Question

Discuss briefly de-mineralisation of water by ion exchange resin.

Answer 1

Pure de-mineralised (de-ionized) water is free from all soluble mineral salts, it is obtained by passing water successively through a cation exchange ( in the $H^{+}$ from ) and an anion exchange (in the $O{H}^{-}$from) resins.

Initally, water is passed through cation exchange resin, in this cation exchange process, $H^{+}$ exchange for $N{a}^{+},C{a}^{2+},M{g}^{2+}$ and other cation present in water.

$2RH\left(s\right)+M^{2+}\left(aq\right)\rightleftharpoons M{R}_2\left(s\right)+2H^{+}\left(aq\right)$

Here R is resin.

Now, this water coming out from cation exchange resin is acidic, therefore it is passed through the anionic exchange resin, which removes anions like $C{l}^{-},S{O}_4^{2-},HC{O}_3^{-}$ etc. by exchanging them with $O{H}^{-}$ions.

$RN{H}_2\left(s\right)+H_{2}O\left(l\right)\rightleftharpoons RN{H}_3^{+}O{H}^{-}\left(s\right)$

$RN{H}_3^{+}O{H}^{-}\left(s\right)+X^{-}\left(aq\right)$$\rightleftharpoons RN{H}_3^{+}.X^{-}\left(s\right)+O{H}^{-}\left(aq\right)$

In the end the $H^{+}andO{H}^{-}$ thus produced combine amongst themselves to produce water

$H^{+}\left(aq\right)+O{H}^{-}\left(aq\right)\to H_{2}O\left(l\right)$

This complete process is called as demineralisation of water by ion exchange resin