An ion-exchanger is a material that does what the name says. It exchanges ions. Millions of people use ion exchangers everyday - probably without even knowing it. And why? Because...well, the story begins at a different place. Ordinary water, for example, contains huge amounts of ions from the many different salts that are dissolved therein (eg, calcium and sulphate). It's possible to modify the properties of water by using the exchanger to replace one kind of ion with another.

For example, it is possible to adjust the conductivity or the acidity of water. It is also possible to replace harmful ions like plutonium or arsenate with innocuous ones like magnesium or carbonate. As you might imagine, ion-exchangers are, for this reason, widely employed in water treatment plants. Ion-exchangers can also be used to reduce the water hardness and thus improve the performance of soap. Washing powder contains significant amounts of ion-exchangers. Dishwashers have a water-softening device in them, which uses ion exchangers¹. Furthermore, ion-exchangers can also be used to desalinate seawater, to extract silver from photographic solutions and in many other different chemical syntheses.

So, what are ion-exchangers?

Ion-exchangers are usually made of synthetic resins and are available in the form of a grainy, brownish powder. There are many other variations, however, ranging from thin synthetic films to porous mineral materials like zeolites, which look more like chalk. Ion-exchangers (films, grains or zeolites) are usually porous to increase the effective surface area. The ions that are to be replaced - eg, in water - will flow into the pores and not just around the outer surface. Due to the aforementioned porosity, the effective surface is often millions of times bigger than the ordinary outer surface of the powder (cf. also BET - Measuring Areas Using Molecules). This effective surface (ie, the walls of the pores) can also be chemically modified to have a more homogenous covering and a specific charge (positive or negative). Ions with opposite charges are then found loosely bound, or adsorbed, to the walls due to electrostatic attraction.

Often people don't think of an ion-exchanger as being a powder. Instead, people often imagine some kind of gadget with an input funnel and an output tap, like a weird kind of filter, with valves and tubes. Such gizmos exist, and are basically pipes containing ion exchanger powder (or grains). When the liquid containing the ions (normally water) is passed through the tube, the ions are exchanged. There are many different sizes and designs of such ion-exchanger paraphernalia, ranging from small, portable, half-metre-sized tubes to big, industrial-size, ten-metre-tall towers. They are all basically long tubes with one input, for the water with the 'bad' ions, and one output, for the water with the 'good' ions.

The ion-exchangers - the powders - can, however, also be used as they are, without any pipes and valves, just by pouring them into water, for example. Such ion-exchangers, like zeolite A, the mineral type of ion-exchanger, are often mixed together with soap in washing powders; this exchanger reduces the water hardness by replacing magnesium and calcium ions with sodium ions. The ion exchanger itself is not water soluble and is washed away along with lint and other smut.

The working mechanism of an ion-exchanger

The working principle of an ion-exchanger is - be it in a tube gizmo or alone in water - in all cases the same.

Ion exchanger schematic

The first thing to do before employing the exchanger is to load it with the appropriate ions, which are going to replace the inappropriate ones (for simplicity, let's just call these ions 'good' and 'bad'). So, the powder is purged with a solution containing the 'good' ions. If this is done for a long enough time, the entire surface of the exchanger will be covered with the 'good' ions. Much like a sponge filled with ink.

Now it is possible to employ the ion-exchanger. The water with the 'bad' ions in passed through a tube containing the ion-exchanger loaded with the 'good' ions. The water will fill the tube and flow around the grains and into the pores. In effect, all water is in close contact with the surface of the ion exchanger. Inside such a pore a 'bad' ion swimming in the water will eventually detach a 'good' ion which is sticking to the wall of the pore. The 'bad' ion will in its turn stick to the wall at the place that became 'vacant'. As a result, the ions have been exchanged. Now with billions of grains containing billions of pores each covered with billions of ions, one will eventually be able to replace all 'bad' ions by 'good' ones.

Of course, the process is not 100% effective - some of the 'bad' ions will pass through the tube without being exchanged. The effectiveness of an ion exchanger depends on many factors like the strength with which the ions bind to the surface, the temperature and the effective length of the tube. Increasing tube length or passing the solution through many tubes usually solves this problem (the grains and the tubes are usually cheap). If space is limited, the ion-exchangers can also be chemically modified, so that the 'bad' ions stick to the walls with a stronger bond than the 'good' ones; in this way the 'bad' ions become chemically absorbed and cannot be simply washed away by the water passing through the tube (this formulation is of course more expensive).

The whole process of exchanging ions will obviously work well in the beginning, when the exchanger is fully loaded with the 'good' ions. After a while, the exchanger becomes depleted of 'good' ions and will work less and less efficiently. At this point the tube containing the 'bad' ions will have to be replenished with replacement by 'good' ions, as in the first step. Sometimes it is easier, or faster, to replace the entire tube with a previously prepared and replenished one. In some cases it is more appropriate to halt the exchanging process and replenish the tube by purging it with a solution containing the 'good' ions.

Summary

Ion-exchangers are little wonders of chemistry. They can be employed in many different ways - for example, in order to modify properties of water. This is achieved by exchanging the ions contained in the water by other, more appropriate, ions. Before employing an ion-exchanger, it is loaded with the suitable ions, much like a sponge can be loaded with ink. A solution passed through the ion exchanger will take up these ions leaving vacant adsorption sites in the ion-exchanger. Other ions in the solution adhere to the ion exchanger at these positions - so, in effect, the ions in the solution are replaced by ions in the exchanger. The effectivity of the ion exchanger decreases with time, as it becomes depleted of ions. Most ion-exchangers, however, can be easily regenerated.