Factors affecting speciation in solution

Ionic strength, pH and electron activity (pE) are the three major characteristics of the soil solution commonly recognised as affecting metal speciation. However, reaction kinetics and the relative concentrations and complexing affinities of cations and anions may be equally important (but are sometimes overlooked). 

The extent of speciation in solution depends on the stoichiometric coefficients of the components of a species the polyvalent nature and protonation behaviour of anionic complexing ligands the type and relative ability of different cations and anions to form complexes pH ionic strength, and the ratio of the total concentrations of the reactants in solution (the total cation anion ratio). 

The effects of these factors can be illustrated by considering the formation of a 1 1 complex in a hypothetical soil solution at constant ionic strength in the presence of equimolar concentrations of the reactants, in the absence of competing cations and anions, and at pH = pKl = 4 for the dissociation of the monoprotic acid from which the complexing anion is derived (Fig. 9.1). Equilibrium modelling (using TITRATOR Cabaniss, 1987) indicates that 50% of the total cation concentration will not be complexed with the anion unless the pK for the formation of the species is approximately 7.7. The pK value is more than halved, 

With respect to trace metal cations, it is common in soil solutions for the total cation anion ratio of the reactants to be very low because complexing anions are 

The pK of formation of a species can have a significant effect on the variation of the extent of speciation with the total cation anion ratio in solution (Fig. 9.3). For weak complexes (p K 2), there is no effect of total cation anion ratio on speciation in solution at constant pH (Fig. 9.3(a)). If the anion complexes the cation moderately (pK 4), however, the fraction of total cation complexed by the ligand increases almost linearly as the cation anion ratio varies from 1 1 to 1 1000 (Fig. 9.3(b)). In the case of strong complexation (pK 6), species 

---