Permanent Hardness

Removing Permanent Hardness

Permanent hardness comes from dissolved calcium and magnesium chlorides and sulphates — and unlike temporary hardness, boiling has no effect on it. Several chemical and physical methods are used instead.

1. Washing soda (sodium carbonate) treatment. Soluble Ca/Mg chlorides and sulphates are converted to their insoluble carbonates:

2. Calgon's method. Sodium hexametaphosphate, $Na_6P_6O_{18}$ (commercially "Calgon"), dissociates and then complexes the offending cations, keeping them in solution rather than precipitating them:

3. Ion-exchange (zeolite/permutit) method. Sodium aluminium silicate, written simply as $NaZ$, exchanges its sodium for the hardness-causing ions as hard water passes through it:

Once all its sodium is used up, the zeolite is said to be exhausted; it is regenerated by flushing with concentrated NaCl solution, which reverses the exchange:

4. Synthetic resin method. Modern water softening more often uses synthetic cation-exchange resins, which are more efficient than zeolite. These carry $-SO_3H$ groups and are converted to the sodium form ($RNa$) by treatment with NaCl; the resin then exchanges its $Na^+$ for $Ca^{2+}$/$Mg^{2+}$ in the hard water:

and is likewise regenerated with aqueous NaCl.

Fully de-ionised water is obtained by passing water successively through a cation-exchange resin (in the $H^+$ form) and an anion-exchange resin (in the $OH^-$ form):

• Cation exchange: $2RH(s) + M^{2+}(aq) \rightleftharpoons MR_2(s) + 2H^+(aq)$ — this releases $H^+$ ions and so temporarily makes the water acidic. …