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Question
how can demineralised be obtained?
Solution
Demineralised water is obtained by passing water successively through a cation exchange (in the H+ form) and an anion exchange (in the OH– form) resin.
During the cation exchange process, H+ exchanges for Na+, Mg2+, Ca2+, and other cations present in water.
\[\ce{2RH_{(s)} + M^{2+}_{(aq)} ⇌ MR_{2(s)} + 2H^+_{(aq)}}\] ....(1)
In the anion exchange process, OH– exchanges for anions such as
\[\ce{CO^{2-}_3, SO^{2-}_4, Cl^-, HCO^-_3}\] etc. present in water.
\[\ce{RNH_{2(s)} + H_2O_{(l)} <=> RNH^+_3 . OH^-_{(s)}}\]
\[\ce{RNH^+_3OH^-_{(s)} + X^-_{(aq)} ⇌ RNH^+_3 . X^-_{(s)} + OH^-_{(aq)}}\] ....(2)
OH– ions liberated in reaction (2) neutralize H+ ions liberated in reaction (1), thereby forming water.
\[\ce{H^+_{(aq)} + OH^-_{(aq)} -> H_2O_{(l)}}\]
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