Concentration Stability Constants Research Articles

Recommendations for best practice for iron speciation by competitive ligand exchange adsorptive cathodic stripping voltammetry with salicylaldoxime

The method of competitive ligand exchange followed by adsorptive cathodic stripping voltammetry (CLE-AdCSV) allows for the determination of dissolved iron (DFe) organic speciation parameters, i.e., ligand concentration (LFe) and conditional stability constant (log KFe′Lcond). Investigation of DFe organic speciation by CLE-AdCSV has been conducted in a wide range of marine systems, but aspects of its application pose challenges that have yet to be explicitly addressed. Here, we present a set of observations and recommendations to work toward establishing best practice for DFe organic speciation measurements using the added ligand salicylaldoxime (SA). We detail conditioning procedures to ensure a stable AdCSV signal and discuss the processes at play during conditioning. We also present step-by-step guidelines to simplify CLE-AdCSV data treatment and interpretation using the softwares ECDSoft and ProMCC and a custom spreadsheet. We validate our application and interpretation methodology with the model siderophore deferoxamine B (DFO-B) in a natural seawater sample. The reproducibility of our application and interpretation methodology was evaluated by running duplicate titrations on 19 samples, many of which had been refrozen prior to the duplicate analysis. Nevertheless, 50% of the duplicate analyses agreed within 10% of their relative standard deviation (RSD), and up to 80% within 25% RSD, for both LFe and log KFe′Lcond. Finally, we compared the sequential addition and equilibration of DFe and SA with overnight equilibration after simultaneous addition of DFe and SA on 24 samples. We found a rather good agreement between both procedures, with 60% of samples within 25% RSD for LFe (and 43% of samples for log KFe′Lcond), and it was not possible to predict differences in LFe or log KFe′Lcond based on the method applied, suggesting specific association/dissociation kinetics for different ligand assemblages. Further investigation of the equilibration kinetics against SA may be helpful as a potential way to distinguish natural ligand assemblages.

Determination of stability constants of the NiCO3 0, ZnCO3 0, and CdCO3 0 complexes using solubilities of corresponding metal carbonates in aqueous solutions

The solubilities of NiCO3, ZnCO3, and CdCO3 in NaClO4–NaHCO3–Na2CO3 solutions are studied experimentally at 25°C and ionic strength of 0.1. The resulting data are used to determine the concentration stability constants of NiCO3 0, ZnCO3 0, and CdCO3 0 complexes in aqueous solutions (they are equal to 810, 12700, and 17900, respectively; log $$K'_{MCO_3^0 } $$ = 2.90 ± 0.14, 4.10 ± 0.20, and 4.25 ± 0.04) and to calculate their thermodynamic values (6.45 × 103, 1.01 × 105, and 1.43 × 105, respectively). The logarithms of stability constants of carbonate complexes are found to be inversely dependent on the solubility products of their metal carbonates.

Complexation of hexavalent U, Np, and Pu with cyclopropanecarboxylate anions. Crystal structure of the uranyl complexes {[UO2(bipy)(cpc)]2O2} and [UO2(cpc)2(H2O)2] (bipy = 2,2′-Bipyridine, cpc− = cyclopropanecarboxylate anion)

The complexation of hexavalent U, Np, and Pu with cyclopropanecarboxylate anions, cpc−, in aqueous solutions was studied. The stepwise concentration stability constants of the complexes PuO2(cpc)i2−i (i = 1, 2, 3) are as follows: logK1,2,3 = 2.63 ± 0.20, 1.61 ± 0.20, 1.43 ± 0.20, respectively; the overall concentration stability constant of the complex PuO2(cpc)3− is logβ3 = 5.67 ± 0.60. The complexing properties of the cpc− anion are very close to those of butyrate and isobutyrate anions. Two crystalline uranyl compounds were synthesized: {[UO2(bipy)(cpc)]2O2} (bipy = 2,2′-bipyridine) and [UO2(cpc)2(H2O)2]. The specific feature of the first complex is that it contains peroxide ion. Its appearance may be due to the formation of the cationic moiety via hydrolytic uranyl dimer. The second compound forms a 3D structure, with the complexes linked via hydrogen bonds.

Speciation study of aluminium in beverages by Competitive Ligand Exchange–Adsorptive Stripping Voltammetry

Competitive Ligand Exchange-Adsorptive Stripping Voltammetry (CLE-AdSV) was used for determining the speciation of aluminium in commonly consumed beverages (water, tea, infusion, coffee, orange juice, tomato juice, beer and red wine). Aluminium determination involves the adsorption of Al-complexes with the ligand cupferron onto a hanging mercury drop electrode. All samples were studied at pH 6.5 with an accumulation step at -0.60 V (all potential values in the paper are given versus the Ag/AgCl, [KCl]=3 M reference electrode) during 60 s, and a final cupferron concentration of 4 × 10(-4)M. These conditions were used to establish (i) the concentration of electro-labile aluminium, (ii) the range of ligand concentrations and (iii) the conditional stability constants of beverage samples using titration procedures. The results based on Ruzic plots were compared to computer simulation with Visual MINTEQ. This comparison suggests that labile monomeric Al-forms and soluble organic complexes of low molecular weight can be quantified by the CLE-AdSV procedure. Overall the relative uncertainties on the determination of the electro-active Al fraction and the complexing parameters, i.e., concentration and conditional stability constant of natural ligands in the samples, are less than 15%. Thanks to these results, information on Al bioavailability in beverages was collected and discussed. This study also illustrates the value of computer simulations when complex, time-consuming voltammetric techniques are applied.

Cylindrospermopsis raciborskii (Cyanobacteria) exudates: Chemical characterization and complexation capacity for Cu, Zn, Cd and Pb

Cylindrospermopsis raciborskii is a cosmopolitan and potentially toxic planktonic Cyanobacteria that produces and exudes copious amounts of dissolved organic materials. This organism dominates the eutrophic reservoir Barra Bonita (Brazil), where it normally blooms throughout the year. This investigation focused on the characterization of such exudates analyzing their capacity to complex copper, zinc, lead and cadmium through the determination of ligand concentration (CL) and conditional stability constant (logK’ML), as well as elemental composition (C, H, N and S), the content of carbohydrates, proteins, lipids and dissolved organic carbon (DOC). The dissolved organic material was fractionated into 3 molecular weights (>30 kDa; 30–10 kDa; 10–3 kDa) and each fraction was analyzed. The results showed that in the >30 kDa and 30–10 kDa fractions carbohydrates dominate over proteins and lipids. Different CL and logK'ML were obtained for the different molecular weight fractions of the excreted organic materials, suggesting high diversity of ligands. In the >30 kDa, there were more complexing sites (CL) for Cu, but higher affinity (K′) for Zn. In the 30–10 kDa fraction, the higher CL was for Cd, but the greatest affinities were for Cu and Zn. In the 10–3 kDa fraction, higher CL was obtained for Cd and Zn, while Cu and Cd had the highest strengths of association. In the environment, such diversity of ligands and strengths of association can result in a displacement of metals weakly bound to the EOM, and increase metal buffering capacity of the environment, supporting higher metal inputs before toxic effects are detected in the biota.

Comparison and combined use of linear and non-linear fitting for the estimation of complexing parameters from metal titrations of estuarine samples by CLE/AdCSV

Despite the need to determine the concentration and conditional stability constants (K′) of natural ligands, we are far from achieving a consensus about the mathematical procedure to use with metal titrations due to the complexity of the samples and the wide range of fitting procedures and problems associated with the selection of the sensitivity (S) of the method. Here, we used Competitive Ligand Exchange/Adsorptive Cathodic Stripping Voltammetry (CLE/AdCSV) empiric data from estuarine waters and computer generated titration sets to compare linear methods with iterative correction of S with non-linear fitting adding S as a parameter. We demonstrate for the first time that, independent of the fitting procedure, S cannot be retrieved if all the ligands present in the sample are not included in the speciation model. We also investigated the variables, apart from analytical noise, that can cause flawed non-linear fittings of titration data. Computer generated data under multiple combinations of analytical conditions showed that a long extension of the titration (at least twice the total ligand concentration for estuarine conditions) and an analytical window (as the side coefficient ∝′) centered below the complexing strength of the natural ligands are essential to produce reliable complexing parameters. We verified, using for the first time a combination of experimental and computer generated data, that faulty estimations of S and K′ obtained in empiric titrations of estuarine samples were artifacts of non-linear fitting. Non-linear fitting flaws were caused by a combined effect of the analytical error, the analytical window and the ratio in between the copper concentration and the concentration of the strongest ligands. Here, we recommend for the study of estuarine waters to complement non-linear fitting with iterative linear fitting in order to avoid severe overestimations of S and the conditional stability constant of strong metal ligands.

Complexation of An(VI) (An = U, Np, Pu, Am) with 2,6-pyridinedicarboxylic acid in aqueous solutions. Synthesis and structures of new crystalline compounds of U(VI), Np(VI), and Pu(VI)

Complexation of An(VI) (An = U, Np, Pu, Am) with 2,6-pyridinedicarboxylic (dipicolinic) acid in aqueous solutions was studied. All these actinides form with dipicolinic acid anion, PDC2− 1: 1 and 1: 2 complexes. The PDC2− ion coordinates to actinide(VI) ions in solutions in tridentate fashion. In 1: 2 complexes, the f-f transition bands in the electronic absorption spectra are very weak, which is associated with approximate central symmetry of the coordination polyhedron (CP) of the An atom. The apparent stability constants of Pu(VI) complexes were measured in a wide pH range, and the concentration stability constants of An(VI) (An = U, Np, Pu, Am) were determined. The crystalline complexes [Li2AnO2(PDC)2]·2H2O (An = U, Np, Pu) and [AnO2(PDC)]n (An = Np, Pu) were synthesized, and their structures were determined by single crystal X-ray diffraction. The X-ray data confirmed the conclusion that CP of An atoms in the complex ions AnO2·(PDC)22− is centrosymmetrical. In the isostructural series of [Li2AnO2(PDC)2]·2H2O, the actinide contraction is manifested in shortening of the An-O distances in the “yl” groups in going from U to Pu.

Acid-base properties of thianaphthene-annulated porphyrazine and tetra(pyrazino)porphyrazine complexes with aluminum group metals

Acid-base properties of Al(III), Ga(III), and In(III) complexes with tert-butyl-substituted thianaphthene-annulated porphyrazine and tetra(2,3-pyrazino)porphyrazine in proton-donor medium (CH2Cl2-CF3COOH) were studied by spectrophotometric titration. The concentration stability constants of the singly protonated complexes were determined. The effects of the metal nature and aromatic heterocyclic fragment on the basicity of meso-nitrogen atoms were analyzed. Negative inductive effect of the sulfur atom in the thianaphthene fragment reduces the basicity of the meso-nitrogen atoms as compared to analogous porphyrazine and phthalocyanine complexes. Acid-base transformations of the thianaphthene-annulated tetra(2,3-pyrazino)porphyrazine involve both meso-nitrogen atoms and those in the pyrazine rings.

Interaction of An(VI) (An = U, Np, Pu) and Np(V) with 2,3-pyridinedicarboxylic (quinolinic) acid (H2Quin): Complexation in aqueous solutions, synthesis and structure of the complexes [UO2(HQuin)2], [(NpO2)2(HQuin)2(C6H4NO3)2]·2H2O, and [PuO2Quin(H2O)

Complexation of An(VI) (An = U, Np, Pu), and Np(V) with 2,3-pyridinedicarboxylic (quinolinic, H2Quin) acid in aqueous solutions was studied. Np(V) can form 1: 1 and 1: 2 complexes, and An(VI), also 1: 3 complexes (at pH ≳ 6 and [H2Quin] ≳ 0.1 M). Quinolinate ion can coordinate to actinide(VI) and (V) ions in solutions in different modes. The apparent stability constants of the complexes in a wide pH range and the concentration stability constants of the An(VI) complexes were measured. In the series from Pu(VI) to U(VI), the stability of the complexes slightly increases. Crystalline complexes [UO2(HQuin)2], [(NpO2)2(HQuin)2(HL)2]·2H2O (HL is N-protonated 2-hydroxypyridine-3-carboxylic acid anion), and [PuO2Quin(H2O)] were synthesized, and their structures were determined by single crystal X-ray diffraction. Different types of coordination of quinolinate ions to actinide ions are also observed in the crystalline complexes.

Interaction of tetravalent actinides and Np(V) with some hydroxy carboxylic acids

Interaction of actinides(IV) with hydroxyisobutyric acid (HHIB) in aqueous solutions and in the course of crystallization of solid compounds was studied. The complexes MLn(4-n)+ (M = U, Np, Pu; L− is hydroxyisobutyrate anion; n = 1, 2, 3) exist in solution. Their apparent stepwise stability constants K′i were measured, and the overall concentration stability constants β3 of the complexes ML3+ were calculated. For U(IV) and Np(IV), logβ3 is close to 13.3–13.4, and for Pu(IV), logβ3 = 14.5 ± 0.9 (ionic strength I = 0.1–0.3). In the course of crystallization in air, complexes of U(IV) with hydroxyisobutyric acid, as well as those with citric acid, undergo oxidative degradation, which can be accompanied by complete oxidation of U(IV). The crystalline compounds formed in the process are oxalates of U(IV) or U(VI). The complexation of Np(V) with HHIB was studied. NpO2+ forms with HHIB the complexes NpO2L and NpO2L2−. Their concentration stability constants are logK1 = 2.04 ± 0.15 and logK2 = 0.71 ± 0.10 (I = 0.4), i.e., logβ2 = 2.75 ± 0.25.

Distinct trends in dissolved Fe speciation between shallow and deep waters in the Pacific Ocean

The chemical speciation of dissolved Fe was investigated in the Pacific Ocean to elucidate the regional variability of organic ligands. The concentration and conditional stability constants (K′Fe′L, with respect to inorganic Fe) of organic ligands were analyzed by a Competitive Ligand Equilibration-Adsorptive Cathodic Stripping Voltammetric (CLE-ACSV) method using 2-(2-thiazolylazo)-p-cresol (TAC). In the surface waters of the Pacific Ocean, a low concentration of dissolved Fe was observed (0.20±0.14nM, n=8) with an excess of organic ligands (0.27±0.14Eq of nM Fe, n=7); thus, most of the dissolved Fe was estimated to be complexed with these ligands. A 2D cross-section diagram along 160°W and 170°W showed a roughly increasing trend in the concentration of organic ligands toward the north of the deep waters, suggesting the accumulation of ligands in the biogeochemical cycle of deep water. In the equatorial Pacific and central subtropical North Pacific, the concentration of organic ligands was observed to exceed that of dissolved Fe throughout the water column. On the contrary, the concentration of dissolved Fe at depths ranging from 1000 to 3000m was higher than that of organic ligands in the western North Pacific, suggesting that the excess dissolved Fe can exist as colloidal Fe and/or organic/inorganic complexes with the ligands that were not detectable by our method. A greater concentration of excess dissolved Fe relative to the concentration of organic ligands was also observed in the deep (below 1000m) waters of the subtropical South Pacific. These differences regarding the saturation state of organic ligands ([L]/[D–Fe]) between the surface and deep waters probably reflect different sink and source mechanisms of organic ligands within the water column. The extent of the organic complexation of Fe varies with the biogeochemical processes in the ocean interior and plays a key role in oceanic Fe cycling.

Cathodic processes in copper(II) solutions containing gluconic acid

The kinetics of cathodic processes proceeding in the acidic 0.01 M Cu(II) solutions containing gluconic acid and 0.5 M Na2SO4 as the supporting electrolyte is studied. According to the spectrophotometric data, in the moderately acidic solutions, a monoligand complex of CuL+ predominantly forms. Its concentration stability constant is 102.2 M−1. In the cathodic voltammograms, a well-defined plateau of the limiting current is observed. The height of the plateau obeys the Levich equation. The effective diffusion coefficient decreases from 4.2 × 10−6 to 2.5 × 10−6 cm2/s with increasing complexation degree of the system. An analysis of normalized Tafel plots showed that the exchange current density of Cu2+ + e → Cu+ process decreases with increasing concentration of ligand or with increasing pH value. Thereby, the cathodic chargetransfer coefficient remains constant (0.33 ± 0.02). A comparison of the kinetic data with the results of deposit surface examination points to significant surface activity of the ligand. The gluconate chemisorption can be accompanied by the incorporation of the fragments, which were formed as a result of its destruction, into the electrodeposits.

Acid-base and complexing properties of 10-carboxymethyl-9-acridone

Acid-base and complexing properties of 10-carboxymethyl-9-acridone in a multicomponent solvent H2O-methanol-acetonitrile-dioxane were studied at 22°C, ionic strength I = 0.1 (NaClO4), and different water-organic component ratios. The concentration protonation constants (logKHL = 3.11), dissociation constants (pKHL = 4.14), and stability constants of complexes (logKMgL = 2.18, logKCaL = 2.09, and logKSrL = 1.96) were determined by extrapolation to zero content of organic solvent.

Significance of data treatment and experimental setup on the determination of copper complexing parameters by anodic stripping voltammetry

Different procedures of voltammetric peak intensities determination, as well as various experimental setups were systematically tested on simulated and real experimental data in order to identify critical points in the determination of copper complexation parameters (ligand concentration and conditional stability constant) by anodic stripping voltammetry (ASV). Varieties of titration data sets (Cu measured vs. Cu total) were fitted by models encompassing discrete sites distribution of one-class and two-class of binding ligands (by PROSECE software). Examination of different procedures for peak intensities determination applied on voltammograms with known preset values revealed that tangent fit (TF) routine should be avoided, as for both simulated and experimental titration data it produced an additional class of strong ligand (actually not present). Peak intensities determination by fitting of the whole voltammogram was found to be the most appropriate, as it provided most reliable complexation parameters. Tests performed on real seawater samples under different experimental conditions revealed that in addition to importance of proper peak intensities determination, an accumulation time (control of the sensitivity) and an equilibration time needed for complete complexation of added copper during titration (control of complexation kinetics) are the keypoints to obtain reliable results free of artefacts. The consequence of overestimation and underestimation of complexing parameters is supported and illustrated by the example of free copper concentrations (the most bioavailable/toxic specie) calculated for all studied cases. Errors up to 80% of underestimation of free copper concentration and almost two orders of magnitude overestimation of conditional stability constant were registered for the simulated case with two ligands.

Competing ligand exchange-solid phase extraction method for the determination of the complexation of dissolved inorganic mercury (II) in natural waters

A method employing dual competitive ligand exchange followed by solid phase extraction (CLE-SPE) for characterizing the complexation of inorganic Hg(II) in natural waters is described. This method employs parallel use of two competing ligands: diethyldithiolcarbamate (DEDC), which forms hydrophobic complexes with Hg(II), and thiosalicylic acid (TSA), which forms hydrophilic complexes with Hg(II). Inorganic mercury complexed by natural and competing ligands are separated based on hydrophobicity using C 18 solid phase extraction columns. Data modeling allows for the calculation of the concentration and conditional stability constants of natural ligands capable of complexing Hg(II) in both the operationally defined hydrophilic and hydrophobic fractions. The use of multiple ligand concentrations, and thus multiple analytical windows, to characterize different ligand classes within both of these two fractions is described. Studies of the kinetics of the ligand exchange involved, potential for changes in the stability of natural ligands during freezing and thawing, potential breakthrough during solid phase extraction, as well as the method's precision and estimation of error, are presented and discussed. Results from the application of the method to natural freshwaters demonstrated that in the limited samples collected over 99.99% of the ambient inorganic mercury is strongly complexed by ligands with conditional stability constants ( K HgL cond , Hg 2+) on the order of 10 30, values similar to that of reduced sulfur ligands. At ambient conditions 85–90% of the mercury exists in hydrophobic complexes in these freshwaters, but strong Hg-binding ligands exist in both the hydrophobic and hydrophilic fractions.

Properties of U(VI)/U(IV) and Fe(III)/Fe(II) pairs and equilibrium U(VI) + 2Fe(II) ⇄ U(IV) + 2Fe(III) in solutions of unsaturated heteropolytungstates K10P2W17O61 and K8SiW11O39

The unsaturated phosphotungstate K10P2W17O61 and silicotungstate K8SiW11O39 are strong and selective complexants of An(IV) in aqueous solution. In presence of these complexants, the redox properties of actinides are changed drastically due to stabilization of the tetravalent actinides. Redox potentials of Fe(III)/Fe(II) and U(VI)/U(IV) pairs were measured in K10P2W17O61 and K8SiW11O39, solutions with various HClO4 concentration and absolute complexes stability constants of Fe(III) ions with phosphotungstate and silicotungstate ions were determined: logβabs Fe(III)=17.4 ± 0.8 and 20.1 ± 0.8 respectively. The equilibrium constant of reaction between U(VI) and Fe(II) in 1 M HClO4 was determined. The equilibrium constant, K eq, depends on excess of heteropolyanion, and is close to 104 in 1 M HClO4 at [P2W17O61]free ∼ 5 × 10-4 M.

NpO 2 + -UO 2 2+ cation-cation interaction in NaCl-KCl-CsCl and LiCl-KCl-CsCl eutectic melts

Absorption spectra of the NpO 2 + ion in UO 2 2+ -containing NaCl-KCl-CsCl (0.300:0.245:0.455) and LiCl-KCl-CsCl (0.575:0.165:0.260) eutectic melts in the region of the 5f-5f transitions, 700–2000 nm, were studied at 650 and 500°C, respectively. Processing of the spectra allowed the previously discovered phenomenon, cation-cation interaction of actinides in melts, to be interpreted for the first time in terms of the mass action law. It was shown that, at the concentrations of the interacting components studied (CNp = 0.2 M and CU = 0.1–2 M), a 1: 1 NpO 2 + ·UO 2 2+ cation-cation complex is formed. Its concentration stability constant was estimated at 0.06(6) and 0.11 1 mol−1 for the NaCl-KCl-CsCl and LiCl-KCl-CsCl systems, respectively.

Concentration and Thermodynamic Stability Constants of Magnesium(II) and Calcium(II) Tartrate Complexes in Aqueous Solutions

The formation of calcium and magnesium tartrate complexes was studied by potentiometric titration at ionic strengths of 0.1 and 0.3 (in the presence of tetraethylammonium chloride) at a temperature of 298.15 K. The composition and the stability constants of the complexes formed in the system were determined. The known stability constants were used to optimize the conditions of calorimetric determination of the heats of formation of calcium and magnesium tartrates in an aqueous solution.

Determination of copper complexation in surface microlayer of Daya Bay and Jiaozhou Bay

Copper complexing ligands concentration (C C) and conditional stability constant of the sea surface microlayer (SML) and subsurface layer (SSL) samples, collected from Baya Bay of Guangdong Province and Jiaozhou Bay of Shandong Province, were determined by anodic stripping voltammetry (ASV) technique. C C of Daya Bay in the SML ranged from 6.19×10−7 to 3.52×10−7 mol/L.C C of Jiaozhou Bay in the SML ranged from 5.30×10−7 to 3.03×10−7 mol/L in August 1998 and from 3.46×10−7 to 1.36×10−7 mol/L in May 2000. TheC C concentrations were higher in all surface microlayer samples, compared with corresponding subsurface samples. The average enrichment factors of copper complexation in the SML observed above were 1.58, 1.41 and 1.56, respectively. The enrichment ofC C concentration in the microlayer demonstrated that organic ligands had certain buffer action on the toxicity of the enriched trace metals in the same layer. Conditional stability constants of the SML in Jiaozhou Bay were lower than those of the SSL; however, it showed just opposite for Daya Bay.BOD (biological oxygen demand) andCOD (chemical oxygen demand) indicated the organic matter content, also showed enriched in the SML.C C had an obvious relationship withBOD orCOD.

Metal Ion Buffer Systems for Ionometry in Water-Ethanol Mixtures and Micellar Medium of Sodium Dodecyl Sulfate

The concentration dissociation constants of nitrilotriacetic acid and the concentration stability constants of Cd(II) complexes with the nitrilotriacetic acid anion in water-ethanol mixtures and of Cu(II) and Cd(II) complexes with o-phenanthroline and 2,2′-bipyridine in micellar solutions of sodium dodecyl sulfate were determined potentiometrically. Based on the concentration constants determined, Cd2+ and Cu2+ buffer solutions were developed for calibration of a Cd2+-selective electrode in water-ethanol mixtures and Cd2+ and Cu2+-selective electrodes in micellar solutions of sodium dodecyl sulfate within the ranges pCu 9.7–13.5 and pCd 7.5–11.8.