8.1 Removal of Hardness

  Hydrogen

• By Boiling: During boiling, the soluble $Ca(HCO_3)_{2}$ is converted into insoluble $CaCO_3$ and $Mg(HCO_3)_2$ is changed to insoluble $Mg(OH)_2$ .It is because of high solubility product of $Mg(OH)_2$ as compared to that of $MgCO_3$, that $Mg(OH)_2$ is precipitated. These precipitates can be removed by filtration. Filtrate thus obtained will be soft water. $$Ca{(HC{O_3})_2}\mathop \to \limits^{boil} CaC{O_3} \downarrow + {H_2}O + C{O_2} \uparrow $$$$Mg{(HC{O_3})_2}\mathop \to \limits^{boil} Mg{(OH)_2} \downarrow + 2C{O_2} \uparrow $$

• By Clark’s Process: Amount of lime is added to hard water which precipitates out calcium carbonate and magnesium hydroxide which can be filtered off. $$Ca{(HC{O_3})_2} + Ca{(OH)_2} \to 2CaC{O_3} \downarrow + 2{H_2}O$$$$Mg{(HC{O_3})_2} + 2Ca{(OH)_2} \to 2CaC{O_3} \downarrow + Mg{(OH)_2} \downarrow + 2{H_2}O$$

Permanent Hardness can be removed

• By Washing Soda: It converts soluble calcium and magnesium compounds into insoluble carbonates. $$\begin{equation} \begin{aligned} CaC{l_2} + N{a_2}C{O_3} \to CaC{O_3} \downarrow + 2NaCl \\ CaS{O_4} + N{a_2}C{O_3} \to CaC{O_3} \downarrow + N{a_2}S{O_4} \\\end{aligned} \end{equation} $$

• Permutit Process: The sodium ions of permutit are exchanged with calcium and magnesium ions when hard water is passed through it. $$N{a_2}A{l_2}S{i_2}{O_8}.x{H_2}O + C{a^{2 + }} \to CaA{l_2}S{i_2}{O_8}.x{H_2}O + 2N{a^ + }$$ This process is also called Ion Exchange Method/ Zeolite. Permutit is regenerated for further use by treating with an aqueous sodium chloride solution.

• Calgon: Calcium and magnesium salts present in hard water react with calgon $(NaPO_3)_{n}$ to give a complex salt, $n$ can be as large as $1000$. $$2CaS{O_4} + N{a_2}\left[ {N{a_4}{{\left( {PO} \right)}_3}{]_6}{\text{ (Calgon)}} \to N{a_2}} \right[C{a_2}{\left( {PO} \right)_3}{]_6} + 2N{a_2}S{O_4}{\text{ }}$$

• Ion Exchange Resin: They are giant organic molecules of high molecular masses which are of two types-

1. Cation Exchanger- $C{a^{2 + }} + 2{R^ - }{H^ + } \to Ca{R_2} + 2{H^ + }\left( {R \to Resin} \right)$

2. Anion Exchanger- $ROH + C{l^ - }\left( {orS{O_4}^{2 - }} \right) \to RCl\left( {or{R_2}S{O_4}} \right) + O{H^ - }$