Free metal concentration

Ruzic [278 ] considered the theoretical aspects of the direct titration of copper in seawaters and the information this technique provides regarding copper speciation. The method is based on a graph of the ratio between the free and bound metal concentration versus the free metal concentration. The application of this method, which is based on a 1 1 complex formation model, is discussed with respect to trace metal speciation in natural waters. Procedures for interpretation of experimental results are proposed for those cases in which two types of complexes with different conditional stability constants are formed, or om which the metal is adsorbed on colloidal particles. The advantages of the method in comparison with earlier methods are presented theoretically and illustrated with some experiments on copper (II) in seawater. The limitations of the method are also discussed. 

Figure 22. Plot of normalised surface concentration versus c to illustrate that the lack of supporting dissociation by complexes can limit the application of the FIAM. Parameters as in Figure 20, but the total metal concentration M is varied to keep a constant free-metal concentration = 10 6molm-3...

The presence of ligands such as chloride, phosphate and citrate that form soluble complexes with Fe or Fe causes the free metal concentration in solution to decrease and thus enhances the solubility of the solid. In this case, for an Fe oxide, (L = ligand). 

All the data cited clearly demonstrate how important it is to know the in-vivo free metal concentration for understanding the metal effect on enzyme capacity. They also warn against to thoughtless interpretation and extrapolation of results. 

Other proteins play important roles in controlling free metal concentrations in the cytosol. Of particular importance are ferritin and metallothionein. Ferritin is the major iron storage protein in the cell. It is critical that cytosolic iron is kept at low levels, because iron can catalize the Fenton reaction, which generates the most toxic of the ROS—the hydroxyl radical. Copper and superoxide can also participate in the fenton reaction. Metallothioneins are another important family of proteins that helps control cytosolic concentrations of metals such as Cu and Cd. Glutathione is another peptide that controls free Hg and Cd levels in the cell (Figure 21.7). 

Fig. 10.2. Relationship between values of (a) pH of agar and zinc phosphate (= 5mM) solubilization measured as clear halo diameters in the agar around colonies of mycorrhizal fungi (see Fomina et al., 2005a) (b) pH of fungal culture filtrates and pyromorphite solubilization measured as the ratio (%) of free metal concentration in the liquid phase to that in the initial solid phase (initial (pyromorphite) = 0.5 mM) (c) as above but excluding oxalate over-excreting Beauveria caledonica (see Fomina et al., 2004).

Apparently the toxicity of copper to plankton depends upon the free metal concentration, as is shown in experiments with varying chelator concentrations (7, ). Calculations using the REDEQL computer model for metal speelation (, 9 ) related data from toxicity experiments to free metal concentrations. Partial growth inhibition is found in the activity -11 -9... 

Plots (b) and (e) show the free metal surface of the system (plotted as pM). These show pM increasing (i.e. free metal concentration decreasing) very rapidly as one moves more into the CLASP-w region (i.e. trace metal in presence of large ligand excess). The metal-buffering capability of the system is clearly depicted in the contour map (e). For example, the lines, or portions of lines, which are parallel to the C axis, describe... 

There is a little evidence in the literature suggesting that both calculated values of free metal concentration ( 1) and values measured with ISE s ( 8, 9) can be correlated with the biological availability of copper. Additionally, Sunda et (27) have... 

Equations 14 and 15 equate [MLl] and [ML2] to concentrations of metal bound at FA site one and site two respectively, and as before, [M] is equal to the free metal concentration in solution during the metal ion titration. Values of [LI] and [L2] are the unbound ligand concentrations or the concentrations of each fluorescent site capable of binding metal. 

From the overall formation constants (see Equations 17-5 through 17-8), the concentrations of the complexes can be expressed in terms of the free metal concentration [M], to give... 

A number of mechanisms lead to metal immobilization. Although these can reduce the external free metal concentration, solubilization may be promoted in some circumstances because of the shift in equilibrium, which will result in the release of more metal species into solution. 

Fig. 17. Schematic representation of elution profiles produced by the binding of background metal ions to macromolecular ligands. [M] - = total metal concentration [M] = free metal concentration [ML] = concentration of metal-ligand complex [L] = free ligand concentration [M]o = concentration of free metal ion in the eluent. Elution proceeds from frame 1 to 3. The bottom illustration is a schematic representation of resulting chromatogram obtained by monitoring the metal content of the column effluent. See text for discussion.

Essentially all of the arguments made earlier relate to lower metal concentration experiments where it can be assumed that only monomeric species exist and the distribution is independent of the metal concentration. At higher concentrations, it is possible to have polymeric metal complexes present. In this latter case, it is impossible to eliminate the free metal concentration from the expressions for the distribution ratio, and the extraction curve will depend on the metal concentration. Nevertheless, the stabihty constants for such complexes could also be determined in such systems by performing several solvent extraction experiments at different total metal concentrations and then evaluating all of the curves together. 

Replacing the free metal concentration through the complexation function [M] = Cm/Fm, where Fm is defined by Eq. (5.126) and taking into account that the... 

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