Question

how can demineralised be obtained?

Answer 1

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.

\[\ce{H^+_{(aq)} + OH^-_{(aq)} -> H_2O_{(l)}}\]