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

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

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R Rakesh

The treatment process of permanent hard water using synthetic resin is based on cations exchange (Ca^{2+},Mg^{2+},Na^{+}) and anion exchange ((Cl^{-},SO_{4}^{2-},HCO_{3}^{-}) ;present in water by H^{+}ions and OH^{-}ions.

There are two types-

(i) Cation exchange resins

(ii)anion exchange resins

Cation exchange resin is large organic molecules contain  -SO_{3}H group and are water-insoluble.  Ion exchange resin (RSO3H) is changed to RNa by treating it with NaCl. The resin exchanges Na^{+} ions with Ca^{2+} and Mg^{2+} ions present in hard water to make the water soft. 

2RNa(s)+M^{2+}(aq)\rightarrow R_{2}M(s)+2Na^{+}(aq)

There are cation exchange resins in H+ form. The resins exchange H+ ions for Na+, Ca2+, and Mg2+ ions.

2RH+M^{2+}\rightleftharpoons MR_{2}+2H^{+}

Anion exchange resins exchanges  OH^{-} ions for anions like   ((Cl^{-},SO_{4}^{2-},HCO_{3}^{-})  

\\RNH_{2}+H_{2}O\rightleftharpoons RNH_{3}^{+}.OH(s)\\RNH_{3}^{+}.OH^{-}+X^{-}\rightleftharpoons RNH_{3}^{+}.X^{-}+OH^{-}

Water first passes through the cation exchange process. The water obtained is acidic in nature and this acidic water then passed through anion exchange process where OH^{-} ions neutralize the  Hions

 

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