180
C.M.G. van den Berg
This ratio is directly related to the ratio of the a-coefficients, where only the one for
ML is unknown. A tentative value for aML can therefore be calculated from the ratio
of the labile over the total metal concentration. This value is only a first approximation as its validity depends on whether the concentration of the natural ligand, [Lt1, is
greater than that of the metal. The ligand concentration itself, and a value for the conditional stability constant, Kk, is determined by titration of the sample with metal ions.
8.8
Calibration of aMAL
The complex stability of MAL can be calculated if the value for the conditional stability constants, KMAL, is known. If this is not known, the conditional stability constant
has to be calibrated against a known ligand. This calibration is done by adding EDTA
and measuring the decrease in the peak height for the metal, which itself is a measure
of the concentration of MAL. EDTA is normally selected for this calibration because
its complex stability is known for a large number of metals and the extent of side-reactions with the major ions in seawater can be fairly accurately calculated. The theory
for the calibration is presented in Table 8.3.
The effect of adding EDTA to sea water also containing the competing ligand
nitrosonaphthol (NN) on the peak height for iron is shown in Fig. 8.2: it can be seen
that the peak height (representing the concentration of FeNN) diminishes when EDTA
is added because the EDTA is competing with the ligand NN for a limited amount of
iron. The decrease in the peak height causes the ratio of ip I io to decrease (see Table 8.3),
and this can be used to calculate a value for aFeNN, and then for f3~eNN3 as the concentration of NN is known (in this case it was assumed that complexes of the type FeNN 3
are formed). The value for aMAL (aFeNN) is then used to determine the complex stability of the unknown ligands.
8.9
Determination of Ligand Concentrations and Conditional Stability
Constants Using CSV with Ligand Competition
Addition of the competing ligand AL to a sample causes some of the metal initially
complexed by L to become labile by complexation with AL until the tendency to form
complexes between AL and M is exactly balanced by that between the natural ligand
(L) and M. At that point the ratio of the labile over the dissolved metal concentration
is given by (Table 8.2):
The fraction of metal which is made labile (i.e. that which is bound by AL) by the
addition of the competing ligand, is determined by the ratio of the complex stability
of MAL, which is defined by aMAL (Table 8.2), over the complex stability of all the complexes in the water, which is defined by the sum of all the a-coefficients. The theory is
shown in Table 8.2.
C.M.G. van den Berg
This ratio is directly related to the ratio of the a-coefficients, where only the one for
ML is unknown. A tentative value for aML can therefore be calculated from the ratio
of the labile over the total metal concentration. This value is only a first approximation as its validity depends on whether the concentration of the natural ligand, [Lt1, is
greater than that of the metal. The ligand concentration itself, and a value for the conditional stability constant, Kk, is determined by titration of the sample with metal ions.
8.8
Calibration of aMAL
The complex stability of MAL can be calculated if the value for the conditional stability constants, KMAL, is known. If this is not known, the conditional stability constant
has to be calibrated against a known ligand. This calibration is done by adding EDTA
and measuring the decrease in the peak height for the metal, which itself is a measure
of the concentration of MAL. EDTA is normally selected for this calibration because
its complex stability is known for a large number of metals and the extent of side-reactions with the major ions in seawater can be fairly accurately calculated. The theory
for the calibration is presented in Table 8.3.
The effect of adding EDTA to sea water also containing the competing ligand
nitrosonaphthol (NN) on the peak height for iron is shown in Fig. 8.2: it can be seen
that the peak height (representing the concentration of FeNN) diminishes when EDTA
is added because the EDTA is competing with the ligand NN for a limited amount of
iron. The decrease in the peak height causes the ratio of ip I io to decrease (see Table 8.3),
and this can be used to calculate a value for aFeNN, and then for f3~eNN3 as the concentration of NN is known (in this case it was assumed that complexes of the type FeNN 3
are formed). The value for aMAL (aFeNN) is then used to determine the complex stability of the unknown ligands.
8.9
Determination of Ligand Concentrations and Conditional Stability
Constants Using CSV with Ligand Competition
Addition of the competing ligand AL to a sample causes some of the metal initially
complexed by L to become labile by complexation with AL until the tendency to form
complexes between AL and M is exactly balanced by that between the natural ligand
(L) and M. At that point the ratio of the labile over the dissolved metal concentration
is given by (Table 8.2):
The fraction of metal which is made labile (i.e. that which is bound by AL) by the
addition of the competing ligand, is determined by the ratio of the complex stability
of MAL, which is defined by aMAL (Table 8.2), over the complex stability of all the complexes in the water, which is defined by the sum of all the a-coefficients. The theory is
shown in Table 8.2.