Question

What is the cause of the permanent hardness of water?

Answer 1

Permanent hardness:

1. The presence of magnesium and calcium soluble salts in the form of chlorides and sulphates in the water.
2. It is usually not removed by the boiling process.
3. The cause of the permanent hardness of water is dissolved chloride and sulphate ions.
4. These are removed by the ion-exchanger process as cation exchanger resins.
5. The safety limits of calcium and magnesium is $40-80\mathrm{ppm}$ and $20-30\mathrm{ppm}$.

Cation exchanger process:

1. The resins contain organic molecules with ${\mathrm{SO}}_3\mathrm{H}$ group and are water-insoluble.
2. When treated with $\mathrm{NaCl}$, ion exchange resin (${\mathrm{RSO}}_3\mathrm{H}$) becomes $\mathrm{RNa}$.
3. ${\mathrm{Na}}^{+}$ions are exchanged with ${\mathrm{Ca}}^{2+}$and ${\mathrm{Mg}}^{2+}$ions to soften hard water. $2\mathrm{RNa}\left(\mathrm{s}\right)+{\mathrm{M}}^{2+}\left(\mathrm{aq}\right)\to {\mathrm{R}}_2\mathrm{M}\left(\mathrm{s}\right)+2{\mathrm{Na}}^{+}\left(\mathrm{aq}\right)$, where R is resin anion
4. Cation and anion exchangers are used to obtain free soluble water.
5. Cation exchange process: Process of exchanging ${\mathrm{H}}^{+}$ for ${\mathrm{Ca}}^{2+}$and ${\mathrm{Mg}}^{2+}$and other cations present in water $2\mathrm{RH}\left(\mathrm{s}\right)+{\mathrm{M}}^{2+}\left(\mathrm{aq}\right)\rightleftharpoons {\mathrm{MR}}_2\left(\mathrm{s}\right)+2{\mathrm{H}}^{+}\left(\mathrm{aq}\right)$
6. Anion exchange process: A process where ${\mathrm{OH}}^{-}$exchanges anion species such as ${\mathrm{Cl}}^{-}$, ${\mathrm{SO}}_4^{2-}$ $$\begin{gathered} {\mathrm{RNH}}_2\left(\mathrm{s}\right)+{\mathrm{H}}_2\mathrm{O}\left(\mathrm{l}\right)\rightleftharpoons {\mathrm{RNH}}_3^{+}.{\mathrm{OH}}^{-}\left(\mathrm{s}\right) \\ {\mathrm{RNH}}_3^{+}.{\mathrm{OH}}^{-}\left(\mathrm{s}\right)+{\mathrm{X}}^{-}\left(\mathrm{aq}\right)\rightleftharpoons {\mathrm{RNH}}_3^{+}.{\mathrm{X}}^{-}\left(\mathrm{s}\right)+{\mathrm{OH}}^{-}\left(\mathrm{aq}\right) \end{gathered}$$
7. Dilute acid and alkali solutions are used to regenerate the exhausted cation and anion exchange resin beds ${\mathrm{H}}^{+}\left(\mathrm{aq}\right)+{\mathrm{OH}}^{-}\left(\mathrm{aq}\right)\to {\mathrm{H}}_2\mathrm{O}\left(\mathrm{l}\right)$