Gaines and Thomas Convention

Ion Exchange Activity Conventions and Binary Exchange Conditional Constants  

C is concentration in aqueous phase molality m is approximate by molar concentration c and X , y- and W are molar, equivalent, and weighted fractions, respectively, in the exchanger phase, g and g are, respectively, moles per exchanger unit mass of adsorbate and sites occupied. Superscripts W and S are omitted for simplicity. 

Following a procedure similar to that outlined in Section 5.3.2.1, the following expressions can be found for thermodynamic quantities (Bond 1995)  

This convention has been widely employed in soil chemistry (Bond 1995 Vulava et al. 2000 Escudey et al. 2001,2002 Appel et al. 2003 Ma, Dong, and Zhou 2005 Gacitda, Antildn, and Briceno 2008). 

In their paper (Krishnamoorthy and Overstreet 1949), they considered reasonable for clay minerals a coordination index Z = 4, thus will have a value of 1 for univalent ions, 1.5 for divalent ions, and 2 for trivalent ions. Consequently, for a binary exchange K+-Mg +, the weighted fractions will be 

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Evangelou, V. P. and R. E. Phillips. 1989. Theoretical and experimental interrelationships of thermodynamic exchange parameters obtained by the Argersinger and Gaines and Thomas conventions. Soil Sci. 148 311-321. 

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