118 THE THEORY OF IONIZATIONCOMPLEX IONS
Ammoniates. Review what was said in Chapter II about water
of crystallization and hydrates (pp 62-65). Two definite crystalline
compounds of sodium carbonate and water are the monohydrate
and the decahydrate. Both of these will dissolve in water, but
both solutions are absolutely identical. Furthermore, either one
or the other of these hydrates can be caused to crystallize from
the solution by adjusting the temperature.
In solution, undoubtedly some water is in combination with
the salt, but it is impossible to say how much, because there is no
physical means of distinguishing the water thus held in com-
bination from the solvent water.
There are other substances than water, notably ammonia,
which form compounds similar to hydrates, in this case ammo-
niates. Numerous solid compounds containing ammonia of
crystallization are known, for example ammonio-copper sulphate
CuSO 4 4NH 3 -H2O, which is the subject of one of our later prepa-
rations (page 227). This substance is easily soluble in water con-
taining a little excess of ammonia, and it is possible to measure
how much ammonia is held to the salt, or rather to the positive
ion, in the solution. This may be done, for example, by passing
an electric current through the solution and measuring the pro-
portionate amounts of copper and ammonia which travel with
the positive current. Results show that 4 moles of NH3 travel
with each mole of Cu++ ions. The formula of the deep blue am-
monio-copper ion is therefore Cu-4NH 3 ++.
When a moderate amount of ammonia is added to a copper salt
solution, a light blue precipitate is formed which is really a basic
salt but which for simplicity we shall treat as the simple hydroxide.
Cu++ sor~
2NH4OH ^± 2OH " 2NH 4 +
jr
Cu(OH) 21
Addition of more ammonia quickly causes this precipitate to
redissolve, giving an intensely deep blue solution
Cu(OH) 2 ^ Cu++ + 2OH-
4NH 4 OH ;=± 4NH 3 + 4H 2 O
Cu4NH 3 ++