HYDROLYSIS 115HYDROLYSIS
The ionization of water is so slight that often it can be totally
disregarded. It cannot be neglected, however, in solutions of
salts when either the acid or the base — or both, from which the
salt is derived, is extremely weak.
Sodium cyanide is the salt of the weak hydrocyanic acid,
HCN (ionization = 0.01 per cent in 0.1 equivalent solution),
and the strong base, sodium hydroxide. A solution of this
salt shows an alkaline reaction to litmus, thus demonstrating
that the solution contains an appreciable quantity of OH~ ions.
This is the result of hydrolysis, and the process may be explained
as follows:
Na+ CN'
H+
HCNThe salt, in accordance with the general rule for salts, will exist
in solution in the ionized condition. Water is in equilibrium with
a very small number of its own ions. But even the small num-
ber of H+ ions thus furnished to the solution is more than can
exist in presence of the large concentration of CN~ ions of the
salt. Undissociated hydrocyanic acid, HCN, must form; but
since this removes some of the H+ ions, the equilibrium between
water and its ions is temporarily destroyed. The equilibrium
must be reestablished through the ionization of more water.
This cycle of reactions repeats itself a great many times until
complete equilibrium among all the components is established.
When this condition is reached, as really happens in a very short
time, there has been a considerable accumulation of OH~ ions
and of an equivalent amount of un-ionized HCN.
In order to fully comprehend the extent and the limitation of
this hydrolysis, we should consider the reverse reaction which
occurs when solutions of hydrocyanic acid and sodium hydrox-
ide are mixed:
Na+ OH"
HCN ;=± CN" H+
H 2 OThe few ions furnished by the acid combine at once with OH"
ions of the base to form water, and this removal of H+ ions allows