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Explain demineralization of water by ion exchange method

Mumbai University > First Year Engineering > Sem1 > Applied Chemistry 1

Marks: 5M

Year: May 2013

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Ion exchange resins are used for softening of water.

They are organic polymers with long chains and cross linked structure having functional groups through which various ions are exchanged.

The resins are porous and insoluble in water.

The principle of ion exchange method is based on the ability of ion exchange resins to exchange their functional group like H+ with cat ions like $Ca^{++}$ , $Mg^{++}$ , $Na^+$ and $(OH)^-$ with all anions present.

The process involves passing raw water through cationic exchange resin and followed by passing it through the anionic exchange resin.as the raw water passes through cation exchange resin, sulphates, chlorides , bicarbonates get converted into sulphuric , hydrochloric and carbonic acids.

$$R-H_2+MgCL_2 → R-M_g+2HCL$$

The acidic water emerges from cation exchange bed and is passed through the anion exchange bed where the anions are exchanged for the OH ions of resin.

$$R-(OH)_2+H_2SO_4 → R-SO_4 + 2H_2O \\ R-(OH)_2+2HCL → R-Cl_2 + 2H_2O$$

The water emerging from anion exchange bed is free from both cations and anions and hence completely demineralized. However, water May contain some dissolved gas which are further removed by passing it through degasifier where it is heated and the escaping gas are removed by applying vacuum. (Diagram)

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