Saved Bookmarks
| 1. |
Discuss the principle and method of softening of hard water by synthetic ion exchange resins. |
|
Answer» Solution :(iii) Ion-exchange method : This method is also called zeolite/permutit process. Hydrated sodium aluminium silicate is zeolite/permutit. For the sake of simplicity, sodium aluminium silicate `(NaAlSiO_2)` can be written as NaZ. When this is added in HARD water, exchange REACTIONS take place. `2NaZ_((s)) + M_((aq))^(2+) to MZ_(2+(aq)) to NZ_(2(s)) + 2Na_((aq))^(+)` (M=Mg, Ca) Permutit/zeolite is said to be exhausted when all the sodium in it is used up. It is regenerated for further use by treating with an aqueous sodium chloride solution. `MZ_(2(s)) + 2NaCl_((aq)) to 2NaZ_((s)) + MCl_(2(aq))` (iv) 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. |
|