Modern inorganic chemistry

GROUP VI 275

sodium ions. If we shake up the solid ion-exchanger with hard

water, the sodium ions are replaced, i.e. exchanged, with ions of

greater charge, for example those of calcium and magnesium, and

hence the water is softened.

In practice, the exchanger is used in granules packed in a vertical

column, through which the water flows. The capacity for exchange

is considerable but when the column is exhausted i.e. 'filled* with

calcium and magnesium ions, it can be regenerated by passing a

concentrated solution of a sodium salt, for example sodium chloride,

through it, the exchange equilibrium now favouring replacement of

the calcium and magnesium by sodium ions since the latter are

present in a much higher concentration.

PURE WATER

The type of exchanger used to soften water is more correctly called
a cation-exchanger but it is also possible to make synthetic onion-
exchangers in which negative ions are mobile and can be exchanged.
By using hydrogen ions instead of sodium ions on the cation-
exchanger (i.e. by regenerating it with hydrochloric acid instead of
sodium chloride) and a hydroxyl ion amon-exchanger, the cations
and anions present in water can be replaced by hydrogen and
hydroxyl ions respectively. These ions unite to form unionised
water. Thus any soluble salts can be removed completely from
water by using two exchangers in series (or mixed in one column).
Hence this is a method of obtaining pure water and can be used
instead of distillation.
Pure water for use in the laboratory can be obtained from tap
water (hard or soft) by distillation; if water of great purity is required,
distillation must be carried out in special apparatus, usually made
of quartz, not glass or metal; precautions must be taken to avoid
any spray getting into the distillate. Water which is sufficiently pure
for most laboratory purposes can, however, be obtained by passing
tap water through cation-exchangers and anion-exchangers as
described above, when the water is^4 dekmised'.

THE ESTIMATION OF WATER

In a substance such as a salt hydrate (for example BaCl 2 .2H 2 O)

water can be determined by heating until it is all driven off. Provided

that only water is evolved on heating, the difference in weight gives

the water content. If water is mixed with other decomposition