182
C.M.G. van den Berg
The part of the natural ligands L which was initially complexed by M is thus liberated by the addition of the competing ligand. Titration of the ligands L with metal ions
while monitoring the concentration of MAL (= the labile metal concentration) with
CSV causes the response to increase in a non-linear fashion until the ligand is saturated when a linear response is obtained: first the ligand is titrated which was freed
upon addition of AL along with any excess ligand. The total ligand concentration is
determined. The curved response is confirmation that the complexes are chemically
reversible: metal is released when the competing ligands are added, and the complex
is formed again when metal is added.
The ligand titration is carried out with equilibration after each addition. For this
reason the sample is typically subdivided into ten 10 ml aliquots. The procedure is as
follows: to 100 ml seawater pH buffer is added (usually pH around 8), and a low concentration of the competing ligand is added sufficient to make less than half the metal
concentration labile. Metal additions are made to ten Teflon ~ or polystyrene cups or
vials to cover a concentration range between 0 and at least twice the expected ligand
concentration, and 10 ml of the seawater/reagent mixture is pipetted into each cup. The
cups are covered and allowed to equilibrate for at least 3 h, usually overnight. Starting
from the lowest concentration, the aliquots are transferred to a voltammetric cell, oxygen is removed by 5 min purging with nitrogen gas, and the CSV response responding
to the labile metal concentration is determined. The sensitivity is calibrated using the
final (linear) part of the titration (where the natural ligands are saturated), and can
be corroborated by further metal additions to the last aliquot.
8.10
How to Evaluate the Ligand Titrations: van den Berg/Ruzic Plots
An example of a complexing ligand titration is shown for a titration with iron of a
seawater sample originating from a depth of 50 m in the north-western Mediterranean (van den Berg 1995). The labile iron concentration is monitored by CSV in the
presence of nitrosonaphthol (NN) (Fig. 8.3). The CSV scans for this titration are shown
in Fig. 8.3a. The peak height for iron increases slowly with the iron concentration until the naturalligand(s) have been saturated when the peak height increases more rapidly; at iron concentrations approximately twice the ligand concentration the increase
of the peak height is approximately linear with the iron concentration. This part of
the titration can be used to obtain the sensitivity.
There is a direct relationship between the sensitivity, the peak height and the labile
metal concentration as the reduction current (ip) is due to the reduction of adsorbed
MAL (e.g. FeNN in the titration with iron). The sensitivity is calibrated by a standard
metal addition which provides the link between the peak height and the concentration.
Because the standard addition distributes itself over complexes with AL as well as slightly
increasing the concentration of inorganic metal, the sensitivity (S) is calibrated with
respect to the sum of these two species rather than just to the concentration of MAL:
S = ip/ ([MAL] + [M'l)
For this reason the labile metal concentration is defined as [labile metal] =
([MAL] + [M'l), which is calculated directly from [labile metal] = ipS (Table 8.3).
C.M.G. van den Berg
The part of the natural ligands L which was initially complexed by M is thus liberated by the addition of the competing ligand. Titration of the ligands L with metal ions
while monitoring the concentration of MAL (= the labile metal concentration) with
CSV causes the response to increase in a non-linear fashion until the ligand is saturated when a linear response is obtained: first the ligand is titrated which was freed
upon addition of AL along with any excess ligand. The total ligand concentration is
determined. The curved response is confirmation that the complexes are chemically
reversible: metal is released when the competing ligands are added, and the complex
is formed again when metal is added.
The ligand titration is carried out with equilibration after each addition. For this
reason the sample is typically subdivided into ten 10 ml aliquots. The procedure is as
follows: to 100 ml seawater pH buffer is added (usually pH around 8), and a low concentration of the competing ligand is added sufficient to make less than half the metal
concentration labile. Metal additions are made to ten Teflon ~ or polystyrene cups or
vials to cover a concentration range between 0 and at least twice the expected ligand
concentration, and 10 ml of the seawater/reagent mixture is pipetted into each cup. The
cups are covered and allowed to equilibrate for at least 3 h, usually overnight. Starting
from the lowest concentration, the aliquots are transferred to a voltammetric cell, oxygen is removed by 5 min purging with nitrogen gas, and the CSV response responding
to the labile metal concentration is determined. The sensitivity is calibrated using the
final (linear) part of the titration (where the natural ligands are saturated), and can
be corroborated by further metal additions to the last aliquot.
8.10
How to Evaluate the Ligand Titrations: van den Berg/Ruzic Plots
An example of a complexing ligand titration is shown for a titration with iron of a
seawater sample originating from a depth of 50 m in the north-western Mediterranean (van den Berg 1995). The labile iron concentration is monitored by CSV in the
presence of nitrosonaphthol (NN) (Fig. 8.3). The CSV scans for this titration are shown
in Fig. 8.3a. The peak height for iron increases slowly with the iron concentration until the naturalligand(s) have been saturated when the peak height increases more rapidly; at iron concentrations approximately twice the ligand concentration the increase
of the peak height is approximately linear with the iron concentration. This part of
the titration can be used to obtain the sensitivity.
There is a direct relationship between the sensitivity, the peak height and the labile
metal concentration as the reduction current (ip) is due to the reduction of adsorbed
MAL (e.g. FeNN in the titration with iron). The sensitivity is calibrated by a standard
metal addition which provides the link between the peak height and the concentration.
Because the standard addition distributes itself over complexes with AL as well as slightly
increasing the concentration of inorganic metal, the sensitivity (S) is calibrated with
respect to the sum of these two species rather than just to the concentration of MAL:
S = ip/ ([MAL] + [M'l)
For this reason the labile metal concentration is defined as [labile metal] =
([MAL] + [M'l), which is calculated directly from [labile metal] = ipS (Table 8.3).