Demineralised water is free from all soluble mineral salts. It is obtained by passing water successively through a cation exchange and an anion exchange resin. Resins have giant molecules with -SO3H or -COOH groups.
First, the hard water is passed through the cation exchanger, which removes cations like Na+, Mg2+, Ca2+,etc. by exchanging them with H+ ions.
Now, this water coming out from cation exchanger is acidic, therefore it is passed through the anionic exchanger which removes anions like OH-, NO3-, etc. by exchanging with OH- ions.
At the end, the H+ and OH- thus produced, combine amongst themselves to produce water
This is demineralisation of water by ion exchange resin.
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