Animated Solution for Chemistry - s and p-Block Elements: Which one of the following methods is most suitable for preparing deionised water?

The quest for absolutely pure water is a fundamental challenge in chemistry. When we talk about deionised water, we are referring to water that has been stripped of every single soluble mineral salt. It contains no stray cations like sodium ($N{a}^{+}$), calcium ($C{a}^{2+}$), or magnesium ($M{g}^{2+}$), and no anions like chloride ($C{l}^{-}$) or sulfate ($S{O}_4^{2-}$). It is just pure $H_{2}O$.

In this problem, we are asked to identify the most suitable method for preparing this highly purified water. Let's evaluate the given options and understand the chemistry behind each process.

First, let's consider Clark's method. This is a classic technique used to remove temporary hardness from water. It involves adding a carefully calculated amount of lime, or calcium hydroxide ($Ca(OH{)}_2$), to the water. This causes the soluble bicarbonates to precipitate out as insoluble carbonates. However, Clark's method does nothing to remove permanent hardness or other dissolved salts. The water is far from being deionised.

Next, we have Calgon's method. Calgon is the trade name for sodium hexametaphosphate ($N{a}_6{P}_6{O}_{18}$). When added to hard water, it forms highly stable, soluble complexes with calcium and magnesium ions. While this prevents the ions from interfering with soaps (effectively softening the water), the ions are still physically present in the solution. Therefore, Calgon's method does not yield deionised water.

What about the Permutit method, also known as the Zeolite process? This method uses hydrated sodium aluminium silicate to perform an ion exchange. As hard water flows through the zeolite bed, the troublesome $C{a}^{2+}$ and $M{g}^{2+}$ ions are trapped, and sodium ions ($N{a}^{+}$) are released into the water in their place. The resulting water is soft, but it is now laden with sodium salts! Because it still contains a significant concentration of ions, it fails the test for deionisation.

Synthetic Resins

This brings us to the Synthetic resin method, which is a two-stage ion-exchange process designed for total purification.

In the first stage, the water is passed through a cation exchange resin. This resin contains large organic molecules with acidic groups (like $-S{O}_{3}H$). As the water flows through, every single cation (whether it's $N{a}^{+}$, $C{a}^{2+}$, or $M{g}^{2+}$) is captured by the resin, and in exchange, hydrogen ions ($H^{+}$) are released into the water.

In the second stage, this acidic water is passed through an anion exchange resin. This resin contains basic groups (like $-N{H}_{3}OH$). It captures all the anions (like $C{l}^{-}$, $S{O}_4^{2-}$, or $HC{O}_3^{-}$) and releases hydroxide ions ($O{H}^{-}$) in their place.

Finally, the magic happens. The released hydrogen ions and hydroxide ions immediately react with each other:

By systematically replacing every mineral cation with $H^{+}$ and every mineral anion with $O{H}^{-}$, the synthetic resin method leaves behind nothing but pure water molecules.

While methods like Clark's, Calgon's, and Permutit are excellent for softening water, they leave behind various dissolved ions. Only the synthetic resin method is capable of completely removing all mineral salts, making it the most suitable and efficient method for preparing deionised water.