question_answer 1)

  Discuss briefly demineralisation of water by ion exchange resin.

Answer:

  The distilled water free from all soluble minerals is termed demineralised water. Demineralisation is done by ion exchange resins which are synthetic substances. These are of two types: Cation exchange resins and (i) Anion exchange resins. (ii) The cation exchange resins are granular insoluble organic acid. Resins have giant molecules with \[-S{{O}_{3}}H\] or\[COOH\]groups, while anion exchange resins contain giant organic molecules with basic groups derived from amines. The hard water is first passed through a bed of cation exchanger which removes the cations like \[N{{a}^{+}},M{{g}^{2+}}\],\[C{{a}^{2+}}\] etc., by exchanging with \[{{H}^{+}}\] ions. \[\underset{\operatorname{Re}\sin }{\mathop{2RCOOH}}\,+C{{a}^{2+}}\to {{(RCOO)}_{2}}Ca+2{{H}^{+}}\] The water coming from cation exchanger is acidic. This water is then passed through anion exchanger which removes anions like\[C{{l}^{-}},SO_{4}^{2-},NO_{3}^{-}\], etc., by exchanging with \[O{{H}^{-}}\] ions. \[R-N{{H}_{3}}OH+C{{l}^{-}}\to R-N{{H}_{3}}Cl+O{{H}^{-}}\]The \[O{{H}^{-}}\] ions combine with \[{{H}^{+}}\] ions to form water. \[{{H}^{+}}+O{{H}^{-}}\to {{H}_{2}}O\] This process gives demineralised water.