Abstract. Competitive ligand exchange–adsorptive cathodic
stripping voltammetry (CLE-AdCSV) is used to determine the conditional
concentration ([L]) and the conditional binding strength (logKcond) of
dissolved organic Fe-binding ligands, which together influence the
solubility of Fe in seawater. Electrochemical applications of Fe speciation
measurements vary predominantly in the choice of the added competing ligand.
Although different applications show the same trends, [L] and logKcond
differ between the applications. In this study, binding of two added ligands
in three different common applications to three known types of natural
binding ligands is compared. The applications are (1) salicylaldoxime (SA)
at 25 µM (SA25) and short waiting time, (2) SA at 5 µM (SA5), and
(3) 2-(2-thiazolylazo)-ρ-cresol (TAC) at 10 µM, the latter two
with overnight equilibration. The three applications were calibrated under
the same conditions, although having different pH values, resulting in the
detection window centers (D) DTAC > DSA25 ≥ SA5 (as logD
values with respect to Fe3+: 12.3 > 11.2 ≥ 11). For the model ligands, there is no common trend in the results of
logKcond. The values have a considerable spread, which indicates that
the error in logKcond is large. The ligand concentrations of the nonhumic model ligands are overestimated by SA25, which we attribute to the lack
of equilibrium between Fe-SA species in the SA25 application. The
application TAC more often underestimated the ligand concentrations and the
application SA5 over- and underestimated the ligand concentration. The
extent of overestimation and underestimation differed per model ligand, and
the three applications showed the same trend between the nonhumic model
ligands, especially for SA5 and SA25. The estimated ligand concentrations for
the humic and fulvic acids differed approximately 2-fold between TAC and SA5
and another factor of 2 between SA5 and SA25. The use of SA above 5 µM suffers from the formation of the species
Fe(SA)x (x>1) that is not electro-active as already suggested by
Abualhaija and van den Berg (2014). Moreover, we found that the reaction
between the electro-active and non-electro-active species is probably
irreversible. This undermines the assumption of the CLE principle, causes
overestimation of [L] and could result in a false distinction into more than
one ligand group. For future electrochemical work it is recommended to take the above
limitations of the applications into account. Overall, the uncertainties
arising from the CLE-AdCSV approach mean we need to search for new ways to
determine the organic complexation of Fe in seawater.
Comparing CLE-AdCSV applications using SA and TAC to determine the Fe binding characteristics of model ligands in seawater
