Discuss the principle and the method of softening of hard water by synthetic, ionexchange resins.

Explanation

The water which is free from all the dissolved mineral salts is called deionised water.
Deionised water can be prepared in two steps.
(1) Removal of cations : Hard water is passed through a tank containing cation exchange resin. Then the `Ca^(+2)` and `Mg^(+2)` ions present in the water are replaced by `H^(+)` ions from the resin.
`2R COOH + Ca^(+2) rarr [R (COO)]_(2)Ca+2H^(+)`
(2) Removal of anions : Now the water coming form the cation tank is passed through a tank containing anion exchange resin. This resin absorbs anions like `Cl^(-), SO_(4)^(2-)` etc. from the hard water and `OH^(-)` ions are released.
`RNH_(3)^(+) + Cl^(-) rarr RNH_(3)^(+) Cl^(-)+OH^(-)`
`2RNH_(3)^(+) + SO_(4)^(-2) rarr (R - NH_(3)^(+))_(2)SO_(4)^(-2)+2OH^(-)`
The `H^(+)` and `OH^(-)` ions unite to form Deionised water.
After sometime the cation resin and the anion resin lose their capacities to remove the cations and anions from the hard water. Then the resins are said to be exhausted. Then they are to be regenerated. The cation resin is regenerated by passing a solution of `H_(2)SO_(4)`. Anion resin is regenerated by passing a moderately concentrated solution of NaOH.