Determination Of Stability Constants Research Articles

Stability constants determination of successive metal complexes by hyphenated CE‐ICPMS

The study of radionuclides speciation requires accurate evaluation of stability constants, which can be achieved by CE-ICPMS. We have previously described a method for 1:1 metal complexes stability constants determination. In this paper, we present its extension to the case of successive complexations and its application to uranyl-oxalate and lanthanum-oxalate systems. Several significant steps are discussed: analytical conditions choice, mathematical treatment by non-linear regression, ligand concentration and ionic strength corrections. The following values were obtained: at infinite dilution, log(beta(1) degrees (UO(2)Oxa))=6.93+/-0.05, log(beta(2) degrees (UO(2)(Oxa)(2) (2-)))=11.92+/-0.43 and log(beta(3) degrees (UO(2)(Oxa)(3) (4-)))=15.11+/-0.12; log(beta(1) degrees (LaOxa(+)))=5.90+/-0.07, log(beta(2) degrees (La(Oxa)(2) (-)))=9.18+/-0.19 and log(beta(3) degrees (La(Oxa)(3) (3-)))=9.81+/-0.33. These values are in good agreement with the literature data, even though we suggest the existence of a new lanthanum-oxalate complex: La(Oxa)(3) (3-). This study confirms the suitability of CE-ICPMS for complexation studies.

Kinetic and mechanistic investigations in homogeneous catalysis using operando UV/vis spectroscopy

This study presents three examples in which the methodological approach of operando UV/vis spectroscopy is demonstrated using both submersible optrodes and the stopped flow/diode array technique. First, the submersible optrode is applied for the monitoring of stoichiometric reactions, that is the hydrogenation of cyclooctadiene (cod) in the rhodium diphosphine complex [Rh(Me-Du-PHOS)(cod)]BF 4 (Me-DuPHOS = 1,2-bis(-2,5-dimethyl-phospholano)benzene), which is a typical catalyst precursor in asymmetric hydrogenation. Second, the determination of stability constants for rhodium diphosphine arene complexes is shown. The third example presents UV/vis spectroscopy of zirconium and hafnium complexes with ethylene as a gaseous reaction partner using the stopped flow/diode array technique.

Study of cadmium–humic interactions and determination of stability constants of cadmium–humate complexes from their diffusion coefficients obtained by scanned stripping voltammetry and dynamic light scattering techniques

Diffusion coefficients of Cd–humate complexes are dependent on pH and [Cd]/[Humic] Acid (HA)] ratio in a Cd–HA system. These two factors mainly control the mass transport and complexation kinetics of Cd that may influence bioavailability and toxicity of Cd species in environmental systems. Determination of diffusion coefficients of Cd–HA systems by Scanned stripping voltammetry and dynamic light scattering techniques can provide a better understanding of the systems and can be very useful for extracting other speciation parameters of the systems. This study revealed that Cd 2+ ion along with small dynamic Cd complexes was predominantly present in a Cd–HA system at pH 5 with high diffusion coefficients. HA molecules were in aggregated form at pH 5. However, HA molecules were in disaggregated form at pH 6 and concentrations of Cd 2+ ion and small Cd-dynamic complexes decreased with a decrease in diffusion coefficients of Cd complexes at this pH due to formation of Cd–humate complexes. No further decrease in the hydrodynamic radii of HA was observed with the increase of pH from 6 to 7. The Cd–humate system partially lost its lability at pH 7. Conditional stability constants were calculated for Cd–humate complexes by combining the diffusion coefficient data obtained by two techniques. The log K values calculated in this study are in good agreement with the data available from the literature.

Cyclodextrin inclusion of four phenylurea herbicides: determination of complex stoichiometries and stability constants using solution 1H NMR spectroscopy

An inclusion complex formation between α- and β-cyclodextrin and four phenylurea analogues, namely metobromuron, monolinuron, monuron and fenuron, is reported. Complex formation was established using solution 1H NMR spectroscopy. Complex stoichiometries were determined by the method of continuous variation using the chemically induced shifts of both the host and guest protons. An analysis of the spectroscopic data revealed the stoichiometry as 1:1 in all cases while a further analysis of the same data yielded values for the association constant K ranging from 208 to 2749 M− 1. From the observed chemical shifts it was deduced that in all cases, only the guest aromatic ring enters the host cavities, the substituted urea moiety protruding from the secondary rim in the case of α-cyclodextrin, but from the primary rim in the case of β-cyclodextrin.

Ferrocene–β‐Cyclodextrin Conjugates: Synthesis, Supramolecular Behavior, and Use as Electrochemical Sensors

Ferrocene with a beta-cyclodextrin unit bound to one or both cyclopentadienyl rings through the secondary face were conveniently synthesized by regiospecific copper(I)-catalyzed cycloaddition of 2-O-propargyl-beta-cyclodextrin to azidomethyl or bis(azidomethyl)ferrocene. The supramolecular behavior of the synthesized conjugates in both the absence and presence of bile salts (sodium cholate, deoxycholate, and chenodeoxycholate) was studied by using electrochemical methods (cyclic and differential pulse voltammetry), isothermal titration calorimetry, and NMR spectroscopy (PGSE, CPMG, and 2D-ROESY). These techniques allowed the determination of stability constants, mode of inclusion, and diffusion coefficients for complexes formed with the neutral and, in some cases, the oxidized states of the ferrocenyl conjugates. It was found that the ferrocenyl conjugate with one beta-cyclodextrin unit forms a redox-controllable head-to-head homodimer in aqueous solution. The ferrocene-bis(beta-cyclodextrin) conjugate is present in two distinguishable forms in aqueous solution, each one having a different half-wave oxidation potential for the oxidation of the ferrocene. By contrast, only one distinguishable form for the oxidized state of the ferrocene-beta-cyclodextrin conjugate is detectable. The redox-sensing abilities of the synthesized conjugates towards the bile salts were evaluated based on the observed guest-induced changes in both the half-wave potential and the current peak intensity of the electroactive moiety.

Electrospray: From ions in solution to ions in the gas phase, what we know now

There is an advantage for users of electrospray and nanospray mass spectrometry to have an understanding of the processes involved in the conversion of the ions present in the solution to ions in the gas phase. The following processes are considered: Creation of charge droplets at the capillary tip; Electrical potentials required and possibility of gas discharges; Evolution of charged droplets, due to solvent evaporation and Coulomb explosions, to very small droplets that are the precursors of the gas phase ions; Production of gas phase ions from these droplets via the Ion Evaporation and Charge residue models; Analytical uses of ESIMS of small ions, qualitative and quantitative analysis; Effects of the ESI mechanism on the analysis of proteins and protein complexes; Determination of stability constants of protein complexes; Role of additives such as ammonium acetate on the observed mass spectra.

Structures and Energetics of Erbium Chloride Complexes in Aqueous Solution

A series of aqueous, constant ionic strength solutions containing 0.5 m Er is studied as a function of chloride/perchlorate ion concentration using high-energy X-ray scattering (HEXS) to probe the metal ion coordination environment. Perchlorate is seen to form an outer-sphere complex only in the end member (4.0 m perchlorate) of the series. Chloride ions are seen to bind as both inner- and outer-sphere complexes. A quantitative analysis of the scattering data is used to determine stability constants that depend on whether the complexation with chloride ions is assumed to add to or to replace the waters bound in the first coordination sphere. Published stability constants obtained from liquid-liquid extraction clarify the mechanism involved and, together with the HEXS data, present a consistent picture of the chemistry. The stability constants determined from the HEXS data, beta(1) = 0.38(8) and beta(2) = 0.014(9), are small, less than the available thermal energy in solution at room temperature. The combination of chemical separations and HEXS data for the analysis supports the argument that the complexes formed are real under the experimental conditions. Details of the structural correlations and interactions are discussed for applications including separations and environmental modeling.

Kinetic and equilibrium studies of copper-dissolved organic matter complexation in water column of the stratified Krka River estuary (Croatia)

An interaction of dissolved natural organic matter (DNOM) with copper ions in the water column of the stratified Krka River estuary (Croatia) was studied. The experimental methodology was based on the differential pulse anodic stripping voltammetric (DPASV) determination of labile copper species by titrating the sample using increments of copper additions uniformly distributed on the logarithmic scale. A classical at-equilibrium approach (determination of copper complexing capacity, CuCC) and a kinetic approach (tracing of equilibrium reconstitution) of copper complexation were considered and compared. A model of discrete distribution of organic ligands forming inert copper complexes was applied. For both approaches, a home-written fitting program was used for the determination of apparent stability constants ( K i equ), total ligands concentration ( L iT ) and association/dissociation rate constants ( k i 1, k i - 1 ). A non-conservative behaviour of dissolved organic matter (DOC) and total copper concentration in a water column was registered. An enhanced biological activity at the freshwater–seawater interface (FSI) triggered an increase of total copper concentration and total ligand concentration in this water layer. The copper complexation in fresh water of Krka River was characterised by one type of binding ligands, while in most of the estuarine and marine samples two classes of ligands were identified. The distribution of apparent stability constants (log K 1 equ : 11.2–13.0, log K 2 equ :8.8–10.0) showed increasing trend towards higher salinities, indicating stronger copper complexation by autochthonous seawater organic matter. Copper complexation parameters (ligand concentrations and apparent stability constants) obtained by at-equilibrium model are in very good accordance with those of kinetic model. Calculated association rate constants ( k 1 1:6.1–20 × 10 3 (M s) − 1 , k 2 1: 1.3–6.3 × 10 3 (M s) − 1 ) indicate that copper complexation by DNOM takes place relatively slowly. The time needed to achieve a new pseudo-equilibrium induced by an increase of copper concentration (which is common for Krka River estuary during summer period due to the nautical traffic), is estimated to be from 2 to 4 h. It is found that in such oligotrophic environment (dissolved organic carbon content under 83 µM C, i.e. 1 mg CL − 1 ) an increase of the total copper concentration above 12 nM could enhance a free copper concentration exceeding the level considered as potentially toxic for microorganisms (10 pM).

Determination of Protonation Constants and Stability Constants of Lanthanides with Derivative of H4DOTA Complexes by the UV-VIS Spectrophotometry and Potentiometry

Methods of static potentiometry and UV-VIS spectrophotometry were applied for the determination of protonation constants of (10-[4-aminobenzyl(hydroxyl)phosphonylmethyl]-1,4,7,10-tetraazacyclo-1,4,7-triacetic acid) (DOTA(NH2)) and stability constants of holmium and yttrium complexes with DOTA(NH2). The DOTA(NH2) protonation constants of [HA] (log 1 = 12.7); [H2A] (log 2 = 21.4) and [H3A] (log 3 = 27.9) were experimentally determined by the static potentiometry. The stability constants of Ho-DOTA(NH2) (log KHo DOTA(NH2) = 18.0) and Y-DOTA(NH2) (log KY-DOTA(NH2) = 17.8) complexes were determined by UV-VIS spectrophotometry using the competitive reaction of the DOTA(NH2) – metal complexes with Arsenazo III.

Metal complexes stability constant determination by hyphenation of capillary electrophoresis with inductively coupled plasma mass spectrometry: The case of 1:1 metal-to-ligand stoichiometry

Nuclear energy development has raised new issues like radionuclides biogeochemistry. The modelling of their biochemical properties involves the accurate determination of thermodynamical data, like stability constants. This can be achieved by using hyphenated capillary electrophoresis (CE)–ICPMS and the method was applied successfully on 1:1 lanthanum–oxalate and uranyl–oxalate complexes. Several significant steps are discussed: choice of analytical conditions, electrophoretic mobility calculation, mathematical treatment of experimental data by using linear regressions, ligand concentration and ionic strength corrections. The following values were obtained with a good precision for lanthanum–oxalate and uranyl–oxalate complexes: log( K°(LaOxa +)) = 6.10 ± 0.10 and log( K°(UO 2Oxa)) = 6.40 ± 0.30, respectively, at infinite dilution. These values are consistent with the literature data, showing CE–ICPMS potential for metal complexes stability constants determination.

Bismuth Film Electrode in Metal Complexation Studies: Stripping Analysis of the Pb(II)‐, Cd(II)‐, and Zn(II)‐Binding with Phthalate

AbstractThe possibilities of bismuth film electrode for complexation studies by anodic stripping voltammetry (ASV) and constant current stripping chronopotentiometry (CCSCP) are tested for Pb(II), Cd(II), and Zn(II) in the presence of phthalate. In all cases, labile complexes are detected. The regular signals obtained for the Pb(II)‐phthalate system allow an accurate determination of stability constants by means of the DeFord‐Hume method. As for Cd(II)‐ions, the two overlapping signals usually present in the absence of ligands suffer a similar evolution with the addition of phthalate. Then, the use of an intermediate potential between both signals allows a reasonable estimation of stability constants. In contrast, the two overlapping signals obtained for Zn(II) behave in a quite irregular way in the presence of phthalate, thus hindering any quantitative analysis of the formed complexes.

Determination of stability constants of metal complexes with IC-ICP-MS

A method was recently developed to determine the conditional stability constant, Kf′, using ion chromatography (IC) hyphenated to inductively coupled plasma mass spectrometry (ICP-MS) (C. Huang and D. Beauchemin, J. Anal. At. Spectrom., 2006, 21, 317–320 and C. Huang, D. Beauchemin, J. Anal. At. Spectrom., 2006, 21, 1419–1422).1,2 It involves the separation of a metal complex from the free metal ion by ion exchange with on-line detection of eluting metal-containing species by ICP-MS. The relative amounts of the complexed and free metals were simply obtained from the area of the corresponding peaks in the time-resolved chromatograms. A plot of the logarithm of the peak area ratio of the complex over the free metal as a function of the logarithm of the analytical concentration of the ligand gave log Kf′ as the intercept and the chelation number, n, as the slope. Although this method was demonstrated to allow the accurate determination of the conditional stability constant of a simple system (EDTA complexes of Co and Zn),1,2 the corresponding chelation numbers were significantly off from the expected unity value. A close scrutiny of the method revealed the main source of the discrepancy: it assumed that the ligand was in excess, which was not the case. Furthermore, it neglected other complexes than that with EDTA4−, such as those with HEDTA3−, H2EDTA2−, etc. In this work, no asuumption is made and a linear regression of the logarithm of the peak area ratio of the complex over the free metal versus the logarithm of the concentration of the uncomplexed chelating reagent is shown to accurately give both log Kf′ as the intercept and the chelation number as the slope. This revised method was successfully applied to Co-EDTA and Zn-EDTA complex systems with 0.1 M NH4NO3 as the supporting electrolyte.

A bis(3-hydroxy-4-pyridinone)-EDTA derivative as a strong chelator for M3+ hard metal ions: complexation ability and selectivity

The study of chelating compounds is very important to solve problems related to human metal overload. 3-Hydroxy-3-pyridinones (HP), namely deferiprone, have been clinically used for chelating therapy of Fe and Al over the last decade. A multi-disciplinary search for alternative molecules led us to develop poly-(3-hydroxy-4-pyridinones) to increase metal chelation efficacy. We present herein a complexation study of a new bis-(3-hydroxy-4-pyridinone)-EDTA derivative with a set of M(3+) hard metal ions (M = Fe, Al, Ga), as well as Zn(2+), a biologically relevant metal ion. Thus a systematic aqueous solution equilibrium study was performed using potentiometric and spectroscopic techniques (UV-Vis, NMR methods). These set of results enables the establishment of specific models as well as the determination of thermodynamic stability constants and coordination modes of the metal complexes. The results indicate that this ligand has a higher affinity for chelating to these hard metal ions than deferiprone, and that the coordination occurs mostly through the HP moieties. Furthermore, it was also found that this ligand has a higher selectivity for chelating to M(3+) hard metal ions (M = Fe, Al, Ga) than Zn(2+).

Determination of stability constants of metal(II)-methylcysteine and metal(II)-nitrilotriacetate-methylcysteine complexes with using a paper electrophoretic method

A method involving the use of paper ionophoresis is described for the study of the equilibria in mixed ligand complex systems in solution. Present method is based on the movement of a spot of metal ion under the electric field with the complexants added in the background electrolyte at pH 8.5. The concentration of primary ligand nitrilotriacetate (NTA) was kept constant while that of the secondary ligand methylcysteine was varied. The plot of log [methylcysteine] versus mobility were used to obtain information on the formation of mixed complexes and to calculate its stability constants. The binary equilibria metal(II)-methylcysteine and metal(II)-NTA have also been studied, since this is a prerequisite for the investigation of mixed complexes. The stability constants of the complexes Pb(II)-NTA-methylcysteine and UO2 (II)-NTA-methylcysteine were found to be 3.03 ± 0.05 and 3.42 ± 0.09 (log K values) at ionic strength 0.01 M and a temperature of 35°C.

Determination of stability constants of metal-methionine and metal-methionine-NTA (binary and mixed) complexes by ionophoretic technique

Electrophoretic technique has been used for the study of Fe III , Cu ΙΙ , Ni II , Co II -methionine binary and Fe III , Cu II , Ni II , Co II -methionine-NTA (nitrilotriacetic acid) ternary complexes. The stability constants of metal-methionine binary complexes are found to be 10 6.03 , 10 5.23 , 10 5.13 , 10 4.23 and the stability constants of metal-methionine-NTA ternary complexes have been found to be 10 6.35 , 10 6.04 , 10 5.87 , 10 5.80 for Fe III , Cu II , Ni II and Co II complexes respectively at μ = 0.1 M (HClO 4 ) and 25 °C.

Determination of stability constants between complexing agents and At(I) and At(III) species present at ultra-trace concentrations

A competition method is proposed to determine the complexation constants between At(I) and At(III) species and complexing agents. The method, tested with an inorganic ligand, thiocyanate ion (SCN −), and an organic macromolecule, thiacalix[4]arenetetrasulfonate (LH 4) is based on solid/liquid separation or liquid/liquid extraction. For the solid/liquid separation, the cationic exchanger Dowex 50X8 was used. The interaction of At(I) and At(III) with the cationic exchanger is specific but could not be described by the expected cation exchange process. Most probably, At(I, III) interacts with a “strong” site (in weak amount) to form a surface complex at the surface of the resin organic skeleton. For the liquid/liquid separation, chloroform, toluene and hexane were used. All solvents extract astatine species with distribution coefficients varying between 0.7 and 120. The extraction process was shown to be independent of aqueous phase characteristics (pH, ionic strength) and was explained by the solvation of astatine species by the organic solvent. The effect of the addition of the thiacalix[4]arenetetrasulfonate on the solid/liquid or liquid/liquid distribution coefficients could be well described by the formation of a 1:1 complex with stability constants of log β 1 = 4.5 ± 0.4 and 3.3 ± 0.3 for At(I) and At(III), respectively. For the thiocyanate ion, the data measured in the presence of the organic solvents could be explained by the formation of both 1:1 and 1:2 At:SCN complexes. In the case of the solid/liquid separation, data analysis was hampered by the probable formation of a ternary complex between At(I, III), SCN − and the functional groups of the resin. As for the calixarene, the interaction strength appeared slightly higher for At(I) (log β 2 = 5.9 ± 0.3 and log β 1 = 3.8 ± 0.2 for 1:2 and 1:1 complexed species, respectively) than for At(III) (log β 2 = 5.3 ± 0.2 and log β 1 = 2.8 ± 0.2 for 1:2 and 1:1 complexed species, respectively).

Ultrasonic method of determination of stability constants of charge transfer complexes of certain carbonyl compounds and diethylamine in n -hexane

The ultrasonic velocities (U), densities (ρ) and viscosities (η) were measured for solutions containing equimolar concentrations of diethylamine (donor), nine aldehydes and nine ketones (acceptors) in n-hexane at 303 K. Acoustical parameters such as adiabatic compressibility (β), free length (L f), viscous relaxation time (τ), and molecular interaction parameter (χU) have been computed. These values indicate the formation of charge transfer complexes between carbonyl compounds and amine. Formation constant (K) values of the complexes have been evaluated using the equation proposed by Kannappan. The constant values of free energy of activation (ΔG ) and relaxation time indicate the formation of similar charge transfer complexes in these systems. However, the variation in free energy of formation (ΔG°F) values suggests that their thermodynamic stability depends on the structure of donor and acceptor.

Modeling Copper(II) Complexation in a Peat Soil Based on Spectroscopic Structural Information

The speciation of Cu in soils and surface waters is largely influenced by complexation reactions with natural organic matter (NOM). In this study, ion selective electrode data for the binding of Cu2+ to a forest peat soil were collected as a function of equilibration time, pH (2.4–6.6), and total Cu(II) concentration (1–54g Cu kg−1 dry soil). As a first step, a one‐site Langmuir isotherm was successfully fitted to the Cu adsorption data for the complete concentration range at pH 4.6. In a second step, structural information extracted from extended x‐ray absorption fine structure (EXAFS) spectroscopy, showing that Cu(II) forms five‐membered rings with possible combinations of amine, carboxyl, and carbonyl functional groups in NOM, were used as input for chemical speciation calculations (using the chemical equilibrium model MINTEQA2). In agreement with the EXAFS results, a model consisting of one RNH2, forming monodentate complexes (Cu2+ + RNH2 ↔ RH2NCu2+; log stability constant KRH2NCu2+ = 9.2; −log acid dissociation constant [pKa] = 9.0 for RNH3+), and two adjacent RCOO− groups, forming bidentate complexes (Cu2+ + 2RCOO− ↔ Cu(OOCR)2; log stability constant β(RCOO)2Cu = 4.7; pKa = 4.5 for RCOOH), gave the best fit to the experimental data. Determined stability constants for Cu(II)–amine and Cu(II)–carboxyl complexes were in good agreement with well‐defined Cu complexes with amino acids and carboxyls, respectively.

Encapsulation of methyl and ethyl salicylates by β-cyclodextrin: HPLC, UV–vis and molecular modeling studies

The complexation of methyl salicylate (MS) and ethyl salicylate (ES), non-steroidal analgesic, anti-inflammatory and antirrheumatic drugs with β-cyclodextrin (βCD) has been studied from thermodynamic and structural points of view. The complexation with βCD has been investigated using reversed-phase liquid chromatography. Retention behavior has been analyzed on a reverse-phase column Luna 18(2) 5 μm. The mobile-phase was methanol:water in different ratios (55:45 to 70:30) in which βCD (1–9 mM) was incorporated as a mobile-phase additive. The decrease in retention times with increasing concentrations of βCD enables the determination of the apparent stability constant of the complexes. Values at 30 °C with 55% methanol were K MS:βCD: 15.84 M −1 and K ES:βCD: 12.73 M −1 for MS and ES, respectively. The apparent stability constants decrease as the polarity of the solvent decreases. The low solubility of MS and ES in aqueous solution has been improved by complexation with βCD (1–9 mM). The stability constants of the complexes obtained from the phase-solubility diagrams using a UV–vis spectrophotometric method were Ḱ MS:βCD: 229 M −1 and Ḱ ES:βCD: 166 M −1. In addition, semi-empirical quantum mechanics calculations using AM1 and PM3 methods in vacuum were performed. The energetically favorable inclusion structures were identified and the most favorable orientation for the inclusion process was found to be the head-down orientation for both complexes. Enthalpy for encapsulation processes was found to be favorable (Δ H° < 0), while entropy (Δ S° < 0) and Gibbs free energy were unfavorable (Δ G° > 0). By means of HPLC and UV–vis measurements and quantum mechanics calculations, it was found that MS and ES form a 1:1 inclusion complex with βCD. The theoretical results are in agreement with the experimental parameters associated with the encapsulation process.

Sulphur-enriched thiacalix[4]arenes in the cone conformation: synthesis, crystal structures and cation binding properties

The synthesis and crystal structure of p-tert-butylthia- and p-tert-butylcalix[4]arene derivatives 1–7 substituted at the narrow rim by diethylthiophosphate ester groups, -PS(OEt)2 are reported. In these compounds the phosphorus atoms are directly attached to the phenolic O atoms of the calix units and the S atoms of the thiophosphate may serve as a two-electron donor to a metal ion. Their crystal structures were solved, which revealed their cone conformation. Their cation-binding properties have been established by liquid-liquid extraction of metal picrates from water into dichloromethane and stability constant determination in acetonitrile using UV-absorption spectrophotometry. Quantitative extraction was achieved for Ag+ (%E = 99) with the tetra-substituted thiacalix[4]arene derivative. The complexes were found to be of 1:1 stoichiometry and the location of the metal cations was shown to be nearby the sulphur atoms by 1H NMR.