Acids of Selenium by Hydriodic Acid. 255 
taining it by means of the vacuum pump upon a thick felt of 
asbestos in a perforated crucible or cone of large filtering sur- 
face. With a properly prepared filter of this description there 
is no difficulty in separating the selenium in a very few 
moments so completely that it is possible to determine the 
iodide remaining dissolved in the excess of potassium iodide 
with all the accuracy characteristic of this most exact of titra- 
tion processes. We find, however, that when the difficulty of 
determining the end-reaction in the titration of the iodine by 
the thiosulphate is overcome, it becomes apparent that the 
reaction upon which Muthmann and Schaefer depend is not 
perfect. Hither the reduction of the selenious acid to selenium 
is not complete, or else the iodine remains in combination to a 
slight extent with the selenium and so fails to appear in the 
filtrate. This is evident from the results of the experiments 
of Table I, in which the selenious acid and potassium iodide 
acidulated with hydrochloric acid were brought together, the 
liquid thrown upon the asbestos filter, the selenium washed 
until free from soluble iodine, and the filtrate containing the 
iodine treated as usual with sodium thiosulphate in presence of 
starch. The details of treatment are described sufficiently in 
the table. The selenium dioxide was prepared for the work 
from the so-called pure elementary selenium by dissolving it 
in strong nitric acid, evaporating off the excess of the last, treat- 
ing the solution of the residue in water with barium hydrox- 
ide, filtering to remove selenic acid formed in the oxidation 
and traces of sulphuric acid possibly present as an impurity, 
recovering the selenium dioxide by evaporation, and purifying 
it by subliming and re-subliming it in acurrent of dry air until 
it was clean and white. 
TasB_eE I, 
Volume 
HCl used. before 
SeO, taken. KI used. (Sp. gr. 1°20) filtering. SeO. found. Error. 
grm. grm. em? cm? grm. grm. 
0°0499 ] 5 100 0°0479 0°0020— 
0°0499 ib 5) 100 0°0477 0°0022— 
0°2035 3 5 100 0°1896 0°0139— 
From these figures it is plain that iodine was not found in 
the filtrate in amount corresponding to the selenium dioxide 
present. In the following experiments of Table II. an excess 
of the thiosulphate was added before filtering off the selenium 
so that there should be every opportunity for the iodine and 
thiosulphate to interact before the removal of the selenium. In 
two experiments the proportion of hydrochloric acid was 
