| 9% 
Acids of Selenium by Hydriodic Acid. 257 
Taste lit. 
HCl in Total 
SeO. KI in flask. volume Time SeO, 
taken. flask. (Sp.gr. 1:20) boiled. in found. Error. 
erm. orm, em? em? minutes. erm. grm. 
0°0499 1 9) 60 9) 0°0497 0°0002 — 
0°0499 ik 5) 60 5 0°0497 0°0002— 
0°0499 IL 5 60 10 0°0496 0°00038 — 
0°2000 3 5 60 10 0°1995 0°0005 — 
0°2000 3 5 60 10 0°1991 0°0009 — 
0°2023 3 9) 60 10 0°2018 0°0005 — 
0°5018 3 3) 60 10 0°4635 0'0383 — 
These results are all fairly good, though alla little deficient, 
for amounts of selenium dioxide up to 0°2 grm; but when the 
amount of the dioxide reaches 0°5 grm. the iodine found in the 
distillate and in solution in the residue falls far below the 
theory based upon the assumption that the products are selen- 
ium, iodine, and water. The selenium in the residue was left 
after the boiling in fine dense crystalline condition in the 
experiments with the smaller amounts, so that it did not inter- 
fere with the titration of the free iodine; but in the last experi- 
ment, in which approximately 0°5 gram. of the dioxide was 
treated, the selenium remained in pasty form adhering to the 
flask, Subsequent examination proved that the pasty selenium 
held iodine, which was liberated slowly to water and more 
rapidly to an aqueous solution of potassium iodide. The 
largest errors have been found (excepting that of the last 
experiment from the discussion) when the free iodine was 
filtered off from the reduced selenium; better results were 
obtained when the precipitated selenium was first treated with 
the thiosulphate before filtering; and in the distillation pro- 
cess the best approximations are made to true indications. It 
is obvious that as the proportion of selenium and iodine increase, 
the tendency to form a combination is more manifest. The 
error thus introduced in the determination of the selenium 
dioxide by the distillation process is allowable up to the limit 
of 0°2 grm. 
Potassium iodide in hydrochloric acid acts much less readily 
upon selenic acid than upon selenious acid. When the hydro- 
chlorie acid is present in small proportions in the mixture of 
selenic acid and the iodide the reduction is very imperfect, but 
it tends to approach completion as the strength of hydrochloric 
acid is increased. 
It is obvious, in the light of the previous experiments with 
selenious acid, that it is unreasonable to expect the full libera- 
tion of iodine in the action of selenic acid upon the iodine 
