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Discuss briefly de-mineralisation of water by ion exchange resin. |
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Answer» Solution :SYNTHETIC resins method : Nowadays hard water is softened by using synthetic cation exchangers. This method is more efficient than zeolite process. Cation exchange resins : It contain large organic molecule with -`SO_3H` group and are water insoluble. Ion exchange resin `(RSO_3H)` is changed to RNa by treating it with NaCl. The resin exchanges `Na^+` ions with `Ca^(2+)` and `Mg^(2+)` ions present in hard water to make the water soft. Here R is resin anion. `2RNa_((s)) + M_((aq))^(2+) to R_2M_((s)) + 2Na_((aq))^(+)` The resin can be regenerated by adding aqueous NaCl solution. Pure de-mineralised (de-ionized) water free from all soluble mineral salts is obtained by passing water successively through a cation exchange in the `H^+` form) and an anion exchange in the `OH^-` form) resins. `2RH_((s)) + M_((aq))^(2+) hArr MR_(2(s)) + 2H_((aq))^(+)` In this cation exchange process, `H^+` exchanges for `Na^(+) , Ca^(2+), Mg^(2+)` and other cations present in water. This process results in proton release and thus makes the water acidic. Anion exchange process : `RNH_(2(s)) + H_2O_((l)) hArr RNH_(3)^(+) . OH_((s))^(-)` `RNH_3^(+) . OH_((s))^(-) + X_((aq))^(-) hArr RNH_3^(+) . X_((s))^(-) + OH_((aq))^(-)` `OH^-` exchanges for ANIONS like `Cl^(-), HCO_3^(-) , SO_4^(2-)` etc. present in water . `OH^-` ions, thus, liberated neutralise the `H^+` ions set free in the cation exchange. `H_((aq))^(+)+ OH_((aq))^(-) to H_2O_((l))` The exhausted cation and anion exchange resin beds are regenerated by treatment with DILUTE acid and alkali solutions respectively. |
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