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

Explanation

Synthetic ion exchange resins are of two types :
Cation exchange resins and anion exchange resins.
Cation exchange resins are either carboxylic acids or sulphonic acids having the general formula, R-COOH or R-`SO_(2)OH` where R represents the giant hydrocarbon framework. These resins exchange their `H^(+)` ions with `Ca^(+)` and `Mg^(2+)` ions present in hard water.
`underset("Cation exchange resin")(2R-COO^(-)H^(+))+ underset("From hard water")(CaCl_(2))to underset("Exhausted resin")((RCOO)_(2)Ca) + 2H^(+) + 2Cl^(-)`
`underset("(Cation exchange resin)")(2R-SO_(2)O^(-)H^(+))+underset("(From hard water)")(MgSO_(4)) to underset("(Exhausted resin)")((RSO_(2)O)_(2)Mg)+2H^(+)+SO_(4)^(2-)`
Anion exchange resins, on the other hand, are substituted ammonium hydroixdes having the general formula , `R+overset(+)NH_(3)OH^(-)` where R denotes the giant hydrocarbon framework. These resins exchange their `OH^(-)` ions with `Cl^(-)` and `SO_(4)^(2-)` ions present in hard water.
`underset("(Anion exchange resin)")(R+overset(+)NH_(3)OH^(-))+underset("(from hard water)")(Cl^(-))to underset("(Exhausted resin)")(R-overset(+)NH_(3)Cl^(-))+OH^(-)`
`underset("(Anion exchange resin)")(2R-overset(+)NH_(3)OH^(-))+underset("(From hard water)")(SO_(4)^(2-)) to underset("(Exhuasted resin)")((R-overset(+)NH_(3))_(2)SO_(4))+2OH^(-)`
Simultaneously , the `H^(+)` ions produced from cation exchange resins and `OH^(-)` ions produced from anion exchange resins combine to form `H_(2)O`
Process. The hard water is first passed through cation exchanged resin and then through anion exchange resin.
The resulting water is freed from both cations and anions and hence is called demineralised water or deionised water and is as good as distilled water.