Mar. 25, 1914 
Reduction of Arsenic Acid 
5i7 
sulphurous acid is very slight and that the essential reaction involves the formation 
of tetrathionic acid, as follows: 
As( 0 H) 5 + 2 H 2 S 2 0 3 =As( 0 H) 3 +H 2 S 4 0 6 + 2 H 2 0 . 
A notable formation of any other acids of sulphur would necessarily result in a 
markedly higher figure for titration I. 
Corroborative evidence of the essential transformation of thiosulphuric acid to 
tetrathionic acid is given by titration III, for Stiasny and Das have shown that an 
alkali tetrathionate, heated with sodium or potassium carbonate, is nearly quantita¬ 
tively reconverted to thiosulphate. Titration III shows reformation of a quantity 
of thiosulphate equivalent to (1) 35.25 c. c. and (2) 35.55 c. c. of iodin solution, com¬ 
pared with an originally introduced quantity of thiosulphate equivalent to 39.50 c. c. 
of iodin, which amount of thiosulphate, as already noted, after oxidation to tetra¬ 
thionate, digestion with sodium carbonate, and repeated titration, required 30.45 
c. c. of iodin solution. 
To further prove the actual reduction of arsenic acid and also that 
such reduction is brought about by thiosulphuric acid in the absence 
of hydriodic acid, the theoretically possible action of which is not rig¬ 
orously excluded by the conditions of experiments Nos. 1 and 2, the 
following experiment was performed: 
Experiment No. 3.—A mixture of arsenic acid, sodium thiosulphate, and hydro¬ 
chloric acid, made exactly as described in experiments Nos. 1 and 2, was allowed to 
stand for five minutes. After the addition of methyl orange, normal caustic soda 
was run in until only faint acidity remained, as shown by the orange tint of the solu¬ 
tion. After the addition of a little sodium acetate and a drop or two of acetic acid to 
insure a distinctly acid reaction to litmus paper the solution was titrated cold with 
uranium acetate, using potassium ferrocyanid as indicator. The end point was 
reached upon the addition of 1 c. c. of uranium-acetate solution. Five c. c. of the 
arsenic-acid solution was then added and titration continued. The end point was 
again reached upon the addition of 10 c. c. more of uranium acetate, or a total of 11 
c. c. Lastly, 5 c. c. of the arsenic-acid solution, treated in a parallel manner, but 
without any addition of sodium thiosulphate, required 10.75 c. c * °f uranium-acetate 
solution. The previous conclusions regarding the nature and extent of the action 
upon arsenic acid were therefore confirmed. 
As previously indicated, a small amount of the thiosulphuric acid 
suffers decomposition into sulphur dioxid and, presumably, sulphur. 
The sulphur does not become evident under the conditions observed, 
being partly held in colloidal solution, but for the most part reacting 
with tetrathionic acid to form pentathionic acid, as shown by Stiasny 
and Das in their investigations already mentioned. The presence of 
pentathionic acid was here shown in a similar manner on several of the 
mixtures while they still remained clear by neutralizing with caustic 
alkali, using methyl orange as indicator. As the neutral point was 
reached, a distinct opalescence appeared which was not affected by 
hydrochloric acid, but which was dissolved after a time by excess of 
caustic alkali. 
The action of thiosulphuric acid upon arsenic acid appears, therefore, 
at least under the particular conditions studied, to be closely parallel 
to the action of thiosulphuric acid upon bichromic acid as described by 
Stiasny and Das. 
For obvious reasons it is not likely that the reaction here noted, 
apparently for the first time, will afford the basis for a desirable volu¬ 
metric method for use in the laboratory. It may be of value as a con¬ 
venient means for reducing arsenic acid to arsenious acid preliminary to 
precipitation by hydrogen sulphid. As a basis for a field test, in default 
of anything better, it does offer some promise, and experiments in that 
direction are now under way. 
