Fulvic acids binding curves

Our results show that conditional stability constants for binding of copper by natural organic ligands are highly dependent on the pH at which the measurement was conducted and the portion of the titration curve which was examined. Published stability constants for fulvic acids often vary considerably and Cheam (22) has reported that these apparent differences can be reasonably... 

One example of this approach has been published by Sposito and coworkers (Sposito and Holtzclaw, 1977 Sposito et al., 1977), who have published proton binding data and a discrete ligand model for fulvic acids derived from sewage sludge. The experimental data, which appear to have been carefully obtained, contain some peculiar anomalies that are difficult to explain. For instance, when low fulvic acid concentrations are titrated with strong base, the low pH region of the titration curve indicates that some functional groups are reprotonated as base is added. Sposito and co-workers have attributed this phenomenon to counterion condensation. The same experimental observation was also reported by Perdue et al. (1980). 

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