Gapon convention

The reaction written in this fashion corresponds to the Gapon convention, in which case the activities / may be calculated equivalently by either Equation 9.27 or 9.28, since the exchanging site maintains unit charge. 

No value for dmq/dmp is needed to evaluate the and Freundlich models. For the Langmuir model and the ion exchange model under the Gaines-Thomas and Gapon conventions,... 

The heterovalent exchange reaction (K-Ca exchange) is Ca-soil + 2K <=> 2K-soiI + Ca2+, except for the Gapon convention where it would be Ca1/2-soil + K4 o K-soil + 1/2 Ca2+. 

In Eq. 3.27, the fact that the molar and equivalent exchangeable fractions are the same if the Gapon convention is used. In other words, [lyi-X] = Pi, and [Na-X] = Pnj. For a homo valent exchange, i = 1, the preceding equation results... 

This expression is called the Gaines-Thomas convention (Gaines and Thomas 1953). If using the molar concentration instead, it is identical to the Vanselow convention (Vanselow 1932). Gapon (1933) proposed the following form ... 

Adsorption of a dissolved ionic species is always part of an exchange reaction that involves a competing ionic species. The desorbing species creates the vacant site to be occupied by the adsorbing one. Soil scientists use a variety of ion exchange models (e.g., the Gaines-Thomas, Gapon, Vanselow, and Rothmund-Kornfeld models) in which different conventions are used to write the concentrations of dissolved and adsorbed species. These models are adequately described and compared elsewhere (cf. Bolt 1979 Sposito 1981, 1989 Appelo and Postma 1993). 

Equations 3.21 and 3.22 are the Gapon (1933) convention. In this case, the molar and equivalent exchangeable fractions are the same because both are based on a single exchange site with the subscript of X, i, in Eq. 3.17 equal to 1. 

For a binary exchange reaction as Equation 5.14, except for Gapon and Cernik conventions. 

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