What is cation exchange?

1.      What is cation exchange?

A process in which cations in solution are exchanged with cations held on the exchange sites of mineral and organic matter, particularly on the surfaces of colloids of clay and humus.

In clay minerals, a certain amount of ionic substitution occurs. For example some Si⁴⁺ ions in the silicate layers can be replaced by Al³⁺ ions as these two ions are of similar size. Substitution of Al³⁺ ions in the aluminium hydroxide layer by Mg²⁺ ions also take place. As these substitutions involve the replacement of one cation by another of lower valency the clay structure becomes negatively charged. This charge is compensated by positive ions, e.g. K⁺, Na⁺, Mg²⁺, Ca²⁺ and H⁺, absorbed at the edges of the clay minerals or between the layers. These ions are loosely held in the structure and as they can readily be replaced, one by another, are known as “cation exchange”.

            

References:

1-      http://www.stadiumturf.com/cation_exchange.htm

2-      http://www.waters.com/waters/nav.htm?cid=10083845&locale=en_US

3-      http://scholar.lib.vt.edu/ejournals/JARS/v30n4/v30n4-deul.htm

4-      Paul Rado, F.I. Ceram, An Introduction To The Technology Of Pottery; PERGAMON PRESS.