5i6 
Journal of Agricultural Research 
Vol. I, No. 6 
(1) a tenth-normal (oxidimetric) solution of arsenic acid prepared by 
oxidizing arsenious acid with nitric acid and expelling excess of the latter, 
(2) a tenth-normal solution of sodium thiosulphate, (3) a twentieth-normal 
solution of iodin, free from iodate, and (4) normal hydrochloric acid. 
The equivalents of the solutions were as follows: 
Ten c. c. of the solution of arsenic acid reduced, after Williamson, with 
hydrochloric acid and potassium iodid and then rendered alkaline with 
an excess of sodium bicarbonate required 19.74 c * c * of the iodin solution. 
Twenty c. c. of the solution of sodium thiosulphate required 39.50 c. c. 
of the iodin solution. To the solution of sodium tetrathionate thereby 
resulting there were added 10 grams of dry sodium carbonate and the 
solution, loosely covered, was heated one hour upon a steam bath. It 
was then cooled, diluted, acidified to litmus paper with acetic acid, and 
without delay titrated with iodin solution, of which 39.45 c. c. were 
required. 
In the experiments to be described a measured quantity of arsenic acid 
was diluted to 25 c. c. and was mixed—whether previously neutralized or 
not appeared to be immaterial—with 20 c. c. of thiosulphate added from 
a burette, and then with 10 c. c. of normal hydrochloric acid added from 
a pipette. When containing moderate amounts of arsenic, the mixtures 
remained perfectly clear for possibly 15 minutes, disengaging but a slight 
odor of sulphur dioxid. After a variable time an opalescence would appear, 
rapidly increasing and becoming yellow and accompanied by a pronounced 
odor of sulphur dioxid. For quantitative work the action obviously must 
be stopped before the separation of sulphur and arsenious sulphid becomes 
perceptible. From the considerable number of experiments only enough 
will be described to show the natuie of the reactions occurring. 
Experiment No. 1.—Ten c. c. of the solution of arsenic acid, 15 c. c. of water, 20 c. c. 
of the solution of sodium thiosulphate, and 10 c. c. of hydrochloric acid were mixed 
and allowed to stand for 1% minutes. The solution was then titrated with the iodin 
solution, using starch indicator (titration I), after which sodium bicarbonate was 
added, avoiding unnecessary excess, and titration with iodin continued (titration II). 
The end point of titration II was but briefly persistent, owing to the tendency of 
sodium tetrathionate to be oxidized to sulphate by iodin in alkaline solution. Next, 
10 grams of dry sodium carbonate were added and the solution, loosely covered, was 
heated for 1 hour on a steam bath. Then it was cooled, somewhat diluted, acidified 
to litmus paper by acetic acid, and immediately titrated again with iodin (titration 
III). The results obtained were as follows: 
Titration 1.20. 50 c. c. of iodin. 
Titration II.19. 75 c. c. of iodin. 
Titration III.35. 25 c. c. of iodin. 
Experiment No. 2. —Experiment No. 1 was duplicated, with the single exception 
that the mixture was allowed to stand but 2% minutes before titration I was started. 
The results were as follows: 
Titration 1.20. 55 c. c. of iodin. 
Titration II.19. 80 c. c. of iodin. 
Titration III.35. 55 c. c. of iodin. 
Titration I removes the excess of reducing agent without affecting any arsenious 
oxid present, provided a sufficient quantity of hydriodic acid be also contained in 
the solution. To insure this condition, it is safer to add a little potassium iodid just 
before beginning titration I, though in case of experiments Nos. 1 and 2 sufficient 
was introduced during the titration itself. Titration II measures the arsenious acid 
formed by the reduction of arsenic acid. 
Comparing now the results of titration II with the iodin equivalent of the arsenic- 
acid solution, it is evident that the reaction is quantitative as respects arsenic. Com¬ 
paring the sums of titrations I and II (x) 40.25 and (2) 40.35 c. c.) with the iodin 
equivalent of the thiosulphate solution (39.50 c. c.), it appears that the formation of 
