Comparing CLE-AdCSV applications using SA and TAC to determine the Fe-binding characteristics of model ligands in seawater

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

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 are compared. The applications are: 1) Salicylaldoxime (SA) at 25µM (SA25) and short waiting time, 2) SA at 5µM (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 log D 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 non humic 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 under estimated the ligand concentration. The extent of overestimation and underestimation differed per model ligand, and the three applications showed the same trend between the non humic 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.

Local Structure of Ca2+ Alginate Hydrogels Gelled via Competitive Ligand Exchange and Measured by Small Angle X-ray Scattering

Alginates, being linear anionic co-polymers of 1,4-linked residues β-d-ManA (M) and α-l-GulA (G), are widely applied as hydrogel biomaterials due to their favourable in vivo biocompatibility and convenient ionic crosslinking. The “egg-box” model is the prevailing description of the local structure of junction zones that form between the alginate chains and divalent cations, such as Ca2+, when ionic gelation occurs. In the present study we address to what extent signatures of lateral dimerization and further lateral association of junction zones also represent a valid model for the gelation of alginate using the recently reported method of competitive ligand exchange of chelated Ca2+ ions as a method for introducing gelling ions at constant pH. Small angle X-ray scattering with a q range from 0.1 to 3.3 nm−1 was employed to determine local structure in the hydrogel, using a custom-made fluid sample cell inserted in the X-ray beam. The scattering volume was intended to be localized to the contact zone between the two injected aqueous alginate solutions, and data was captured to resolve the kinetics of the structure formation at three different conditions of pH. The data show evolution of the local structure for the Ca2+ induced formation of junction zones in an alginate with 68% G residues, characterized by cross-sectional radii that could be accounted for by a two-component, broken rod like model. The evolution of the two component weight fractions apparently underpinned the connectivity, as reflected in the rheological data.

Competitive Ligand Exchange and Dissociation in Ru Indenyl Complexes

Kinetic profiles obtained from monitoring the solution phase substitution chemistry of [Ru(η⁵-indenyl)(NCPh)(PPh₃)₂]⁺ (<b>1</b>) by both ESI-MS and ³¹P{¹H} NMR are essentially identical, despite an enormous difference in sample concentrations for these complementary techniques. These studies demonstrate dissociative substitution of the NCPh ligand in <b>1</b>. Competition experiments using different secondary phosphine reagents provide a ranking of phosphine donor abilities at this relatively crowded half-sandwich complex: PEt₂H > PPh₂H >> PCy₂H. The impact of steric congestion at Ru is evident also in reactions of <b>1</b> with tertiary phosphines; initial substitution products [Ru(η⁵-indenyl)(PR₃)(PPh₃)₂]⁺ rapidly lose PPh₃, enabling competitive recoordination of NCPh. Further solution experiments, relevant to the use of <b>1</b> in catalytic hydrophosphination, show that PPh₂H out-competes PPh₂CH₂CH₂CO₂Bu<i>^t</i> (the product of hydrophosphination of <i>tert</i>-butyl acrylate by PPh₂H) for coordination to Ru, even in the presence of a ten-fold excess of the tertiary phosphine. Additional information on relative phosphine binding strengths was obtained from gas-phase MS/MS experiments, including collision-induced dissociation (CID) experiments on the mixed phosphine complexes [Ru(η⁵-indenyl)PP’P’’]⁺, which ultimately appear in solution during the secondary phosphine competition experiments. Unexpectedly, unsaturated complexes [Ru(η⁵-indenyl)(PR₂H)(PPh₃)]⁺, generated in the gas-phase, undergo preferential loss of PR₂H. We propose competing orthometallation of PPh₃ is responsible for the surprising stability of the [Ru(η⁵-indenyl)(PPh₃)]⁺ fragment under these conditions.

Competitive Ligand Exchange and Dissociation in Ru Indenyl Complexes

Kinetic profiles obtained from monitoring the solution phase substitution chemistry of [Ru(η⁵-indenyl)(NCPh)(PPh₃)₂]⁺ (<b>1</b>) by both ESI-MS and ³¹P{¹H} NMR are essentially identical, despite an enormous difference in sample concentrations for these complementary techniques. These studies demonstrate dissociative substitution of the NCPh ligand in <b>1</b>. Competition experiments using different secondary phosphine reagents provide a ranking of phosphine donor abilities at this relatively crowded half-sandwich complex: PEt₂H > PPh₂H >> PCy₂H. The impact of steric congestion at Ru is evident also in reactions of <b>1</b> with tertiary phosphines; initial substitution products [Ru(η⁵-indenyl)(PR₃)(PPh₃)₂]⁺ rapidly lose PPh₃, enabling competitive recoordination of NCPh. Further solution experiments, relevant to the use of <b>1</b> in catalytic hydrophosphination, show that PPh₂H out-competes PPh₂CH₂CH₂CO₂Bu<i>^t</i> (the product of hydrophosphination of <i>tert</i>-butyl acrylate by PPh₂H) for coordination to Ru, even in the presence of a ten-fold excess of the tertiary phosphine. Additional information on relative phosphine binding strengths was obtained from gas-phase MS/MS experiments, including collision-induced dissociation (CID) experiments on the mixed phosphine complexes [Ru(η⁵-indenyl)PP’P’’]⁺, which ultimately appear in solution during the secondary phosphine competition experiments. Unexpectedly, unsaturated complexes [Ru(η⁵-indenyl)(PR₂H)(PPh₃)]⁺, generated in the gas-phase, undergo preferential loss of PR₂H. We propose competing orthometallation of PPh₃ is responsible for the surprising stability of the [Ru(η⁵-indenyl)(PPh₃)]⁺ fragment under these conditions.

On the nature of dissolved copper ligands in the early buoyant plume of hydrothermal vents

Environmental contextCopper released by deep-sea hydrothermal vents has been recognised to be partly stabilised against precipitation by its complexation with strong Cu binding ligands. Yet, the sources and nature of these compounds in such environments are still not fully understood. This study shows that the Cu ligands detected are hydrothermally sourced and could be mainly inorganic sulfur species. AbstractThe apparent speciation of Cu in the early buoyant plume of two black smokers (Aisics and Y3) from the hydrothermal vent field Lucky Strike (Mid-Atlantic Ridge) was investigated using competitive ligand exchange-adsorptive cathodic stripping voltammetry (CLE–AdCSV). We have assessed the apparent Cu-binding ligand concentration ([L]) and the corresponding conditional stability constant (log K′) for 24 samples. At the smoker Aisics, [L] ranged from 18.2 to 2970 nM. Log K′CuL ranged from 12.4 to 13.4. At Y3, the binding capacity of natural ligands was from 32.5 to 1020 nM, with Log K′CuL ranging from 12.5 to 13.1. Total dissolved Cu ranged from 7.0 to 770 nM and from 12.7 to 409 nM at Aisics and Y3, respectively. Our results show that the amount of ligand L increases with dissolved Mn (dMn) concentrations, suggesting a hydrothermal origin of the Cu-binding ligands detected. In addition, such high concentrations of Cu-binding ligands can only be explained by an additional abiotic source differing from organic processes. Based on the massive in situ concentrations of free sulfides (up to 300 µM) and on the striking similarities between our log K′CuL and the log K′Cu(HS) previously published, we infer that the Cu-binding ligands could be predominantly inorganic sulfur species in the early buoyant plume of the two vent sites studied.

Competitive ligand exchange of crosslinking ions for ionotropic hydrogel formation

We describe a new approach to form hydrogels of ionotropic polymers using competitive displacement of chelated ions. This strategy enables control of ion release kinetics within an aqueous polymer solution and thus control over gelation kinetics across a wide range of pH.