562 
Proceedings of the Royal Society of Edinburgh. [Sess. 
XLII. — On the Precipitation of Soluble Chlorides by Hydro- 
chloric Acid. By John Gibson, Ph.D., and R. B. Denison, 
D.Sc., Ph.D. 
(Read June 22, 1908. MS. received August 20, 1910.) 
In a communication made to this Society on December 6, 1897, a tendency 
towards maximum specific electrolytic conductivity was shown to be a char- 
acteristic of many chemical reactions.* This tendency is very marked in 
reactions in which strong acids play a part. Thus, to take a well-known 
example, chlorine decomposes water under the influence of light with pro- 
duction of hydrogen chloride and oxygen, but as the hydrogen chloride 
accumulates in the solution, the decomposition of the water is more and 
more retarded. This photo-chemical reaction is a reversible one, for in the 
case of highly concentrated solutions the hydrogen chloride is oxidised to 
water and free chlorine. 
The equation is usually written — 
4HC1 + 0 2 ^2H 2 0 + 2C1 2 . (1) 
but it may also be written — 
(x + 4)HC1 + ?/H 2 0 + 0 2 ===== ^HCl + (y+ 2)H 2 0 + 2C1 2 . . (2) 
It has been found by one of us that the equilibrium point of this reaction 
corresponds closely with the point of maximum specific conductivity. 
According to Kohlrausch, this maximum is reached when the hydrochloric 
acid contains about 18‘25 per cent. HC1, so that the conductivity falls when 
either water or hydrogen chloride is added to a solution having this concen- 
tration. In the case of solutions of hydrogen chloride containing 19 per 
cent. HC1 along with a little free chlorine, a trace of free chlorine was 
found to persist even when the solution was exposed to sunlight. On the 
other hand, in solutions containing 17 per cent. HC1 or less, the hydrogen 
chloride is not oxidised. On the contrary, a trace of free chlorine soon 
disappears. 
For the purpose of our discussion, it is important to note that equations 
(1) and (2) read from left to right imply a dilution, but from right to left a 
concentration of the hydrochloric acid. The hydrochloric acid may, there- 
fore, be said to tend towards maximum specific conductivity, for the reaction 
always proceeds so that the conductivity approaches the maximum either 
by increasing or by lessening the concentration of the hydrogen chloride in 
the solution. 
* Proc. Roy. Soc. Edin., vol. xxii. p. 33. 
