1895-96.] Prof. Crum Brown on Theory of Salt-Solution. 57 
Experiment illustrating the Modern Theory of Salt- 
Solution. By Professor Crum Brown. 
(Read January 20, 1896.) 
It is well known to all chemists that sulphuretted hydrogen does 
not give a precipitate of zinc sulphide in a solution of zinc chloride 
or zinc sulphate if a sufficient (quite small) quantity of a strong 
acid, such as hydrochloric or sulphuric acid, has been added to the 
solution. It is also quite well known that if we add a sufficient 
quantity of a solution of sodium acetate to the clear solution con- 
taining the zinc salt sulphuretted hydrogen and strong acid, we 
throw down the whole of the zinc as sulphide. This used to he 
explained as follows : — Hydrochloric acid acts on zinc sulphide thus, 
ZnS + 2HC1 = H 2 S + ZnCl 2 , and therefore the opposite action 
ZnCl 2 + H 2 S = ZnS + 2HC1 cannot take place in the presence of 
hydrochloric acid. But as acetic acid is too weak an acid to act on 
zinc sulphide, the presence of acetic acid does not prevent the pre- 
cipitation of zinc sulphide, and the hydrochloric acid originally 
there, as well as that produced by the reaction, acts on the sodium 
acetate to form sodium chloride and acetic acid. On the modern 
theory the explanation is this : — The zinc sulphide is attacked not 
by the hydrochloric acid, but by the hydrogen ions; these are present 
in the solution of hydrochloric acid ; and that acid being to a great 
extent “ ionised,” we have a great concentration of hydrogen ions. 
There is a limiting value for the concentration of hydrogen ions, 
above which the action ZnS + 2H* = Zn*' + H 2 S takes place, so that 
zinc sulphide cannot be formed by the action of sulphuretted 
hydrogen on a zinc salt in a solution containing hydrogen ions 
with a concentration above this limiting value. The addition to 
a solution containing hydrogen ions of a salt of a weak acid, such 
as acetic acid, diminishes the concentration of the hydrogen ions. 
Sodium acetate is ionised to a very much greater extent than 
hydrogen acetate, and therefore when we add sodium acetate we 
are adding not only NaA but also ISTa’-f A', and some of the A' ions 
so introduced unite with H* ions to form HA, and so diminish the 
