Kinetics and speciation in solution

The mechanisms of reactions in solution and the factors controlling their kinetics may be divided into five broad categories (Table 9.4 Hering and Morel, 

Outer-sphere complexation M(H20) + L M(H20) L M(H20)n L Water lost by metal 

Inner-sphere complexation M + L ML Equilibrium and formation constants 

Examples of reaction rates for different metals are given in Tables 9.5 and 9.6. Reaction rates that are extremely fast ( 107s 1) or very slow ( 10 8s 1) will not affect assumptions concerning solution equilibrium. However, caution is required in the application of chemical thermodynamics to reactions with intermediate rates (Sposito, 1986 1989). The importance of kinetics in solution speciation depends on the time frame of the experiment or application. Solution reactions that take days to come to equilibrium will tend to have a minor impact on conclusions or predictions concerning long-term behaviour (e.g. soil formation), but could have important implications for short-term situations, such as the growth of an annual pasture or storm water runoff. 

The competition between Cu and Ca for ethylenediamine tetraacetic acid (EDTA) and humic acid is an example of a double-exchange reaction (Table 9.4 Hering and Morel, 1989). If Cu is added to a solution of Ca EDTA and Ca humate, it replaces Ca in the weakly bound Ca humate initially, but, as time progresses, it is able to exchange with EDTA-bound Ca by either a ligand- or 

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