Question

Discuss the principle and method of softening of hard water by synthetic ion exchange resins.

Answer 1

Water that easily forms lather when treated with soap is soft water. Hard water does not form a considerable amount of lather with soap, this happens due to presence of Ca and Mg dissolved salts in water. The hardness due to bicarbonates are temporary and can be removed on boiling or adding slaked lime. Hardness due to sulphates, chlorides and nitrates are permanent.

Complete answer:
To remove permanent hardness we use Synthetic Resins Method:
This method proved to be the most efficient method to remove permanent hardness.
The principle of the method is to exchange the ions (cation and anion both) that form the salts responsible for hardness in water.
Cation exchange resins:
It contains a water insoluble large organic molecule with $- SO3H{\text{ group}}$. This is treated with NaCl to form RNa. And then resin exchange the ions:
$2RNa(s) + {M^{2 + }}(aq) \to MR_2(s) + 2N{a^ + }(aq)$
In cation exchange process, ${H^ + }$ exchange for $N{a^ + },C{a^{2 + }},M{g^{2 + }}$ ions and also other cations. It results in proton release which makes water acidic.
$2RH(s) + {M^{2 + }}(aq) \rightleftharpoons MR_2(s) + 2{H^ + }(aq)$
Anion exchange resins:
$O{H^ - }$ ion exchange for anions like $C{l^ - },HC{O^ - }3,S{O^{2 - }}4$ which are present in hard water.
$O{H^ - }$ along with ${H^ + }$ (obtained from cation exchange method)neutralizes and forms water.
$H{}^ + (aq) + O{H^ - }(aq) \to H_2O(l)$

Note:
We can regenerate resin by adding aqueous NaCl solution. Pure (de-ionized) water is obtained by passing water through a cation exchange and an anion exchange resin.The used exchange resin beds are regenerated by treating them with dilute acid and alkali solution.