How Water is Softened
Hard water caused by the presence of
calcium and magnesium ions (which form insoluble
precipitates with soaps) is softened by exchanging its
calcium and magnesium ions with sodium ions. To
accomplish this, the hard water is passed through a column
of cation exchanger containing sodium ions.
After the column
has been in use for some time, calcium and magnesium
begin to appear in the water leaving the column. Then the
column must be regenerated by passing a concentrated
solution of common salt slowly through the column; the
excess sodium ions displace the ions that produce the
hardness so that, after flushing with water, the bed of
exchanger is ready to be used again. At first, the exchangers
used for this purpose were natural aluminosilicates; but later,
synthetic resins came to be used instead.