Fulvic acids stability

Figure 4. Copper complexation by a pond fulvic acid at pH 8 as a function of the logarithm of [Cu2+]. On the x-axis, complex stability constants and kinetic formation rate constants are given by assuming that the Eigen-Wilkens mechanism is valid at all [M]b/[L]t. The shaded zone represents the range of concentrations that are most often found in natural waters. The + represent experimental data for the complexation of Cu by a soil-derived fulvic acid at various metakligand ratios. An average line, based on equations (26) and (30) is employed to fit the experimental data. Data are from Shuman et al. [2,184]...

Ion-selective electrodes have been used to determine the stability constants for the complexation of copper II ions with soil fulvic acids [4], Two classes of binding sites were found with conditional stability constants of about 1 xf 06 and 8xl03. 

So-called "dissolved" iron(III) consists mainly of colloidal Fe(III) oxides stabilized by humic or fulvic acids Soil Colloids... 

Previous ESR determinations of stability constants for complexes of Mn2+ with commercial fulvic acid preparations (Aldrich) gave log K values... 

Also present in many natural waters are humic/fulvic acid, citric acid, and the like. These organics also can complex actinides. In Figure 15.18, we show the relative stability constants for the first complexation reaction of various ligands with actinides of different oxidation states. Clearly, the carbonate and humate ions along with hydrolysis dominate the chemistry. The tetravalent actinide ions will tend toward hydrolysis reactions or carbonate complexation rather than humate/fulvate formation. 

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. 

Table 10 Ranges of Stability Constants" for Humic and Fulvic Acids of Various Origins with Selected Metal Ions188...

Almost all trace metals of interest have been investigated with respect to their complexation with humic- and fulvic acids and to their stability constants (e.g. Ernst et al., 1975. Mantoura et al., 1978 b Bresnahan et al., 1978 Takamatsu and Yoshida, 1978 Raspor et al., 1984). Results were summarized and compared by Mantoura (1982) and Buffle et al. (1984). 

Bresnahan, W.T., Grant, C.L. and Weber, 3.H., 1978. Stability constants for the complexation of copper (II) ions with water and soil fulvic acids measured by an ion selective electrode. Anal. Chem., 50 1675-1679. 

Schnitzer, M., and E. H. Hansen. 1970. Organo-metallic interactions in soils. 8. An evaluation of methods for the determination of stability constants of metal-fulvic acid complexes. Soil Science 109 333-340. 

Ryan, D. K., and Weber, J. H. (1982). Fluorescence quenching titration for determination of complexing capacities and stability-constants of fulvic-acid. Anal. Chem. 54(6), 986-990. 

Esteves da Silva JCG, Machado AASC, Ramos MA. 1997. Variation of the stability of complexes of Al(3) with a fulvic acid extracted from a humic cambisol soil in the pH range three to five. Environ Toxicol Chem 16 1845-1850. 

Wilkinson, K.J., Joz-Roland, A. and Buffle, J. (1997a) Different roles of pedogenic fulvic acids and aquagenic biopolymers on colloid aggregation and stability in freshwaters. Limnol. Oceanoge, 42, 1714—1724. 

The way in which conditional stability constants are used to calculate the distribution of chemical species can be illustrated by consideration of the forms of dissolved Cu(II) in a dilute, acidic soil solution. Suppose that the pH of a soil solution is 6.0 and that the total concentration of Cu is 0.1 mmol m 3. The concentrations of the complex-forming ligands sulfate and fulvic acid have the values 50 and 10 mmol m 3, respectively. The important complexes between these ligands and Cu are CuS04 and CuL where L refers to fulvic acid ligands (see Section 2.3). These illustrative complexes are not the only ones formed among Cu, S04, or L, nor are the three ligands the only ones that form Cu complexes in soil solution.29 Under the conditions assumed, the equation of mole balance for Cu is (cf. Eqs. 2.11 and 2.30)... 

The importance of complex formation is exemplified by the binding of Mg, Na, Co, Mn, Fe, Cu, and Zn to fulvic acids (FA), and of Fe to humic acids. The binding capacity of these natural acids for metal ions is within the range of 0.2-0.6 mmol/g, and the order of stability of complex formation (M-FA) with some key metals is Fe2+ > Al3+ > Cu2+ > Ni2+ > Ca2+ > Zn2+ > Mn2+ > Mg2+(see Schnitzer, 1970). Interestingly, some cryptogams (i.e., mosses and lichens) capture part of their essential minerals by secreting... 

Schnitzer, M. Hansen, E. H. Organo-metallic Interactions in Soils 8. An Evaluation of Methods for the Determination of Stability Constants of Metal-Fulvic Acid Complexes, Soil. Sci. 1970, 109, 333-340. 

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