C.M.G. van den Berg
equation shown in Table 8.3 was used to calculate the ligand concentration ([Ltl = 9.5 nM)
and the conditional stability constant (log KpeL = 21.5) (Fig. 8.3c). The plot did not show
evidence for the presence of more than one ligand which would manifest itself in two
linear segments joined by a curve. In that case the data can be fitted using two linear
segments, or by using non-linear fitting of the data.
In the example of the titration with iron shown in Fig. 8.3, the dissolved iron concentration was -3 nM, well below the ligand concentration. The stability of the complex was very great with a value for log aPeL = 13.5, much greater than that for inorganic complexation of iron: log aPe = 11.3 at pH 8). Most iron therefore occurred fully
complexed by organic matter in this sample, in spite of a very great stability of the
inorganic (hydroxide) species.
8.10.1
Example of a Ligand Titration with Copper
The organic complexation of copper in natural waters is probably most studied of
all metals because copper is known to be more readily complexed than other divalent metals by organic ligands. Copper is known to occur strongly complexed
by organic matter throughout the oceanic water column (Buckley and van den Berg
1986; Coale and Bruland 1988). Copper complexing ligands are known to be released by algae and bacteria in cultures (Sueur et al. 1982; Harwood-Sears and
Gordon 1990).
A complexing ligand titration with copper of a culture of Emiliania huxleyi with
detection of labile copper by CSV and ligand competition clearly shows curvature. The
scans for this titration are shown in Fig. 8Aa, whereas a plot of the peak height as a
function of the metal concentration reveals the curvature. The ligand concentration
calculated from the data (Fig. 8.3C) shows a ligand concentration of 54 nM, much
greater than the copper concentration of 8 nM originally present in the water (data
from Leal et al. 1999). The complex stability is quite high: in this case the value for
log KCuL was 12.0 but complexes of greater stability have been found in oceanic waters
(Coale and Bruland 1990) and in estuarine waters (van den Berg et al. 1990).
8.11
Effects of the Detection Window
Separate investigations have shown that copper, zinc and nickel occur to various extents complexed in sea water. Different ligand concentrations and different complex
stabilities are detected for different metals, and different ligand concentrations using
different methods for the same metal. This has led to the suggestion that the detected
ligand concentration depends on the detection window of the technique (van den Berg
et al. 1990; van den Berg and Donat 1992). It is likely that natural waters contain several organic complexing ligands and what is detected depends somewhat on the technique. Mostly the stronger complexes should be detected first unless these have been
saturated with metal due to sample contamination. Very strong complexes can be detected by using ligand competition using either a ligand which forms very stable complexes and can therefore compete with strong, natural, complexing ligands, or by us-
equation shown in Table 8.3 was used to calculate the ligand concentration ([Ltl = 9.5 nM)
and the conditional stability constant (log KpeL = 21.5) (Fig. 8.3c). The plot did not show
evidence for the presence of more than one ligand which would manifest itself in two
linear segments joined by a curve. In that case the data can be fitted using two linear
segments, or by using non-linear fitting of the data.
In the example of the titration with iron shown in Fig. 8.3, the dissolved iron concentration was -3 nM, well below the ligand concentration. The stability of the complex was very great with a value for log aPeL = 13.5, much greater than that for inorganic complexation of iron: log aPe = 11.3 at pH 8). Most iron therefore occurred fully
complexed by organic matter in this sample, in spite of a very great stability of the
inorganic (hydroxide) species.
8.10.1
Example of a Ligand Titration with Copper
The organic complexation of copper in natural waters is probably most studied of
all metals because copper is known to be more readily complexed than other divalent metals by organic ligands. Copper is known to occur strongly complexed
by organic matter throughout the oceanic water column (Buckley and van den Berg
1986; Coale and Bruland 1988). Copper complexing ligands are known to be released by algae and bacteria in cultures (Sueur et al. 1982; Harwood-Sears and
Gordon 1990).
A complexing ligand titration with copper of a culture of Emiliania huxleyi with
detection of labile copper by CSV and ligand competition clearly shows curvature. The
scans for this titration are shown in Fig. 8Aa, whereas a plot of the peak height as a
function of the metal concentration reveals the curvature. The ligand concentration
calculated from the data (Fig. 8.3C) shows a ligand concentration of 54 nM, much
greater than the copper concentration of 8 nM originally present in the water (data
from Leal et al. 1999). The complex stability is quite high: in this case the value for
log KCuL was 12.0 but complexes of greater stability have been found in oceanic waters
(Coale and Bruland 1990) and in estuarine waters (van den Berg et al. 1990).
8.11
Effects of the Detection Window
Separate investigations have shown that copper, zinc and nickel occur to various extents complexed in sea water. Different ligand concentrations and different complex
stabilities are detected for different metals, and different ligand concentrations using
different methods for the same metal. This has led to the suggestion that the detected
ligand concentration depends on the detection window of the technique (van den Berg
et al. 1990; van den Berg and Donat 1992). It is likely that natural waters contain several organic complexing ligands and what is detected depends somewhat on the technique. Mostly the stronger complexes should be detected first unless these have been
saturated with metal due to sample contamination. Very strong complexes can be detected by using ligand competition using either a ligand which forms very stable complexes and can therefore compete with strong, natural, complexing ligands, or by us-