Potentiometric and Conformational Studies of the Acid-Base Properties of Fulvic Acid from Natural Waters
Gram quantities of reference fulvic acid (FA) have been extracted from natural waters for a systematic physico-chemical investigation of proton and metal binding properties. Electrophoretic and gel filtration chromatographic studies of FA indicate that during the course of an acid-base titration, marked conformational changes occurred with the molecular radius increasing from 0.65 nm (6.5Å) at pH 1.15 to 1.32 nm (13.2 Å) at pH 9.26. These conformational charges arise from intra-molecular electrostatic replusive forces associated with the build up of charge (Z) on the flexible FA polyanion. This in turn has a marked effect on the acid association constant (k). For this reason, the broad poly-disperse titration curves which are characteristic of FA could not be adequately explained by simple acid-base models which assume single values of k. However, by incorporating both electrostatic and conformational terms into an expanded Tanford model, we obtained a good fit to the titration data. The value of the intrinsic association constant ki (when Z = 0) derived from this model was rather acidic (log ki = 2.3−2.7) suggesting that the FA is strongly electron withdrawing. However, the combined effects of electrostatic and conformational changes is to spread the apparent constants over a wide range with the greatest number of sites having apparent log k = 4−5.
KeywordsHumic Acid Natural Water Fulvic Acid Titration Curve Molecular Radius
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