9.26 What is meant by ‘demineralised’ water and how can it be obtained ?
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.
2RH(s) + M2+(aq) ⇄ MR2(s) + 2H+(aq)........(1)
In the anion exchange process, OH- exchange for anions such as Co2-3 , SO2-4 , CI- , HCO-3 etc. present in water.
RNH2(s) + H2O(I) ⇄ RNH3+ . OH- (s)
RHN3+OH-(s) + X-(aq) ⇄ RNH3+ . X-(s) + OH-(aq).........(2)
OH- ions liberated in reaction (2) neutralize H+ ions liberated in reaction (1)
Thereby Forming water,
H+(aq) + OH-(aq) → H2O(I)
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