256 Gooch and Reynolds—Reduction of the | 
increased tenfold for the purpose of seeing whether the pres- 
ence of a large amount of free acid influences the result. 
TaBieE II. 
Volume 
HCl used. before 
SeO, taken. KIused. (Sp. gr. 1:20) filtering. SeO. found. Error. 
em? 
grm. erm. em? erm. erm, 
0°0499 og: 5 100 0°0489 0°:0010— 
0°0499 1 3 100 0°0485 0:0014— 
0:0499 1 50 100 0°0489 0:0010— 
0°0499 ] 50 100 0°0488 0;0011— 
0°2006 3 5 100 0°1925 0:0081— 
0°2030 3 5) 100 0°1945 0°0085 — 
These results show improvement over those obtained when 
filtration is made before acting with the thiosulphate, but it is 
obvious that the presence of a large proportion of free hydro- 
chloric acid is without effect upon the reaction, and that the 
iodine set free and measured is still deficient in proportion to 
the amount of selenium dioxide present. Plainly the reduction 
of the selenium dioxide is incomplete or else there is formed 
between the selenium and iodine a combination, such as was 
noticed by Hautefeuille* in the interaction of iodine upon 
hydrogen selenide. In either case it should be possible to push 
the reaction farther toward completion by submitting the mix- 
ture of selenious acid, potassium iodide, and hydrochlorie acid 
to distillation. We have used for this purpose an apparatus 
employed and described in connection with previous similar 
work in this laboratory. The distillation flask isa Voit gas- 
washing flask, and this is sealed to the inlet tube of a Drexel 
wash-bottle used as a receiver, to the outlet tube of which is 
sealed a Will and Varrentrapp absorbtion apparatus to serve as a 
trap. The mixture to be distilled was introduced into the flask, 
a solution of 3 grm. of potassium iodide in 100 em* of water 
was put into the receiver and trap and during the distillation a 
slow current of carbon dioxide was passed through the appa- 
ratus to keep the boiling regular. Naturally the acidified solu- 
tion of the iodide in the flask retains with great tenacity traces 
of dissolved iodine, so that, in order to determine all the iodine 
liberated in the reaction, the residue in the flask as well as the 
distillate in the receiver and trap was titrated in the usual way 
with sodium thiosulphate. The details of treatment and the 
results are recorded in Table III. 
* Compt. Rend., Ixviii, 1554. 
