Synthetic Inorganic Chemistry

116 THE THEORY OF IONIZATION

more of the acid to ionize. This cycle of operations repeats itself
until we have the same state of equilibrium as existed in the solu-
tion obtained by dissolving pure sodium cyanide in pure water.
The reaction of neutralization in this case goes about 99 per cent
of the way to completion when equivalent amounts of the acid and
base are mixed. The reverse reaction, that is, the hydrolysis of
sodium cyanide, progresses only about 1 per cent of the way to
completion before the state of equilibrium is reached.
If we consider the case of a salt of a much weaker acid than
hydrocyanic acid, or of a salt of both a very weak acid and a very
weak base, it is fairly obvious that hydrolysis will be much more
extensive.
Aluminum sulphide furnishes a good example of this, for when
it is treated with water its hydrolysis is complete;

2A1+++ 3S"

6H 2 O ^ 6OH" 6H+

1 1

2A1(OH) 3 3H 2 S

1 1

2A1(OH) 3 1 3H 2 S t

Note in this reaction that solid A1 2 S 3 disappears, and solid A1(OH) 31

and gaseous H 2 S f appear.

IONIZATION OF POLYBASIC AdDS
It is usually true with polybasic acids that one hydrogen radical
ionizes with greater facility than the remaining ones. Thus
phosphoric acid ionizes primarily as a monobasic acid
H 3 PO 4 ^ H+ + H 2 PO 4 ~
to the extent of 27 per cent in 0.1 equivalent solution. The fairly
high concentration of H+ ions thus established prevents appre-
ciable ionization of the H 2 PO 4 ~ ion, but if one equivalent of NaOH
is added for each mole of H 3 PO 4 , the H+ ions from the first H
radical are entirely removed and the H 2 PO 4 ~ ion itself ionizes

to the extent of 0.2 per cent. If a second equivalent of NaOH

is now added, the HP0 4 ~~ ion is enabled to ionize to the extent

of 0.0002 per cent.