Answer:
Ion exchange resins are synthetic
substances. These are of two types:
(i) cation exchange
resins and
(ii) anion
exchange resins.
The cation
exchange resins are granular insoluble organic acid resins having giant
molecules with \[-S{{O}_{3}}H\]or \[\text{COOH}\] groups having general formula
\[R-S{{O}_{3}}H\] or \[R-COOH\] where R is a giant hydrocarbon network while
the anion exchange resins contain giant organic molecules with basic groups derived
from amines. These resins remove all soluble minerals from water.
The hard water is
first passed through a bed of cation exchange which removes the
cations like \[N{{a}^{+}},M{{g}^{2+}},C{{a}^{2+}}\] and others by exchanging
with \[{{H}^{+}}\] ions.
\[2\underset{\operatorname{Re}\sin
}{\mathop{RCOOH}}\,+C{{a}^{2+}}\to {{(RCOO)}_{2}}Ca+2{{H}^{+}}\]
\[2RS{{O}_{3}}H+M{{g}^{2+}}\to
(RS{{O}_{3}})Mg+2{{H}^{+}}\]
The water coming
from cation exchanger is acidic on account of free \[{{H}^{+}}\] ions. This
water is then passed through anion exchange which removes anions like\[C{{l}^{-}},SO_{4}^{2-},NO_{3}^{-}\],
etc., by exchanging with \[O{{H}^{-}}\] ions.
\[R-N{{H}_{3}}OH+C{{l}^{-}}\to
R-N{{H}_{3}}Cl+O{{H}^{-}}\]
The \[O{{H}^{-}}\]
ions combine with \[{{H}^{+}}\] ions.
\[{{H}^{+}}+O{{H}^{-}}\to {{H}_{2}}O\]
This process
gives distilled water. This water is also called demineralised water.
The resins can be
regenerated by treated with cone. \[HCl\] and \[\text{NaOH}\]solution
respectively and regenerated exchangers can be used over again and again.
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