Conditional Stability Constant Determination of Metal-Fulvic Acid Complexes

Abstract

Conditional stability constants for metal complexes of a terrestrial fulvic acid were determined using an ion-exchange chromatography—atomic absorbance spectroscopy method. Employing the Scatchard model, conditional stability constants were determined for the metal (II) fulvic acid complexes of cadminum, copper, lead, nickel, manganese, and zinc. The order of metal binding by the fulvic acid was determined to be: Cu > Ni > Pb > Zn > Cd > Mn. Complexes of weakly bound metal ions were determined with an added metal ion concentration of 2 × 10^–5 M to 1 × 10^–4 M while complexes of strongly bound metal ions were determined with an added metal ion concentration of 1 × 10^–5 M to 8 × 10^–4 M. The fulvic acid concentration was kept constant at 4 × 10^–4 M. The effect of pH and ionic strength on the copper-fulvic acid complex also was investigated.     

Complexation Properties of Typical Soil and Peat Humic Acids with Copper(II) and Cadmium(II)

Abstract

Potentiometric titration with ion-selective electrodes was applied to determine characteristic parameters like the degree of complexation and the approximate molecular weight of humic acids from theoretical considerations and conditional stability constants for the complexes of Cu(II) and Cd(II) with humic acids obtained from sources such as garden soil, peat prepared by decomposing water hyacinth in soil and humified water hyacinth. Double-reciprocal and Scatchard plots were constructed to determine the conditional stability constants of the complexes formed. Cu(II) was found to have more affinity for the humic acids than Cd(II) and the stability of the metal complexes in aqueous medium was found to increase with increasing pH. The order of stability of the complexes was M-HA (soil)>M-HA (peat)>M-HA (humified water hyacinth), where M and HA represent metal and humic acids, respectively.     

A study of complexation thermodynamic of humic acid with cadmium (II) and zinc (II) by Schubert's ion-exchange method

The complexation of humic acid from Azraq Oasis with two heavy metal ions Cd(II) and Zn(II) was investigated at pH 4 and 5 under constant ionic strength of 0.1 and at different temperatures (25, 35, 45, 55 and 65 °C). This investigation was done by using Schubert's ion-exchange equilibrium method, and its modified version.The derived conditional stability constants (log β_n) for these two metal-humate complexes were determined; they formed 1:1 and 1:2 complexes. It was found that the conditional stability constants (log β_n) increased by increasing pH and temperatures for all metal-humate complexes. It was found that the conditional stability constant log β₁ for Cd-humate is bigger than Zn-humate at all the desired temperatures and at pH 4 and 5.The derived constants and their temperature dependences have been used to calculate the corresponding thermodynamic parameters ΔG, ΔH, and ΔS, the results indicate that the stability of these complexes derives from very favorable entropy.     

Effects of ionic strength and pH on the stability constants of the complex of Co(II) with soil fulvic acid

The stability constants for tracer concentrations of Co(II) complexes with the red earth fulvic acid were determined at pH 3.8–6.8 and ionic strength 0.0010–1.0 mol/l by using the cation exchange equilibrium method and the radiotracer⁶⁰Co. The effects of ionic strength and pH on the stability constants of 1∶1 Co(II) complexes were investigated, and it was found that the stability constants of complexes of humic substances do not vary with ionic strength and pH in a manner similar to that of simple complexes.     

Use of the cation exchange equilibrium method for the determination of stability constants of Co(II) with soil humic and fulvic acids

The stability constants for tracer concentrations of Co(II) complexes with both the red earth humic and fulvic acids were determined at pH 5.9 and ionic strength 0.010 mol/l by using theArdakani-Stevenson cation exchange equilibrium method and the radiotracer⁶⁰Co. It was found that the 1:1 complexes of Co(II) with the red earth humic and fulvic acids were formed and that the average values of logβ (stability constant) of humic and fulvic acid complexes were 5.76±0.19 and 4.42±0.03, respectively.     

Cadmium(II) binding by soil-derived fulvic acid measured by anodic stripping voltammetry

Titrations of aqueous solutions of soil fulvic acid (30, 45, and 60 mg l^-1) with cadmium ion solutions at pH 6 and 7 reveal unusual shapes in the stripping current (i_s) vs. total cadmium ion (C_cd²⁺) curves. The expected inflections occur in the titration curves at 8–16 μM. at pH 6 and 12–26 μM at pH 7. In addition, there is an initial rapid increase in i_s at very low C_cd²⁺. The initial rapid increase in current is attributed to labile cadmium—fulvic acid complexes that contribute to i_s by rapid dissociation. Subsequent addition of cadmium ion results in moderately labile complexes and i_s becomes partially kinetically controlled. The stripping current was corrected for kinetic current contributions from dissociation of complexes, and total ligand concentrations, conditional stability constants, and upper slopes were calculated from data well past the titration end-point. The use of upper slopes after kinetic current corrections as in situ calibration curves, allowed calculations of equilibrium cadmium concentrations. The data show that both kinetic current corrections and in situ calibration curves are necessary to avoid substantial errors in calculations of equilibrium cadmium concentrations from anodic stripping voltammetric experiments.     

COMPLEXES OF Al(III) WITH HYDROXYAROMATIC LIGANDS

Abstract

In order to assess the aluminium binding ability of humic and fulvic acids, important organic soil constituents, a pH-potentiometric study was made of the proton and aluminium(III) complexes of various bi-, tri- and tetradentate catechol and salicylic acid derivatives at 25°C and at an ionic strength of 0.20moldm^?3 (KC1). The stability data revealed that at low pH the salicylate function, and at high pH the catecholate function, is preferentially bound to the aluminium ion. In the intermediate pH range, mixed hydroxo complexes and other di/oligomeric species are also formed. With an increase of the number of available coordinating sites in the molecule, the tendency to oligomeric complex formation increases, while the tendency to metal ion hydrolysis decreases.     

Kinetic Analysis Applied to Iron in a Natural Water Model Containing Ions,Organic Complexes,Colloids, and Particles

Abstract

Kinetic analysis using complex formation reactions is applied to “mixtures” containing (variously), Fe³⁺, Fe²⁺, iron complexed to a fulvic acid, hydrous oxide colloids, and non-settleable particulate iron. Such mixtures can be directly resolved if the kinetics of complex formation reactions are pseudo first order and differences among rate constants are large enough. At pH = 4, it is found that fulvic acid causes substantial reduction of Fe³⁺ to Fe²⁺ and that it causes complete dissolution and depolymerization of colloidal and particulate iron at 1:1 ratio. Addition of one equivalent of phosphate causes precipitation of ferric phosphate even in the presence of fulvic acid. This system is very useful for modeling natural water because the kinetic technique allows convenient analysis of components of varying particle size. The present results are strongly indicative of the role of fulvic acid in mediating metal ion chemistry in natural water.     

Application of resonance Raman spectrometry to the determination of vitamin B12.

A gel filtration technique is described for the study of the complexation of dissolved metals by humic and fulvic acids. Measurements can be made under realistic conditions of pH and free metal ion concentrations, both of which can be maintained by the use of TRIS as a buffer. The procedure permits the determination not only of the overall stability constant, but of the binding stoichiometries and the intrinsic stability constants associated with the various types of metal binding sites. The procedure has been applied to the investigation of the interaction of fresh-water fulvic acids with Cu, Zn and Ni, and has provided evidence for the existence of two different types of binding sites in the fulvic acid molecule.     

Correlation Between Chromatographic Behaviour and Stability Constant of Metal Ions on Diethylenetriamine Thin Layer

Abstract

The stability constants of the complexes formed between diethylenetriamine and the metal ions -V, Th, As, Pb, Zr and Se have been determined by pH metric method and these values, together with the stability constants for the metal ions -Co, Zn, Cd, Hg, Cu, Ni and Ag already reported, have been utilised to show their correlation with chromatographic behaviour of these metal ions on silica gel G plates impregnated with diethylenetriamine.     

Determination of complexing capacities of ligands in natural waters and conditional stability constants of the copper complexes by means of manganese dioxide

A dispersion of manganese dioxide is used as a weak ion exchanger in the determination of the complexing capacities of natural waters for copper and in the estimation of the stability constants of the complexes formed, at the concentration and pH at which the ligands occur in natural waters. Experiments were done at 25°C and 0.01 M ionic strength. Stability constants varied from 10^7.8 to 10^8.8 at pH 7.6 for ligands in lakes and rivers. The constant for the copper—fulvic acid complex was estimated as 10^7.8 at pH 7.6. Only one complexing site of major importance for natural water environments was observed.     

The determination of the ionisation constants of polymaleic acid and the stability constants of its complexes with copper

The ionisation constants of unfractionated polymaleic acid, H₃A, a synthetic analogue of fulvic acid, and the stability constants of its copper complexes were determined by differential pulse polarography. The ionisation constants are: pK₁ 5.16, pK₂ 7.04 and pK₃ 9.82 at 20° C and ionic strength 0.12 M. The stability constant of the CuHA complex, which is the predominant species at pH 5–9, lies in the range log (CuHA) = 7.15–8.7; the mean value is 8.17.     

Citrate complexes of tin(IV) studied by liquid-liquid extraction with 5,7-dichloro-8-quinolinol

The tin(IV)/5,7-dichloro-8-quinolinol/chloroform extraction system is used to determine the composition and the stability constants of the aqueous complexes of tin(IV) with citric acid, in the pH range 2.0–3.5. The tin(IV):citric acid ratios found are 1:1 and 1:2, depending on the concentration of the acid. The conditional stability constants are (5.5 ± 0.4) × 10³ and (1.3 ± 0.3) × 10³ for the 1:1 and 1:2 complexes, respectively. This extraction system is convenient in studies of aqueous tin(IV) complexes that cannot be extracted into chloroform.     

Comparative study of copper(II) interactions with monomeric ligands and synthetic or natural organic materials from potentiometric data

Potentiometric measurements were used to characterize the complexing properties of (a) a mixture of five monomeric ligands, (b) a synthetic humic-like substance and (c) fulvic acids extracted from soils, with copper (II). In order to compare the binding strengths involved, the same mathematical treatments of the data were used for all measurements. Calculations of the total ligand concentration with the Gran function give underestimated values for the multiligand mixture because of the known presence in the mixture of functional groups with pK^H ; 11 which > cannot be titrated and are revealed only by complexation phenomena. The acid-base properties of the humic substances are better described with a continuous model than with the descrete Henderson-Hasselbach model, because of the inability to distinguish properly between equivalent and independent types of functional groups. In the presence of copper (II), the formation functions show that a mixture of different kinds of complexes with different stability constants is probably formed. Extra protons released during the titration of the various copper (II) systems with alkali are attributed to untitrated functional groups with extremely low protonation constants in the case of the multiligand mixture, but ambiguity remains with the humic substances because their chemical structure is not known. Comparison of binding strengths can be made in terms of global conditional stability constants, by taking into account the three parameters, pH, concentrations of metal and of ligand. It is shown that the mechanisms of complexation are different for monomers and for polymers.     

Use of ion exchange for the determination of stability constants of metal-humic substance complexes

Humic substances (HSs) occur throughout the ecosphere in soils, waters and underground systems. The strong complexation of HSs is of importance in the migration of radionuclies in geological media. Renewed interest in stability constants of complexes of radioelements and radionuclides with humic and fulvic acids has been generated by problems associated with the nuclide migration in the environment. Use of the ion exchange method for the determination of conditional stability constants of metal-HS complexes was examined and reviewed. The complexation of HS to metal ions cannot be described in rigorous mathematical terms because of the ill-defined nature of HSs in contrast with the complexation of single ligands. Furthermore, the advantages and disadvantages of Schubert's and, Ardakani-Stevenson's, curve fitting methods were discussed. The great stabilities of HS complexes to rare earths (Yb(III), Tb(III), Eu(III), Gd(III)), americium(III), cobalt(II), uranyl(VI) and thorium(IV) were revealed.     

Tungsten(VI)-gluconic acid complexes: Polarimetric and13C-N.m.r. Studies in an excess of tungsten(VI)

Summary A polarimetric study of the tungsten (VI)-gluconic acid system in an excess of metal reveals the formation of four stable complexes: two monomers with 1 : 2 and 2: 1 stoichiometries and two dimers of 2:2 composition. The pH ranges of these species, the amount of acid equivalents their formation requires, and their conditional stability constants have also been calculated.The probable coordination of the organic ligand to the metallic centre in solutions containing different metal : ligand ratios has been investigated by¹³C-n.m.r. spectroscopy. The results confirm the formation of different complexes depending on the reagent which is in excess. The behaviour of the system when the metal is in excess is related to that of other polyhydroxylic ligands such as mannitol and sorbitol.     

Copper speciation in aqueous solutions of fulvic acid and related molecular weight distributions

Potentiometric and fluorescence measurements of aqueous solutions of fulvic acid containing Cu(NO₃)₂ or Cu(ClO₄)₂, respectively, were carried out at 25?°C and pH 5.5 to determine naturally occurring Cu species. The fulvic acid used was isolated by XAD-8 from filtrated (0.3 μm) water of a peat bog in the Dachauer Moos near Munich. From the results an operational molecular weight of fulvic acid of about 750 g/ mol was estimated, which was confirmed by molecular weight distributions determined by high-performance size-exclusion chromatography (HPSEC), gel permeation chromatography (GPC) and time-of-flight secondary ion mass spectrometry (ToF-SIMS). Using this molecular weight and assuming that mainly 1:1 Cu-fulvic acid complexes are formed, a conditional stability constant of the Cu-fulvic acid complex of 10^5.9 could be calculated. These data are essential for the assessment of organic carrier-mediated migration of Cu as well as of the toxicological risk potential of Cu in aqueous environment and can be used as input parameters for geochemical modeling of the Cu species distribution in aqueous solutions.     

Stability constants for the complexes of transition-metal ions with fulvic and humic acids in sediments measured by gel filtration

The molecular-weight distributions of fulvic and humic acids in sediments and their complexes with metal ions (Cu(2+), Zn(2+), Mn(2+)) were investigated by gel filtration. In all cases, metal complexes were found in the fulvic and humic acids. In the complexes the metals were bound to the high molecular-weight fulvic and humic polymers. By use of gel filtration, stability constants for the complexes of copper, zinc and manganese with fulvic acids have been measured. Scatchard plots indicate the presence of two classes of binding site with stability constants of 2.3 x 10(7) and 1.4 x 10(6) for copper, 2.1 x 10(6) and 6.6 x 10(4) for zinc and 1.8 x 10(5) and 7.3 x 10(3) for manganese, respectively.     

Synthesis of Two N,N′-bis(N,N-dialkylamide) Derivatives of Diethylenetriaminepentaacetic Acid and the Stabilities of Their Complexes with Gd3+, Ca2+, Cu2+, and Zn2+

Two bis(N,N-dialkylamide) derivatives of DTPA [(carboxymethyl)iminobis (ethylenenitrilo) tetraacetic acid], DTPA-BDMA = the bis(N,N-dimethylamide) and DTPA-BDEA = the bis(N,N-diethylamide) were synthesized. Their protonation constants were determined by potentiometric titration in 0.10 M Me₄NNO₃ and by NMR pH titration at 25.0 ± 0.1 °C. Stability and selectivity constants were measured to evaluate the possibility of using the corresponding gadolinium(III) complexes for magnetic resonance imaging contrast agents. The stability constants of gadolinium(III), copper(II), zinc(II), and calcium(II) complexes with DTPA-BDMA and DTPA-BDEA were investigated quantitatively by potentiometry. The stability constant for gadolinium(III) complexes is larger than those for Ca(II), Zn(II), and Cu(II) complexes. The selectivity constants and modified selectivity constants of the amides for Gd³⁺ over endogenously available metal ions were calculated. Effectiveness of these two ligands in binding divalent and trivalent metal ions in biological media is assessed by comparing pM values at physiological pH 7.4. Spin-lattice relaxivity values R₁ for Gd(III) complexes were also determined. The observed relaxivity values were found to decrease with increasing pH in the acid range below pH 4 and relaxivity values became invariant with respect to pH changes over the range of 4–10. ¹⁷O NMR shifts showed that the [Dy(DTPA-BDMA)] and [Dy(DTPA-BDEA)] complexes had one inner-sphere water molecule. Water proton spin-lattice relaxation rates for the [Gd(DTPA-BDMA)] and [Gd(DTPA-BDEA)] complexes were also consistent with one inner-sphere gadolinium(III) coordination position.     

Determination of stability constants of a copper/citric acid complex by ion-exchange chromatography and atomic absorption spectrometry

The method is based on separation of free copper ion from the complex by ion-exchange chromatography and determination of the free copper ions by atomic absorption spectrometry. Parameters that influence the separation and determination of the free metal are evaluated. Calculation of the conditional stability constant and the number of binding sites per citric acid (L) molecule was based on a Scatchard plot. The conditional stability constants found for the CuL^2? complex were pK = 5.04 at pH 7.0 and pK = 4.76 at pH 6.2 and ionic strength 0.10 M. The number of binding sites per molecule was unity.