Synthetic Inorganic Chemistry

120 THE THEORY OF IONIZATION

When another equivalent of sodium cyanide is added the pre-
cipitate entirely redissolves. In this solution the silver is found
to be in the negative ion, and to be associated with 2 cyanide
radicals.
AgCN — Ag+CN"
CN" Na+

Ag(CN) 2 -

There are thus two possible reactions between Ag+ and CN" ions

Ag+ + CN " ^± AgCN 1 and Ag+ + 2CN " ^ Ag(CN) 2 ~

The effect of increasing CN" ion concentration upon the second
reaction is far greater than on the first. Thus when an excess
of CN~ ion is employed the removal of Ag+ ions in the second
reaction is so complete as to cause the first reaction to go back-
ward.
This example is typical of complex ion formation. The for-
mulas of a few complex ions in the decreasing order of their
stability are:

Co(CN),~"

Fe(CN) 6

Fe(CN) 6 "~

Ag(CN) 2 "

Cu(CN) 2 "

Ni(CN) 4 ~~

CuCl,"

Agl 2 "

AaCl."

yellow

yellow

red

colorless

colorless

colorless

colorless

colorless

colorless

The last in the list is so little stable that it can exist only in a
concentrated chloride solution. Dilution of such a solution
causes precipitation of all the silver as simple silver chloride.
In solution complex ions are in equilibrium with their con-
stituents, for example, Cu-4NH 3 ++ ^ Cu++ + 4NH 3. In fact, an
excess of ammonia is necessary to stabilize the complex in solu-
tion. The ions NO 3 ~, SO 4 ~~, CO 3 , PO 4 are so very stable that
we do not think of them as other than simple ions. Nevertheless
they may be similar in nature to the complex ions just considered
except for the fact that no simple constituents, of which they might
be supposed to be built, have ever been identified.