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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.

Answers (1)
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