Intrinsic binding constant definition

Each of the intrinsic binding constants may be interpreted either as a probability ratio or as a free-energy change for a specific binding process. Both meanings are derived directly from definitions (2.2.6). For instance,... 

An example that conforms to this definition would be benzene-l,3,5-tricar-boxylic acid (Section 5.9). Clearly, owing to the symmetry of the molecule there is only one intrinsic binding constant k, and only one intrinsic binding constant for pairs fcj, or, equivalently, only one pair correlation function... 

In this case there is, by definition, only one (first) intrinsic binding constant, hence (kj) is the same as k but the three pair correlations can be different, hence (gn) is the arithmetic average of the three pair correlations. 

We shall very briefiy present the definitions of these quantities. We use a shorthand notation whenever possible. For instance, we shall use 2(1) rather than the more detailed notation 2(U 0, 0, 0) as we have done in previous sections. Thus the intrinsic binding constants and the corresponding Helmholtz energy changes for binding the zth ligand are... 

Since is presumed to be independent of C, all the intrinsic constants are also independent of C. Note also that the factor 2(0,0, 0) does not appear explicitly in n, although it is contained in the definitions of all the binding constants. 

Typical values of pK[nt and pfor a humic acid are 2.67 and 4.46. The introduction of the electrostatic factor into the equilibrium constant is analogous to the coulombic term used in the definition of the intrinsic surface complexation constants. In addition another binding site (WAH) is recognised which is thought to behave as a weak acidic phenolic functional group. Although this site does not contribute to the titratable acidity and, therefore, no pK is needed for proton dissociation, it is involved in metal complexation reactions. The total number of the three monoprotic sites is estimated from titratable acidity and then paired to represent the humic substance as a discrete non-interacting mixture of three dipro-tic acids, which act as the metal complexation sites. The three sites are... 

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