Fertilizers, deodorant and waste water treatment have them in common: metal chlorides


Potassium chloride (chemical formula KCl), which is found in nature in the form of white or colourless crystals, is used in medicine, scientific applications and food processing. But its best-known straight-forward application is in fertilizers: together with the elements nitrogen and phosphorus, potassium is an essential fertilizer component that makes plants flourish and provides us with rich harvests.

Silver chloride (chemical formula AgCl), a white crystalline powder, is very helpful in wound-healing products and plasters. It is also used in some personal deodorants to neutralize bacteria. Silver chloride is also the main active component in photographic paper and it is used in lenses that automatically darken in the sun. It is added to pottery glazes for more lustre or radiance.

And finally, iron chloride (chemical formula FeCl2 or FeCl3) may seem a somewhat simple substance because it is based on plain iron, but it plays an important role in the cleaning of industrial and household waste water. This iron comes in two forms: it can be bound to either two or three chloride ions. This results in two completely different compounds.

The one with two chlorine ions (FeCl2) is a paramagnetic green crystal that dissolves in water to give a yellow aqueous solution. It is used as a flocculant in waste water treatment, especially for chromate-containing water, e.g. from the leather treatment industries.
When iron is bound to three chlorine ions (FeCl3) the colour ranges from dark green to purple-red, depending on the light conditions: reflected light or transmitted light. The water-based solution is dark brown! It is used in water treatment, to eliminate phosphates. And it is also used as an etchant for copper-based metal printed circuit boards that you find in nearly all electronic apparatus.