Cation Exchange Equations

Several equations describe cation exchange processes. Each has its own set of characteristics and merit. The choice of a particular equation often seems to be subjective, however, and may be based as much on the investigator s background as on any other factor. Lack of familiarity with the units, and of the numerical values of exchange coefficients for other equations are major deterrents to adoption of a more widespread and uniform approach to cation exchange. 

Certain limitations are inherent in most cation exchange equations  

Cation and anion exchange are considered separately acknowledging their simultaneous, presence is rare. 

The cation or anion exchanger is assumed to possess constant exchange capacity. Often, however, die capacity varies with the exchangeable ion, with salt concentration, and with pH. 

Stoichiometric (1 to 1) ion exchange is generally assumed. Apparent exceptions are usually explained by simultaneous adsorption of molecules or by complex ion formation. 

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Cation exchange in soils or clay minerals involves replacement of a given cation on a given mineral surface by another cation. Exchange equations are commonly used to evaluate ion availability to plant roots and/or release of metals to soil water (e.g., heavy metals to groundwater or surface water). There are two major types of cation-exchange reactions in soil systems—homovalent and heterovalent cation exchange. 

Bolt, G. H. 1967. Cation exchange equations used in soil science. A review. Neth, J. Ag. Sci, 15 81-103. 

The cation exchange equation corresponding to this reaction is... 

In equation 15 co-ion exclusion also contributes slightly and, for completeness, should be considered. With the introduction of X as a component, it is a straightforward matter to incorporate cation-exchange equations into general chemical equilibrium models. All of the exchanger species can be defined in terms of X" and one solution component (e.g., NaX = KSiX Na+ X ). 

The soluble anion is usually not explicitly considered in cation exchange equations, because the activities of electrolytes can be expressed in terms of the activities of their constituent ions, that is,... 

Ht pmious section discussed the special case of exchange between cations of equal However, the general form of the cation exchange equation allows for endiange between A " " and where n+ and m+ may not be equal ... 

The AF thereby generated from mineral dissolution becomes available to displace Ca from soil exchange sites. Unfortunately, the mass-action cation exchange equation (5.21) does not adequately describe Ca -AF exchange on layer silicate clays (see Chapter 3). Instead, a Langmuirlike function is found to be useful ... 

As indicated in Section 8.4, the primary difference between various cation exchange equations is their differing treatment of the activities of exchangeable cations. Vanselow, for example, assumed that the activities of exchangeable cations were proportional to their mole fractions. This is equivalent to saying that ions on soil colloid surfaces behave as if in ideal solution (Appendix 3.2). The mole fraction of an ion in a binary system is... 

Table 8.5. A comparison of exchange coefficients for several cation exchange equations, as calculated from the ammonium-calcium exchange data of Table 8.2...

Cation exchange equations contain the ratio of the monovalent cation concentrations to the square root of the divalent cation concentration, or the square of this ratio. The equation may involve ion activities rather than concentrations, and may include corrections for ion pairs. For most field practice, however, the ratios of total ion concentrations alone are sufficient. 

Through the analysis of the equilibrium of the ion-exchange reactions, the selectivity of the zeolites for the examined cation was calculated. Accordingly, given a general cation exchange equation ... 

Example 2.6. Fresh surface water carries large amoimts of suspended particles whose surface is dominated by Ca +, and the sea water includes abundant dissolved Na+. AAffien fresh and sea water mix, cation exchange occurs between Ca and Na+. the cation exchange equation between them is... 

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