Equations for Solid—Solution Interactions

In the initial few weeks following submergence, the properties of the soil smface change markedly as a result of reductive dissolution and precipitation reactions. But in time, a steady- or quasi-steady-state is reached, and then the same rules govern the distributions of exchangeable ions between the soil solid and 

Interchange of Solutes between Solid, Liquid and Gas Phases 

Becanse there is generally a large reserve of exchangeable cations on the solid, a small change in A+ resnlts in little change in the ratio on the left-hand side. Hence the rednced activity ratio (A+)/V(B tends to remain constant. The activity coefficients for the ions in solution can be evaluated with Equation (3.3). 

Becanse of the complexity of soils, there are no general relations between the proportions of two cations on the exchange complex and their reduced activity ratio in solntion. Bnt two equations are commonly used the Gaines and Thomas 

Calculated Changes in Exchangeable Cations Following Soil Reduction. We can use these equations to calculate how the exchangeable cations and the composition of the soil solution will change following soil reduction. As we have seen, precipitation of insoluble reduced compounds is often inhibited until a large supersaturation is reached. Therefore for simplicity I assume no precipitation the effects of precipitation are considered in Chapter 4. 

---