126 PROFESSOR JOHN GIBSON ON 
No change of colour was observed with solutions weaker than 34 per cent. HBr, which 
is the concentration of maximal hydrobromic acid * (Kynax, = 0°7590 ; Prax. = 4°24). 
Benaviour oF AQquEous SoLurions oF HyprocEn [opDIDE. 
In Aqueous solutions of hydrogen iodide the tendency towards increased specific 
conductivity is frequently masked, owing to the strong affinity between dissolved 
oxygen and the hydrogen of hydrogen iodide. 
Aqueous solutions of hydriodic acid do, however, exhibit the tendency to gain 
in specific conductivity. 
A well-known method for the preparation of hydrogen iodide is to suspend iodine in 
water and pass in hydrogen sulphide. This reaction, however, cannot be used to obtain 
solutions of pure hydriodic acid of a higher concentration than that of the maximal 
solution which has a conductivity at 18° C. of K,,,.=°740, [=3-4,* for in ultra- 
maximal solutions of hydrogen iodide an increase in the concentration of hydrogen 
iodide involves a decrease in specific conductivity. So long as the acid is premaximal 
the dissolved iodine is completely converted into hydrogen iodide, and the solution 
becomes colourless. Ultramaximal solutions remain coloured, however vigorous or 
long-continued the current of hydrogen sulphide may be. 
In all these systems where the strong affinities are nearly balanced, the tendency 
towards a gain of specific conductivity becomes effective and determines the position 
of equilibrium. 
The tendency of chlorine to unite with the hydrogen of water is of the same order 
as that of oxygen to unite with the hydrogen of hydrogen chloride, and similarly the 
tendency of iodine to unite with the hydrogen of hydrogen sulphide is of the same 
order as that of sulphur to unite with the hydrogen of hydrogen iodide. 
The tendency towards increased conductivity is masked, in an aqueous solution 
containing hydrogen iodide and oxygen, because the affinity of the hydrogen of 
hydrogen iodide for oxygen is enormously greater than that of the free iodine for the 
hydrogen of water. 
The addition of free iodine to a solution of hydrogen iodide lowers the conductivity 
of the solution. The resulting solution can decompose hydrogen sulphide only so long 
as there is a consequent gain in conductivity. 
In an ultramaximal solution of hydriodic acid the tendency towards increased 
specific conductivity may even induce the action of hydrogen iodide on sulphur, with 
production of hydrogen sulphide, free iodine, and probably HI. 
mH,O + (n+ 2)HI1 +8 — mH,0+nHI+H,8+ I, 
mH,O + (n+ 3)HI +8 = mH,0 + nHI+H,S + HI. 
* According to conductivity determinations made in this laboratory. 
