Measurement of iron chemical speciation in seawater at 4°C: The use of competitive ligand exchange–adsorptive cathodic stripping voltammetry

• Published in: Marine Chemistry
• Main Authors: Hassler, Christel, Legiret, François-Eric, Butler, Edward C.V.
• Format: Article in Journal/Newspaper
• Language: English
• Published: 2013
• Subjects: Fe; chemical speciation; voltammetry; seawater; low temperature; organic ligand; complexation; Southern Ocean; Tac
• Online Access: https://archive-ouverte.unige.ch/unige:26577

Iron is mostly bound to poorly characterised organic ligands, thus, organic ligands are paramount in defining Fe biogeochemical cycling and its control on oceanic primary productivity. Since 1994, Fe chemical speciation has been determined by Competitive Ligand Exchange– Adsorptive Cathodic Stripping Voltametry (CLE–AdCSV) at room temperature. However, chemical speciation is strongly dependent on temperature and some organic ligands can be temperature sensitive. Here, we compare the use of the CLE–AdCSV at room temperature and at 4 C—a temperature closer to that found in the Southern Ocean, one of the largest iron–limited regions. For both temperatures, similar detection limits and total Fe concentrations were found. However, at 4 C the analytical detection window (αFe(TAC)2) was shifted by 1.4-fold towards the detection of weaker ligands, resulting in up to 2-fold lower ligands concentrations as well as a 2 to 5-fold and 10 to70-fold lower conditional stability constants with inorganic Fe (Fe′) and Fe(III), respectively. As a result, the Fe′ concentration at 4 C was 2-fold greater, resulting in direct implication for Fe bioavailability. Results show that difference in Fe chemical speciation at 4C was not solely explained by temperature effect on thermodynamics with the exchange ligands or the diffusion of the electroactive complex toward the Hg drop. Lowering analytical window during analysis at room temperature is proposed as a first estimate of temperature effect on iron chemical speciation.