubic centimeters. Add 

 10 cubic centimeters of potassium iodide (165-1,000) and boil 

 until free iodine is expelled, — solution practically colorless, — 

 with the reduction of arsenic to arsenous acid. 



AsoOo + 4 HI = AS2O3 + 4 I -f 2 H2O. 



Dilute, cool immediately, neutralize approximately three- 

 quarters of the free acid with 20 per cent, sodium hydrate solu- 

 tion, add starch paste, and if any free iodine remains, add 

 dilute (iSr/50) thiosulfate carefully, with vigorous shaking, to 

 the absence of blue color. 



2 1 + 2 Na2S203 = Na2S406 + 2 NaT. 



1 Amer. Jour. Sci., 40, 66 (1890). 



2 Proc. Assoc. Off. Agr. Chem., 2.3, 1G5 (1906). Provisional method of the association. 



