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Clay materials, including soils, have the property of sorbing certain cations and anions and retaining these in an exchangeable state; that is, these ions are exchangeable for other cations and anions by treatment with such ions in a water solution (the exchange reaction may also take place in some nonaqueous environments). The exchange reaction is stoichiometric. A simple and well-known example of the ion exchange reaction is the softening of water by the use of zeolites, permutites, or carbon exchanges. The discovery that soils have the power of exchanging cations with solutions containing other cations is the outgrowth of observations dating back into the remote past. For example, it has been known for centuries that liquid manures become decolorized and deodorized when filtered through soils. Thompson about 1850 is generally credited with first systematically studying cation exchange. He showed that when soils were mixed with ammonia and then leached with water, the greater part of the ammonia was held back. Shortly after Thompson's work, Way, another Englishman, began a detailed study of the phenomenon. It was Way who first showed that cation exchange in soils was restricted to the clay fraction and that it was connected with the silicate compounds in the soil. Following the pioneering labors of Thompson, Way, and others, a large number of investigators, particularly in the field of soil chemistry, have studied various aspects of the exchange reaction. The term base exchange was used for many years to describe the cation reaction long after it was established that the hydrogen ion may take part in the exchange reaction. Indeed the term base exchange is frequently still used at the present time.
Grim, Ralph E.
Research Professor, University of Illinois, Urbana, Ill.