THE REDUCTION OF ARSENIC ACID TO ARSENIOUS 
ACID BY THIOSULPHURIC ACID 
By Robert M. Chapin, 
Senior Biochemist , Bureau of Animal Industry 
While endeavoring to work out a practicable field method for the esti¬ 
mation of the total arsenic—that is, a method which should include both 
arsenites and arsenates—in arsenical baths used for dipping cattle, studies 
were made upon the effect of various reducing agents which are able to 
absorb iodin in acid solution upon the well-known reversible reaction, 
As(OH) 3 + 2l-f 2H 2 0^ ± As(0H) 6 + 2HI. Unless the solution in which this 
reaction is taking place is freely acidified with a strong mineral acid or 
heated, the progress of the reaction from right to left is inconveniently 
slow. It was found that the addition of sodium thiosulphate apparently 
so greatly aided the reduction that it rapidly went to completion, even 
in cold and but slightly acid solutions. From this observation it was 
but one step to discover that the presence of hydriodic acid played no 
part whatever, the reduction of arsenic acid to arsenious acid being 
quickly and completely effected by treatment with a mixture of sodium 
thiosulphate and mineral acid alone. * 
It has long been known that arsenic, like some other metals, may be quan¬ 
titatively precipitated as sulphid by sodium thiosulphate in a boiling acid 
solution. In the present case, however, provided the conditions are right, 
there is no formation of arsenious sulphid. 
The reactions which may follow from the acidification of a solution of 
sodium thiosulphate are complex and variable, depending upon tempera¬ 
ture, dilution, relative proportions of thiosulphate and acid, and pos¬ 
sibly upon the order in which the admixture is made. The matter has 
most recently been discussed by Stiasny and Das, 1 who studied the reac¬ 
tions between such a mixture and potassium bichromate, a problem simi¬ 
lar in nature to the one here under consideration. 
Preliminary experiments showed that (1) the rapidity with which the 
reduction of arsenic acid progresses is mainly dependent upon the con¬ 
centration of hydrogen ions, the organic acids, except oxalic, operating 
very sluggishly, and (2) the nature of the reactions probably depends to 
a considerable extent upon whether arsenic or thiosulphuric acid is in 
excess and is also varied by the order in which the three components, 
arsenic acid, thiosulphate, and mineral acid, are mixed if the operation of 
mixing occupies any considerable time. 
The present series of experiments was limited to the study of the 
reactions occurring when a mixture of arsenic acid, or arsenate, with 
excess of sodium thiosulphate is acidified with an appropriate amount of 
hydrochloric or sulphuric acid, such being the conditions which must 
necessarily prevail in any method for the quantitative estimation of arsenic 
which might be based on the reactions. The solutions employed were 
1 Stiasny, Edmund, and Das, B. M. Reaction between sodium thiosulphate and a mixture of potassium 
bichromate and sulphuric acid. A contribution to the chemistry of chrome tannage. Jour. Soo. Chem. 
Indus., v. 31, no. 16, pp. 753~759- 1912. 
(51s) 
Journal of Agricultural Research, 
Dept, of Agriculture, Washington, D. C. 
Vol. I, No. 6 
Mar. 25, 19x4 
A—4 
