Chemical speciation of iron in seawater using catalytic cathodic stripping voltammetry with ligand competition against salicylaldoxime

• We optimize here a method to determine the chemical speciation of iron in seawater
• The experiments indicate that FeSA is the species that adsorbs on the electrode
• LogK′Fe′SA is 6.52 and log B′Fe′SA2 = 10.72 for SA between 1 and 100 μM SA, with αFe′SA between 3 and 856
• The sensitivity for iron is catalytically enhanced 10-fold by dissolved oxygen
• Log K values are distinct for ligands in seawater, humics and a siderophore

The chemical speciation of iron in seawater is typically determined by cathodic stripping voltammetry (CSV) making use of ligand competition between an electroactive ligand added to obtain the CSV signal and the natural ligand to determine the complex stability of the natural species. Different procedures differ in the added ligand that is selected. Recent findings have suggested that several of these procedures suffer from interference by humic substances, which are now known to be ubiquitous in coastal and ocean waters. We re-optimise here CSV of iron speciation using salicylaldoxime (SA) in seawater, finding differences with the pre-existing method, and a different interpretation for the electroactive species. The main findings are that optimum sensitivity is obtained at ~ 5 × less SA, that the complex responsible for adsorption on the electrode is FeSA, that the FeSA2 species does not adsorb, and that the sensitivity of the method is much improved in the presence of dissolved oxygen (DO) through a catalytic effect (FeII acts as catalyst for the reduction of DO). The complex stability for complexes of Fe′ with SA (FeSA and FeSA2), in pH 8 seawater, is calibrated over a range of SA concentrations between 1 and 40 μM SA against EDTA and between 1 and 100 μM SA without EDTA. Data fitting of the EDTA data gave log K′Fe′SA = 6.50 ± 0.04 and log B′Fe′SA2 = 10.85 ± 0.08. The data fits agree with the formation of an electroactive species FeSA which is superseded by a non-electroactive FeSA2 at [SA] > 5 μM. Independent calibration of these stability constants on the basis of the formation of FeSA in competition only with the hydroxide species of FeIII, between 1 and 100 μM SA, without EDTA, gave values of log K′Fe′SA = 6.52 ± 0.01 and log B′Fe′SA2 = 10.72 ± 0.03. These are the values we propose for the constants as they are independent of any uncertainties in the speciation with EDTA. The similarity of these constants to those determined via calibration against EDTA shows that the speciation of Fe with SA and EDTA is well understood. The re-optimised method is applied to a mixed depth Celtic Sea sample, and two GEOTRACES samples from the Atlantic, at a SA concentration of 5 μM. Ligand concentrations were 1.47 and 1.49 nM in the GEOTRACES water (log K′Fe′L values of 11.1 and 11.9) and 2.53 nM in the Celtic Sea water (log K′Fe′L = 11.5). Application of the method to ligands added to seawater gave log K′Fe′L values of 11.6 ± 0.1 for humic acid (Suwannee River) and 12.2 ± 0.3 for a siderophore (desferrioxamine B). Measurement of the rate of dissociation of the complex of Fe with the natural ligand in Celtic seawater gave a value of kFeL = 0.00133 ± 0.0002 s− 1. The half-life of this reaction is 8.7 minutes. This means that a reaction time of 1 h is required after the addition of SA prior to analysis.

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