Constant M.G. van den Berg

Evidence for organic complexation of iron in seawater

Iron occurs at very low concentrations in seawater of oceanic origin and its low abundance is thought to limit primary production in offshore waters (Martin and Fitzwater, 1988). A new electrochemical method, cathodic stripping voltammetry (CSV), is used here to determine the speciation of iron in seawater originating from the Western Mediterranean taking advantage of ligand competition of an added electroactive ligand with the natural organic complexing matter to evaluate whether iron is organically complexed. The measurements indicate that iron occurs 99% (or 99.9% depending on which value is selected for αFe′) complexed by organic complexing ligands throughout the water column of the Western Mediterranean and by analogy probably also in other oceanic waters. The composition of the organic complexing ligands is as yet unknown, but the data indicate a major source from microorganisms (bacteria or phytoplankton) in and immediately below the fluorescence maximum in the upper water column. The organic complexes are apparently reversible releasing iron when the competing ligand is added and binding more iron when its concentration is increased. The organic complexing ligands occur at concentrations well above those of iron ensuring full complexation of this biologically essential element, and buffer the free iron concentration at a very low level against fluctuations as a result of removal by primary producers or inputs from atmospheric sources. The new data indicate that a re-evaluation of the concept of the bioavailable fraction of iron is required.

DETERMINATION OF THE COMPLEXING CAPACITY AND CONDITIONAL STABILITY CONSTANTS OF COMPLEXES OF COPPER(II) WITH NATURAL ORGANIC LIGANDS IN SEAWATER BY CATHODIC STRIPPING VOLTAMMETRY OF COPPER--CATECHOL COMPLEX IONS

A new method is proposed for the determination of complexing capacities and conditional stability constants for complexes of copper(II) with dissolved organic ligands in seawater. This method is based on ligand competition by the added ligand catechol for free metal ions. The concentration of copper-catechol complex ions is measured with great sensitivity by cathodic stripping voltammetry. The concentration of the free copper ion is calculated from the concentration of copper-catechol complex ions. Ligand concentrations and conditional stability constants are obtained from a titration of the ligands with copper. Two techniques for treatment of the data are compared. A seawater sample, originating from open oceanic conditions, is analysed and two complexing ligands were detected, having concentrations of 1.1 × 10−8 and 3.3 × 10−8 M, and conditional stability constants (log K′CuL) of 12.2 and 10.2, respectively.

Organic complexation and its control of the dissolved concentrations of copper and zinc in the Scheldt estuary

Cathodic stripping voltammetry (CSV) is used to determine total (after UV-irradiation) and labile dissolved metal concentrations as well as complexing ligand concentrations in samples from the river Scheldt estuary. It was found that even at high added concentrations of catechol (1 mm for copper and 0·4 mm for iron) and of APDC (1 mm for zinc) only part of the dissolved metal was labile (5–58% for copper, 34–69% for zinc, 10–38% for iron); this discrepancy could be explained by the low solubility of iron which is largely present as colloidal material, and by competition for dissolved copper and zinc by organic complexing ligands. Ligand concentrations varied between 28 and 206 nm for copper and between 22 and 220 nm for zinc; part of the copper complexing ligands could be sub-divided into strong complexing sites with concentrations between 23 and 121 nm and weaker sites with concentrations between 44 and 131 nm. Values for conditional stability constants varied between (logK′ values) 13·0 and 14·8 for strong and between 11·5 and 12·1 for weaker copper complexing ligands, whereas for zinc the values were between 8·6 and 10·6. The average products of ligand concentrations and conditional stability constants (a-coefficients) were 6 × 102 for zinc and 6 × 106 for copper. The dissolved zinc concentration was found to co-vary with the zinc complexing ligand concentration throughout the estuary. It is argued that the zinc concentration is regulated, in this estuary at least, by interactions with dissolved organic complexing ligands. A similar relationship was apparent between the dissolved copper and the strong copper complexing ligand concentration. The total copper complexing ligand concentrations were much greater than the dissolved copper concentrations, suggesting that only strongly complexed copper is kept in solution. These results provide evidence for the first time that interactions of copper and zinc with dissolved organic complexing ligands determine the geochemical pathway of these metals.

Determination of copper complexation in sea water by cathodic stripping voltammetry and ligand competition with salicylaldoxime

Copper complexing ligands in sea water were determined by cathodic stripping voltammetry (CSV) with ligand competition using salicylaldoxime (SA) at a detection window intermediate to that and overlapping with those currently available using other electroactive ligands. The optimised condition to determine total dissolved copper in sea water by CSV using SA entails an SA concentration of 25 μM, a solution pH of 8.0–8.4, and a deposition potential of -1.1 V; the voltammetric scans were initiated at -0.15 V. The conditional stability constants for copper complexation by SA were calibrated by ligand competition against EDTA in sea water of salinities between 1 and 35. The following empirical relationships were found to hold between these conditional stability constants and the salinity (S, in psu): log K′CuSA = (10.12 ± 0.03) - (0.37+-0.02) log S, and log β′Cu(SA)2 = (15.78 +- 0.08) - (0.53 +- 0.07) log S. The centre of the detection window for detection of copper complexation in sea water using SA can be varied in the range of 3.6–5.8 by varying the SA concentration between 1 and 25 μM; this range lies between that covered by tropolone (2.5–4.5) and 8-quinolinol (5.0–8.4). Use of SA in CSV has the analytical advantage of high sensitivity for copper (3–4 fold greater than using catechol, 8-quinolinol or tropolone), reflected in a limit of detection of 0.1 nM copper at a deposition time of 1 min which can be lowered further by extending the deposition time. Comparative complexing ligand titrations indicated that an equilibration period of 6 h is required to attain equilibrium between the added copper, the natural comlexing ligands and the added SA. The method was tested by determining copper complexation at several windows in samples from the North Sea, the NW Mediterranean Sea and the NE Atlantic Ocean, revealing the presence of several complexing ligands. At a constant detection window the ligand concentration in the upper water column of a station in the NE Atlantic was found to vary between 3 and 8 nM, with a maximum at 78 m depth.

Potentials and potentialities of cathodic stripping voltammetry of trace elements in natural waters

Abstract Recent advances in voltammetry are discussed. The main advances made are related to the deposition step which now normally utilizes deposition by adsorption rather than by plating. This change suggests that equipment could now be usefully redesigned to provide fast wave-forms such as the high-frequency square-wave modulation as standard. An additional advantage of adsorptive deposition over voltammetry is that the opportunity is provided to utilize catalytic effects to enhance the sensitivity further. Voltammetry is especially suited for the automated monitoring of the concentration and chemical speciation of trace elements in natural waters. It is in this area that is has an advantage over multiple-element analytical techniques.

Determination of copper, cadmium and lead in seawater by cathodic stripping voltammetry of complexes with 8-hydroxyquinoline

Abstract Procedures are presented to determine simultaneously copper, lead and cadmium in seawater by differential pulse cathodic stripping voltammetry (DPCSV) preceded by adsorptive collection of complexes with 8-hydroxyquinoline (oxine) onto a hanging mercury drop electrode (HMDE). In preliminary experiments the optimal analytical conditions were found to be an oxine concentration of between 0.8 and 2 × 10 −5 M , a pH between 7.5 and 8, the adsorption potential at −1.1 V, and the initial scanning potential at −0.3 V. The peak heights for low levels (sub-nanomolar) of cadmium and lead increase linearly with the adsorption time between 1 and 12 min. The limits of detection for a 1 min stirred adsorption time are 0.12 n M Cd, 0.3 n M Pb and 0.24 n M Cu; these limits are reduced 10x by increasing the adsorption time to 10 min. In these conditions most seawater samples are amenable for direct voltammetric determination of copper, cadmium and lead using an HMDE.

Determination and data evaluation of copper complexation by organic ligands in sea water using cathodic stripping voltammetry at varying detection windows

Published data of trace metals in sea water show very large differences in strength of organic complexation. These differences are incompatible with the expected order of complexation by reversible and nonspecific complexing ligands. Comparison revealed a linear relationship between the detection windows of the analytical techniques and the detected metal complexation. A possible explanation for this relationship is that the apparent differences between organic complexation of different metals are artifacts when the analytical techniques detect only one or two of many complexing ligands of greatly varying complexing ability in the natural water samples. Detailed complexation measurements of copper in samples from the Sargasso Sea and the North Sea were carried out at several detection windows using cathodic stripping voltammetry and confirmed the presence of several complexing ligands. The detected ligand concentration was found to decrease with increasing magnitude of the detection window, whereas the conditional stability constant was found to increase. The overall effect was that the degree of metal complexation [(ΣαCuLi) − i + 1], where i is the number of α coefficients, increased with the detection window whereas the complexation due to the individual ligands (αCuLi) increased until a maximum had been reached when the ligand concentration was approximately equal to the metal concentration whereafter it decreased at very high detection windows. An additional observation was that the dissociation rate of organically complexed copper in sea water is very slow the process requiring more than 1 h to reach equilibrium with an added competing ligand. The complexation data were used to estimate the total number of ligands and the range of conditional stability constants. The overall complexation of copper in sea water was calculated giving values for pCu between 13.5 and 15.4.

Zinc speciation in the Northeastern Atlantic Ocean

Measurements of zinc and zinc complexation by natural organic ligands in the northeastern part of the Atlantic Ocean were made using cathodic stripping voltammetry with ligand competition. Total zinc concentrations ranged from 0.3 nM in surface waters to 2 nM at 2000 m for open-ocean waters, whilst nearer the English coast, zinc concentrations reached 1.5 nM in the upper water column. In open-ocean waters zinc speciation was dominated by complexation to a natural organic ligand with conditional stability constant (log KZnL′) ranging between 10.0 and 10.5 and with ligand concentrations ranging between 0.4 and 2.5 nM. The ligand was found to be uniformly distributed throughout the water column even though zinc concentrations increased with depth. Organic ligand concentrations measured in this study are similar to those published for the North Pacific. However the log KZnL′ values for the North Atlantic are almost and order of magnitude lower than those reported by Bruland [Bruland, K.W., 1989. Complexation of zinc by natural organic-ligands in the central North Pacific. Limnol. Oceanogr., 34, 269–285.] using anodic stripping voltammetry for the North Pacific. Free zinc ion concentrations were low in open-ocean waters (6–20 pM) but are not low enough to limit growth of a typical oceanic species of phytoplankton.

The determination of the chromium speciation in sea water using catalytic cathodic stripping voltammetry

Abstract A voltammetric procedure to determine chromium in aqueous solution using cathodic stripping voltammetry (CSV) preceded by adsorptive collection of complex species with diethylenetriaminepentaacetic acid (DTPA) on a hanging mercury drop electrode is optimized for sea water. The optimized conditions include a pH of 5.2 and a DTPA concentration of 2.5 mM. Comparison of the chemical speciation of DTPA in sea water and distilled water with the CSV sensitivity indicated that the adsorptive species is a complex of chromium(III_with H2DTPA. The sensitivity of the CSV procedure for chromium(VI) in sea water is a decade lower than in distilled water due to major cation competition (of calcium and magnesium) for DTPA in the sea water. The limit of detection for chromium(VI) in sea water is 0.1 nM at a deposition time of 2 min. Chromium(VI) produces a stable peak using the optimized CSV procedures, whereas the peak due to chromium(III) is unstable due to probable conversion of the chromium(III) complex to an electrochemically inert complex over a period of ≈ 30 min. The different behaviours of chromium(VI) and –(III) were utilized to devise a procedure to determine reactive chromium(III) and chromium(VI) in sea water samples. Experiments with an added chelator indicated that the reactive chromium(III) concentration does not include complexes with organic material. The total dissolved chromium concentration was determined after UV-irradiation of the sample at its original (neutral) pH. Storage experiments indicated that neither the redox speciation of chromium nor its total dissolved concentration in sea water could be stored. The CSV method was applied succesfully to certified sea water samples and to the ship-board determination of chromium(VI) and –(III) in the water column of the N.W. Mediterranean.

Determination of copper in sea water by cathodic stripping voltammetry of complexes with catechol

A reduction current is obtained when an aqueous solution of copper and catechol is subjected to differential-pulse cathodic stripping voltammetry (d.p.c.s.v.) because of the reduction of copper(II)—catechol complex ions which adsorb onto the hanging mercury drop electrode (HMDE). The most likely form of the adsorbed complex ions is CuL2−2 (L being catechol). A.c. polarographic measurements showed that these complex ions adsorb more strongly onto the drop than free catechol ions. Monolayer adsorption density is obtained at 2.1 × 10−10 molecules/cm2, equivalent to a surface area of 78 A2 complex ion, which agrees well with the molecular surface area calculated from the bond lengths. Analytically useful currents are obtained at very low metal concentrations, such as in uncontaminated sea water. The possible interference by other trace metals, major cations, and organic complexing ligands is investigated. Competition for copper ions by natural organic complexing ligands is evident at low concentrations of catechol. Analysis of the dissolved copper concentration in sea water by d.p.c.s.v. at the HMDE (at neutral pH) compares favourably with the d.p.a.s.v. technique at a rotating disk electrode (at low pH) because of the shorter collection period and greater sensitivity.