Fulvic acids metal complexes

Jambu, R, Dupuis,T., and Garais, M. (1975a). Use of differential thermal-analysis to characterize fulvic acid metal complexes and humic acid metal complexes. 2. Application to natural organometallic complexes. J. Therm. Anal. 8(2), 231-237. 

The measured values of the formation constant for Cu-NTA at intermediate pH will then be 10 which is significantly less than the value of 10 measured at high pH. The same is true for the humic substance-metal ion complex equilibrium constants measured at low pH values. Morgan estimates that at pH 2 to 3 the —COO groups, which are important in forming fulvic acid-metal ion complexes, are virtually all in the —COOH or protonated form. Thus to obtain the value of the formation constant at intermediate pH for metal ion fulvic acid complexes, when the carboxyl groups are ionized, we should divide the equilibrium constant values presented in Table 5-13 by the dissociation constant of the —COOH group, that is,... 

As a result of microbial formation of metal-organic complexes with fulvic acids in soils of Tropical Rain Forest ecosystems, the surface and sub-surface runoff waters are enriched in some heavy metals like manganese and copper. A similar tendency has been shown for boron, strontium and fluorine. 

Saar and Weber [1] compared methods based on spectrofluorimetry and ion-selective electrode potentiometry for determining the complexes formed between fulvic acid and heavy metal ions. 

ESR examination of nonchemically isolated fulvic acids showed that Mn2+ was the primary paramagnetic species observable (60, 61). Most likely, the soluble-colloidal fraction we identified in the speciation studies consisted primarily of such complexes. Because the ESR spectral characteristics of Mn in fulvic acid complexes are quite similar to Mn(H20)62+, Alberts et al. (62) suggested that the metal-fulvate interaction was weak. Stronger interaction would be expected to lead to changes in peak shape. This view leaves unexplained the ability of the complexes to survive the isolation procedure s long ultrafiltration steps, because weak interactions are usually associated with reversible complexation. 

Many studies have been carried out concerning the stability constants of humic and fulvic acid complexes.188 190,191 Stability constants vary considerably with pH and ionic strength213 and this, together with the variable nature of the ligands involved, accounts for the range of values reported for individual metal ions in the literature. However, the stabilities of divalent metal complexes generally follow the well-known Irving-Williams order Mg < Ca < Mn < Co < Zn = Ni < Cu < Hg. 

---