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

Explanation

Water which is free from all a solube minerals salts is called demineralised water.
Deminerlised water is obtained by passing water successively throught a cation exchanges and an anino exchange resin
In cation exchange, `Ca^(2+),Mg^(2+), Na^(+)` and other cations present in water are removed by exchanging them with `H^(+) ions` while in anion exchanger, `Cl^(-),HCO_(3)^(-), SO_(4)^(2-)`, etc., present in water are removed by exchanging them with `OH^(-) ions`.
`underset("(Relased in cation exchanger)")(H^(+)) + underset("(Relased in anion exchanger)")(OH^(-))to H_(2)O`
Syntheic ion exchange resins are tow types.
Cation exchange resins contain large organic molecule with `SO_(3)H` group and are water soluble. It is changed to RNa by treating it with NaCl. The resin `Mg^(2+)` and d`Ca^(2+)` ions present in hard water to make the water soft.
`2RNa(s) +M^(2+) (aq) to R_(2)M(s)+ 2Na^(+)(aq) (M= Ca^(2+) or Mg^(2+))`
The resin can be reagenerated by passing NaCl (aqueous solution) in .it
Pure deminerlasid (deionised) water is obtained by passing water successilvely thorugh a cation exchange and anion exchnage resins. In the cation exchange resins. In the cation exchange process,
`2RH(s) + M^(2+)(aq) hArr underset("resin in the" H^(+) "form")underset("Cation exchange")(MR_(2)(s)) + 2H^(+) (aq)`
`H^(+)` exchanges for `Ca^(2+),Mg^(2+)` and other cations present in water.
This process results in proton release and thus, makes the water acidic. In the anion
exchange process.
`RNH_(2)(s) + H_(2)O(l) hArr Roverset(+)N H_(3). OH^(-)(s)`
`Roverset(+)N H_(3). OH^(-)` is substituted ammonium hydroxide anion exchange ressin.
`Roverset(+)N H_(3). OH^(-)(s) + X^(-)(aq) hArr Roverset(+)N H_(3). X^(-)(s) + OH^(-)(aq)`