by Potassium Bromide in Acid Solution. 403 
thus set free may be determined by standard sodium thio- 
sulphate and taken as the measure of the bromine distilled. 
We have found an apparatus previously used in this laboratory 
in similar work (made by sealing the exit tube of a Voit wash- 
bottle, used as a retort, to the inlet tube of a Drexel wash-bottle 
used as a receiver, with a set of Will and Varrentropp absorb- 
tion bulbs sealed to the outlet tube of the receiver, to serve as 
a trap) extremely convenient in the distillation process, and a 
current of carbon dioxide passed slowly through the apparatus 
aids greatly in carrying the bromine to the receiver and in pro- 
moting quiet boiling. We find that the applicability of the 
reaction to quantitative purposes turns upon the adjustment of 
the proportions of the reagents used. The following table 
contains the results obtained by varying the relative amounts 
of acid and bromide and the time of boiling. The selenium 
trioxide recorded as found is calculated upon the assumption 
that selenious acid is the product of the reduction according to 
the equation, 
H,SeO, + 2HBr = H,SeO,+H,0O+Br, 
pe 79-140 — 16 
SeO, taken H.S0, of KBr - Initial Final SeO; 
as H.SeO,. half-strength. taken. volume. volume. found. Error. 
grm. em?, grm. em? em’, grm. grm. 
; 25 0°1140 0°0005 — 
1) 0°1145 9) 1 0) 
() (Oo fis otkos  O-GNE 
30 0:1134 0-0011— 
9 . 
(2) 01145 = 3 Be 23. O-1184 0:0043-4 
27 . 01134 0-0011— 
ee 10 aaah 23  0'1141 0°0004— 
(4) 0114520 1 60 35 01152 00007 + 
(5) 071145 20 Taper eao 35 01144 000014 
(6) 071145 20 of Ace 45  O-1172 0°0087+ 
From these results it is apparent that the amount of iodine 
set free in the receiver is dependent upon the proportion of 
the bromide, the strength of the acid, and the degree of con- 
centration during the distillation. When the proportions of 
sulphuric acid, potassium bromide, and selenic acid are favor- 
able, the bromine liberated is removed rapidly to the distillate, 
leaving the residue perfectly colorless, but as the distillation is 
continued the liquid residue again takes on color and more 
iodine is set free by the action of the distillate upon a clear 
solution of potassium iodide, while selenium is plainly visible 
in the receiver. When the amount of potassium bromide is 
large its effect is to retain bromine in the liquid so obstinately 
that no period of colorlessness intervenes before the second 
stage of color arrives; when its amount is small while that of 
