The binding of Mn 2+ to a well characterized fulvic acid sample is reported based on the competition with K + in an ion exchange equilibration. The free energy of binding of [Formula: see text] to fulvic acid is only 1–2 kJ/equivalent more favourable than that for K + . This suggests an outer sphere electrostatic structure for the complex. The suggestion is confirmed by observation of minimal change in the presence of fulvic acid of the effect of paramagnetic Mn 2+ on the nmr spectra of water. The interpretation of nmr spectra is supported by comparison with nmr measurements of Mn 2+ complexes with simple ligands and contrast with nmr measurements on Fe 3+ – fulvic acid complexing. The latter is confirmed as inner sphere.
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Gamble et al. (1976) studied this question.