CHAPTER 8 . Determination of Organic Complexation
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the known, added, ligand). The principle of the method is that the added ligand is
equilibrated with the sample containing the natural ligands, and the concentration of
metal bound with the added ligand is detected. This method has been called "competitive ligand equilibration" (Rue and Bruland 1995).
The analytical problem is how to specifically detect the metal complexed with the
added ligand. One possibility is to extract the complex with the added ligand (Moffett
and Zika 1987), but it is likely that some of the natural complex is extracted too. CSV
utilizes the principle of complex adsorption in the pre concentration step, and the complex with the added ligand is detected specifically. For this reason CSV is an excellent
method to investigate metal speciation with ligand competition.
8.6
Determination of Labile Metal Concentrations Using CSV
The speciation in the sea water is altered by the added competing ligand (AL)
(see Table 8.2) as the formation of the metal complex MAL causes shifts in the original
speciation. The magnitude of the alteration depends on the stability of the MAL complexes, and can be regulated by varying the concentration of AL or by selecting ligands
forming more or less stable complexes. Thus it is possible to investigate weak or strong
natural complexes selectively by varying the "detection window." The labile metal concentration in CSV therefore equals all metal bound by the added ligand, and the fraction of the total metal that is labile depends on the relative stabilities of the complexes,
and therefore on the magnitudes of the a-coefficients (aMAL and UML). When the complex of the metal with the added ligand is more stable than that with the natural ligand
(UMA> uMd, most of the metal will be bound by AL, and therefore most of the metal
will be "labile;" half of the metal is labile when UMAL = UML. The labile metal concentration can be made to approach the total metal concentration by using a complexing
ligand which forms very stable complexes and thus outcompetes the natural species.
The theory of CSV of metal species is summarised in Table 8.2. It can be seen that
the labile metal concentration is exactly defined and that a clear relationship exists
between the labile metal concentration and the natural complexing ligand(s) in the
seawater. The magnitude of the shifts in the equilibria by the addition of the competing ligand AL are exactly known and are made use of to calculate the complex stability of the natural species.
Measurements of the labile metal concentration are carried out in equilibrium conditions by allowing the added complexing ligand to equilibrate with the metal species
(this can take several hours). The concentration of MAL is normally a significant fraction of the total dissolved metal concentration (typically 50% is complexed by MAL)
and only a very small amount is adsorbed on the electrode, so no changes in the speciation occur during the actual measurement of MAL: the measurement is therefore
independent of kinetic effects of metal species up to time-scales of hours.
8.7
Calculation of aML
The stability of the natural metal complexes is evaluated to a first approximation from
the ratio of the labile and total dissolved metal concentrations (Table 8.2):
179
the known, added, ligand). The principle of the method is that the added ligand is
equilibrated with the sample containing the natural ligands, and the concentration of
metal bound with the added ligand is detected. This method has been called "competitive ligand equilibration" (Rue and Bruland 1995).
The analytical problem is how to specifically detect the metal complexed with the
added ligand. One possibility is to extract the complex with the added ligand (Moffett
and Zika 1987), but it is likely that some of the natural complex is extracted too. CSV
utilizes the principle of complex adsorption in the pre concentration step, and the complex with the added ligand is detected specifically. For this reason CSV is an excellent
method to investigate metal speciation with ligand competition.
8.6
Determination of Labile Metal Concentrations Using CSV
The speciation in the sea water is altered by the added competing ligand (AL)
(see Table 8.2) as the formation of the metal complex MAL causes shifts in the original
speciation. The magnitude of the alteration depends on the stability of the MAL complexes, and can be regulated by varying the concentration of AL or by selecting ligands
forming more or less stable complexes. Thus it is possible to investigate weak or strong
natural complexes selectively by varying the "detection window." The labile metal concentration in CSV therefore equals all metal bound by the added ligand, and the fraction of the total metal that is labile depends on the relative stabilities of the complexes,
and therefore on the magnitudes of the a-coefficients (aMAL and UML). When the complex of the metal with the added ligand is more stable than that with the natural ligand
(UMA> uMd, most of the metal will be bound by AL, and therefore most of the metal
will be "labile;" half of the metal is labile when UMAL = UML. The labile metal concentration can be made to approach the total metal concentration by using a complexing
ligand which forms very stable complexes and thus outcompetes the natural species.
The theory of CSV of metal species is summarised in Table 8.2. It can be seen that
the labile metal concentration is exactly defined and that a clear relationship exists
between the labile metal concentration and the natural complexing ligand(s) in the
seawater. The magnitude of the shifts in the equilibria by the addition of the competing ligand AL are exactly known and are made use of to calculate the complex stability of the natural species.
Measurements of the labile metal concentration are carried out in equilibrium conditions by allowing the added complexing ligand to equilibrate with the metal species
(this can take several hours). The concentration of MAL is normally a significant fraction of the total dissolved metal concentration (typically 50% is complexed by MAL)
and only a very small amount is adsorbed on the electrode, so no changes in the speciation occur during the actual measurement of MAL: the measurement is therefore
independent of kinetic effects of metal species up to time-scales of hours.
8.7
Calculation of aML
The stability of the natural metal complexes is evaluated to a first approximation from
the ratio of the labile and total dissolved metal concentrations (Table 8.2):