Reduction of Arsenic Acid in Analysis. 67 



is boiled, iodine is evolved and the color of the liquid 

 passes from the dark red when the iodine is abundant through 

 the various gradations of tint to a canary yellow, and then, 

 as the sulphuric acid reaches a degree of concentration suffi- 

 cient to determine by its own specific action the liberation of 

 iodine, the color again darkens, and if the process of concentra- 

 tion is continued, and much arsenic is present, crystals of 

 arsenious iodide separate and form more abundantly on cooling. 

 If evaporation is pushed still farther the arsenious iodide begins 

 to volatilize and at the point where the sulphuric acid fumes 

 the liquid loses all color and the arsenic has vanished more or 

 less completely. In one experiment conducted in this man- 

 ner it was found, by the method to be described later, that of 

 0-3861 grm. of arsenic pentoxide originally present with 1 

 grm. of potassium iodide and 10 cm 3 of sulphuric acid [1 : 1] 

 the equivalent of 0*1524 grm. remained. In another similar 

 experiment in which, however, only a few milligrams of arsenic 

 oxide were involved not a trace of arsenic remained at the 

 end. 



It is obvious that two points in this course of action demand 

 examination at the outset. First, means must be found for 

 removing the remnant of free iodine which is withheld by the 

 hydriodic acid ; or of rendering it harmless in the titration 

 process to follow ; and, secondly, the degree to which the solu- 

 tion may be concentrated without loss of arsenic must be fixed. 

 In our work upon the converse of this process, we noted 

 particularly the marked influence of the amount of sulphuric 

 acid present upon the degree of concentration necessary to 

 expel the iodine. We turned attention, therefore, at once to 

 this point in the present case and investigated the effect of 

 varying the proportion of sulphuric acid in solutions contain- 

 ing definite amounts of potassium iodide and potassium arseni- 

 ate. The volume of the solution was made up to about 100 

 cm 3 and concentrated by boiling until the color was faintest. 

 Then, to determine provisionally, and for preliminary purposes, 

 the point at which volatilization of arsenic was likely to occur, 

 the concentration was continued until the arsenious iodide 

 began to separate. The results are tabulated as follows . : 







Volume when 











color was 



Volume when 



KI 



As 2 5 



H 2 S0 4 [1 : 1] lightest. 



Asl. 



i appeared. 



1 grm. 



0-1900 grm. 



20 cm 3 80 cm 3 





33 cm 3 



1 " 



0-1900 " 



15 " 65 " 





25 " 



1 " 



0-1900 " 



10 " 40 " 





19 " 



1 " 



0-1900 " 



5 " 30 " 





11 " 



The amount of sulphuric acid which, considering rapidity 

 in concentrating to the proper point, ease in neutralizing the 



