Modern inorganic chemistry

274 GROUP VI

soft water has the disadvantage that it attacks lead piping to give

the hydroxide, Pb(OH) 2 , which is slightly soluble and may give rise

to lead poisoning, which is cumulative.

METHODS FOR REMOVING THE METAL IONS RESPONSIBLE FOR

HARDNESS IN WATER

Temporary hardness only may be removed:

1. By boiling, as explained above; a method too expensive for use
   on a large scale.


2. By addition of slaked lime, in calculated quantity for the par
   ticular degree of hardness (Clark's method):

Ca(HCO 3 ) 2 + Ca(OH) 2 -> 2CaCO 3 i + 2H 2 O
For temporary hardness due to magnesium carbonate, more
lime is required, since the magnesium precipitates as the hydroxide
(less soluble than the carbonate):

Mg(HCO 3 ) 2 + 2Ca(OH) 2 -> Mg(OH) 2 l + 2CaCO 3 + 2H 2 O
It is thus important to determine the relative amounts of calcium
and magnesium, for addition of too much lime means that calcium
ions are reintroduced into the water, i.e. it becomes hard again, the
hardness being permanent.

Temporary or permanent hardness may be removed:

1. By addition of sodium carbonate, for example.
   Ca(HCO 3 ) 2 + Na 2 CO 3 -> CaCO 3 i + 2NaHCO 3

CaSO 4 + Na 2 CO 3 -» CaCO 3 i + Na 2 SO 4

2. By the use of an ion-exchanger. An ion-exchanger can be a
   naturally-occurring aluminatesilicate, called a zeolite, or its syn-
   thetic equivalent known by a trade name, for example 'Permutit'.
   Such exchangers have large, open three-dimensional structured
   anions with the negative charges at intervals, and balancing cations
   capable of free movement throughout the open structure.
   Alternatively the ion exchanger may be a synthetic polymer, for
   example a sulphonated polystyrene, where the negative charges are
   carried on the —SO 3 ends, and the interlocking structure is built
   up by cross-linking between the carbon atoms of the chain. The
   important property of any such solid is that the negative charge is
   static—a part of the solid—whilst the positive ions can move from
   their positions. Suppose, for example, that the positive ions are