# 9.26  What is meant by ‘demineralised’ water and how can it be obtained?

R Rakesh

Demineralized water means free from all types of cations and anions and also soluble mineral salts.

It can be obtained by passing water successively through a cation exchange (in the $H^{+}$ ion form) and anion exchange (in the form of $OH^{-}$ion) resins:

Cation exchange process-

$2RH(s)+M^{2+}\rightleftharpoons MR_{2}(s)+2H^{+}(aq)$   {  exchange for $Na^{+}, Ca^{2+},Mg{2+}$ and other cation present in water.}

Anion exchange process-

$OH^{-}$  exchanges for anions like $Cl^{-},HCO_{3}^{-},SO_{4}^{2-}$ etc present in the water    $\\RNH_{2}(s)+H_{2}O(l)\rightleftharpoons RNH_{3}^{+}.OH^{-}(s)\\RNH_{3}^{+}.OH^{-}(s)+X^{-}(aq)\rightleftharpoons RNH_{3}^{+}X^{-}(s)+OH^{-}(aq)$

$OH^{-}$ ions liberated in anion exchange neutralizes the  $H^{+}$ ions liberated in cation exchange, thereby forming water

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