Intercomparison of voltammetric techniques to determine the chemical speciation of dissolved copper in a coastal seawater sample

Abstract

The chemical speciation of dissolved copper in a coastal seawater sample was determined using a variety of voltammetric methods by three different research groups. This intercomparison study was performed on a sample from near the mouth of Narragansett Bay, Rhode Island, chosen to be representative of coastal seawater from the East Coast of North America. Three research groups used distinct methods of competitive ligand equilibration/adsorptive cathodic stripping voltammetry (CLE-ACSV) with different competing ligands (salicylaldoxime, benzoylacetone and 8-hydroxyquinoline), two of which employed a range of added ligand concentrations. Two groups used anodic stripping voltammetry (ASV) with a thin mercury film (TMF) rotating glassy carbon disk electrode (RGCDE). Combining these various approaches, resulted in analytical competition strengths ( α Cu(AL) 2 ) ranging from 10 2.7 to 10 6.5 for the CLE-ACSV approaches and 10 1.3 ( α Cu) for the ASV methods. As the value of the analytical competition strength for the added competing ligand increased, the value determined for the natural Cu-binding ligand concentration decreased, while its conditional stability constant, K CuL i ,Cu 2+ cond, increased. This type of behavior is consistent with the natural Cu-binding ligands consisting of a continuum of Cu-binding ligands with increasingly smaller concentrations of increasingly stronger ligands. Results from the different approaches carried out at similar analytical competition strengths compared very favorably. All of the approaches determined that dissolved copper in this sample was overwhelmingly chelated with strong Cu-binding ligands (>99.97%). Thus, although the total dissolved copper concentration was 12.7 nM (10 −7.9 M), the Cu-binding ligands lowered the free cupric ion concentration, [Cu 2+], to ∼10 −13.0 M, a concentration that is non-toxic to even the most sensitive marine microorganisms.