Exchanged Species Card Options (-R)

Exchange Activity Options

The Debye Huckel theory offers a straightforward model to relate concentrations and activities in water. However, for adsorbed cations, there is no unifying theory to calculate activity coefficients and different conventions are in use. The standard state (where the activity of the exchangeable ion is equal to 1), is in all cases an exchanger which is totally occupied by the same cations. The activity of each exchangeable ion is expressed as a fraction of the total, either as a molar fraction or as an equivalent fraction. Furthermore, the total number can be based on the number of exchange sites, or on the number of exchangeable cations. For homovalent exchange it makes no difference what convention is used, but for heterovalent exchange the effect is quite notable.

Gaines-Thomas 

In this option the equivalent fraction of the exchange cations is used, conforming to the Gaines-Thomas convention, which is a rigorous definition of a thermodynamic standard state of exchangeable cations.

Vanselow

In this option molar fractions are used to describe exchange.

Gapon

In this option the activities of the adsorbed ions are expressed as a fraction of the number of exchange sites.