Citrate System Research Articles

Effect of Low Molecular Weight Organic Anions on Adsorption of Aluminum by Variable Charge Soils

To examine the effect of organic anions on adsorption of Al by variable charge soils at different pH values, the adsorption by three soils in the presence of three low-molecular-weight aliphatic carboxylic acids was investigated. The results showed that the effect depended on pH, the type of organic anions and their concentration. The presence of citrate and oxalate led to an increase in the adsorption of Al at low pH and low concentration of organic anions, with citrate showing a stronger effect than oxalate. For example, the maximum increments of Al adsorption in the presence of citrate were 131.9, 104.8 and 32.9% in the Hyper-Rhodic Ferralsol, the Rhodic Ferralsol and the Ferric Acrisol, respectively, whereas in the presence of oxalate it was 36.1% in the Rhodic Ferralsol. At high pH or high concentration of organic anions, they showed an inhibiting effect on the adsorption of Al. For example, citrate caused the increase in Al adsorption by 164.0, 131.0 and 61.0% at pH3.85 and the decrease in Al adsorption by 15.2, 19.5 and 45.6% at pH 4.8 for the Hyper-Rhodic Ferralsol, the Rhodic Ferralsol and the Ferric Acrisol, respectively. In the citrate and oxalate systems, the adsorption of Al increased with the increase in the concentration of organic anions, reaching a maximum values at about 0.4 mmol L−1, and then decreased. When the concentration of organic anions was higher than about 1.0 mmol L−1, both citrate and oxalate inhibited the adsorption of Al. The ability of organic anions in increasing the adsorption at low pH and decreasing the adsorption at high pH followed the same order: citrate > oxalate > acetate. The increase of Al adsorption at low pH is caused by the increase in soil negative surface charge as a result of the adsorption of organic anions by variable charge soils, while the decrease of Al adsorption at high pH and high concentration of organic anions is related to the competition of organic ligands for aluminum ions with soil surface. After the removal of free iron oxides from the soil, Al adsorption decreased in the presence of citrate, the anion species most strongly adsorbed by variable charge soils and complexed with aluminum ions. For example, for the Rhodic Ferralsol and the Ferric Acrisol, the removal of free iron oxides caused a decrease in the adsorption of Al in the presence of citrate at pH4.4 by 26.2 and 21.9%, respectively.

Gibbsite and Goethite Solubility

2‐Ketogluconate (kG) is a microbial byproduct that has been isolated from the rhizosphere of several plants and has been found to accumulate around bacteria adhering to rock surfaces. It has long been suspected that kG may play a role in enhancing the bioavailability of soil nutrients. However, quantitative information relevant to its behavior in soils, its reactions with mineral surfaces, and its aqueous complexation of metal cations is unavailable. The objectives of this research were to examine the influence of kG on gibbsite and goethite solubility and to describe these effects by considering the aqueous complexation of Al3+ and Fe3+ by kG. Secondary to this was to compare the impact of kG on mineral solubility to that of citrate. The equilibrium solubility of gibbsite and goethite was examined at ambient (20–22°C) or controlled (25°C) temperatures as a function of pH, ionic strength, and kG or citrate concentration. Both ligands were observed to significantly enhance mineral solubility (as determined by the total soluble concentrations of Al and Fe), with kG and citrate having similar impacts on gibbsite solubility, but with kG having a lesser impact than citrate on goethite solubility. The solubility data were employed to characterize the aqueous complexation chemistry of Al– and Fe(III)–kG and citrate. In the 4 < pH < 8.5 range, the mononuclear species AlkG2+(aq), Al(OH)2kG0(aq), and Al(OH)3kG−(aq) describe Al–kG speciation, with the latter two species mechanistically described by the bidentate complexation of AlOH2+(aq) and Al(OH)+2aq [also written as AlOH(H−1kG)0(aq) and Al(OH)2(H−1kG)−(aq)]. Iron(III)–kG complexation is described by the FekG2+(aq) and Fe(OH)3 kG22−aq or FeOHH−1kG22−aq species. For the citrate systems, the chemical models required to describe metal speciation differed from those reported in the literature: Alcit0(aq), Al(OH)2cit2−(aq), and Al(OH)3 cit3−(aq) for Al–citrate speciation and Fecit0(aq), Fe(OH)2 cit2−(aq), and Fe(OH)3cit3−(aq) for Fe(III)–citrate.

PH-Specific Aqueous Synthetic Chemistry in the Binary Cadmium(II)−Citrate System. Gaining Insight into Cadmium(II)−Citrate Speciation with Relevance to Cadmium Toxicity

The involvement of Cd(II) in toxic manifestations and pathological aberrations in lower and higher organisms entails interactions with low and high molecular mass biological targets. To understand the relevant chemistry in aqueous media, we have launched pH-dependent synthetic efforts targeting Cd(II) with the physiological ligand citric acid. Reactions of Cd(II) with citric acid upon the addition of NaOH at pH 2.5 and pyridine at pH 3 and the addition of ammonia at pH approximately 7 led to the new complexes [Cd3(C6H5O7)2(H2O)5] x H2O (1) and (NH4)[Cd(C6H5O7)(H2O)] x H2O (2), respectively. Complexes 1 and 2 were characterized by elemental analysis, spectroscopy (FT-IR and NMR), and X-ray crystallography. Complex 1 crystallizes in the monoclinic space group P2(1)/n, with a = 18.035(6) A, b = 10.279(4) A, c = 12.565(4) A, beta = 109.02(1) degrees, V = 2202(2) A3, and Z = 4. Complex 2 crystallizes in the monoclinic space group P2(1), with a = 9.686(4) A, b = 8.484(4) A, c = 7.035(3) A, beta = 110.28(1) degrees, V = 542.3(4) A3, and Z = 2. Complex 1 is a trinuclear assembly with the citrate ligand securing a stable metallacyclic ring around one Cd(II), with the terminal carboxylates spanning into the coordination sphere of two nearby Cd(II) ions. Complex 2 contains mononuclear units of Cd(II) bound by citrate in an overall coordination number of 8. In both 1 and 2, the participating citrates exhibit three different modes of coordination, thus projecting a distinct yet variable aqueous structural chemistry of Cd(II) with physiological substrates. The pH-dependent chemistry and its apparent structural diversity validate past solution speciation studies, projecting the existence of mononuclear species such as the one in the anion of 2. The spectroscopic and structural properties of 2 emphasize the significance of the information emerging from synthetic studies that otherwise would not have been revealed through conventional solution studies, while concurrently shedding light onto the linkage of the requisite chemistry with the potential biological toxicity of Cd(II).

New Trends in the Chemistry of Iron(III) Citrate Complexes: Correlations between X‐ray Structures and Solution Species Probed by Electrospray Mass Spectrometry and Kinetics of Iron Uptake from Citrate by Iron Chelators

Despite the crucial role of "iron(III) citrate systems" in the iron metabolism of living organisms (bacteria as well as plants or mammals), the coordination chemistry of ferric citrate remains poorly defined. Variations in the experimental conditions used for the preparation of so-called ferric citrates (iron salt, Fe:cit molar ratio, base, pH, temperature, solvent) lead to several different species, which are in equilibrium in solution. To date, six different anionic complexes have been structurally characterized in the solid state, by ourselves or others. In the work described herein, we have established the experimental conditions leading to each of them. Five were obtained from aqueous solution. With the exception of a nonanuclear species (of which fragments have been detected), all were identified in aqueous solution on the basis of electrospray ionization mass spectrometry. In addition, the spectra revealed a new trinuclear species, which could not be crystallized. Kinetic studies of iron uptake from citrate species by iron chelators confirmed the results indicated by the ESI-MS studies. These studies also allowed the relative molar fraction of mononuclear versus polynuclear complexes to be determined, which depends on the Fe:cit molar ratio.

Thermodynamic and transport properties of aqueous trisodium citrate system at 298.15 K

In this work, we report density, viscosity, refractive index and water activity data for the aqueous trisodium citrate (C 6H 5Na 3O 7) system at 298.15 K. The parameters of the Pitzer ion-interaction model are evaluated from isopiestic water activity data. The apparent molar volume at infinite dilution was calculated from density data and a new value for the conventional partial molar volume of citrate ion (Cit 3−) was derived. The experimental viscosity data have been fitted to the Kumar equation. Using this equation, the hydration number ( h) and ion–solvent interaction parameter ( B) for trisodium citrate were obtained.

Spectroscopic studies of aqueous gallium(III) and aluminum(III) citrate complexes

Aqueous gallium(III) citrate complexes have been studied in the 10 −2 M concentration range with extended X-ray absorption fine structure (EXAFS) and FTIR techniques. From EXAFS data, one mononuclear and one oligomeric species were identified at different Ga(III) to citrate ratios. The first shell of the mononuclear complex was found to be distorted, with average Ga–O bond lengths of 1.95 and 2.06 Å, in agreement with the solid-state structure of Ga ( Cit ) 2 3 - (Cit = citrate). Also the oligomeric species was found to have a distorted first shell, with average Ga–O bond lengths of 1.95 and 2.04 Å. This complex was found to contain two Ga–Ga distances at 3.03 and 3.56 Å, typical for edge and corner sharing GaO 6 octahedra, respectively. The gallium(III) and aluminum(III) citrate systems were compared by means of FTIR, and were found to be analogous. The IR results suggest that the bond lengths derived from EXAFS for the 1:2 gallium(III) citrate complex also provide a good estimate of the corresponding distances in the mononuclear 1:1 complex. Direct coordination of citrate to the metal ions in the oligomeric gallium(III) citrate complex was indicated from both EXAFS and IR results, and this complex is stoichiometrically analogous to the Al 3(H −1Cit) 3(OH)(H 2O) 4− complex, which has been structurally determined. However, while the formation of the aluminum trimer has been shown to be slow, the gallium trimer was significantly more labile with a rate of formation indicated to be in the order of seconds or faster.

Effect of low-molecular-weight organic anions on exchangeable aluminum capacity of variable charge soils

Low-molecular-weight (LMW) organic acids exist widely in soils and have been implicated in many soil processes, such as nutrient availability, translocation of metals, fate of heavy metals, and mineral weathering. In this paper, the effect of the LMW organic anions on the exchangeable aluminum of two variable-charge soils was examined. The results showed that the organic anions induced an increase or a decrease in the exchangeable Al, and the extent and direction of the effect depended on the nature of organic anions, surface chemical properties of soils, and pH. For example, at pH 4.5, the quantity of exchangeable Al of Oxisol in the control system was 2.65 mmol kg −1, whereas the values in the citrate, oxalate, malonate, malate, tartarate, salicylate, and lactate systems increased by 3.25, 1.93, 1.95, 1.82, 1.28, 0.88, and 0.45 times, respectively. In contrast, the quantity of the exchangeable Al of Ultisol at pH 4.5 in the oxalate and the citrate systems decreased by 8.8 and 19.6%, respectively. The increase in the exchangeable Al was caused mainly by the increase in negative surface charge of the soils due to the specific adsorption of organic anions. The ability of organic anions at low concentrations to increase exchangeable Al for Oxisol followed the order citrate > oxalate and malonate > malate > tartarate > salicylate > maleate > lactate. This order is consistent with that of the effect of the adsorption of anions on the increase in the negative surface charge and/or the decrease in the positive surface charge of the soil. On the other hand, the organic anions could depress the exchangeable Al through the formation of soluble Al–organic anion complexes under certain conditions. The anions with small stability constants of Al–organic anion complexes, such as lactate, caused an increase in exchangeable Al with the change in surface charge of the soils, while those with large stability constants, such as citrate and oxalate, caused an increase in the exchangeable Al at low concentration and a decrease at high concentration.

Reductive Dechlorination of α-, β-, δ-, and γ-Hexachlorocyclohexane Isomers by Hydroxocobalamin in the Presence of Either Dithiothreitol or Titanium(III) Citrate as Reducing Agents

The effect of the reducing potential on the reductive dehalogenation of the different HCH (hexachlorocyclohexane) isomers has not yet been studied. In the present study, the potential for dehalogenation of (alpha-, beta-, delta-, and gamma-HCH isomers by the dithiothreitol (DTT) and titanium(III) citrate (reducing potential at pH 7, -0.33 and -0.48 V, respectively), with and without the addition of hydroxocobalamin was investigated. In the presence of DTT without catalyst, there was no disappearance of any of the HCH isomers studied after 1 h of treatment. However, disappearance of the gamma- and alpha-HCH isomers was observed during the same time period when titanium(III) citrate was used as the reductant in the absence of catalyst (62.9 and 16.6% disappearance, respectively). Addition of the hydroxocobalamin to the DTT system favored mainly the disappearance of gamma- and alpha-HCH (92.9 and 30.8% disappearance after 1 h, respectively); disappearance of delta-HCH and beta-HCH was small (11.9%) or negligible, respectively. Addition of the hydroxocobalamin to the titanium(III) citrate system favored the degradation of all HCH isomers under study: beta- and alpha-HCH completely disappeared to undetectable levels (<0.1%) after 1 and 2 min, respectively; degradation of delta-HCH and beta-HCH was slower than that of the other two isomers, although they had almost completely disappeared (99.9 and 99.6% disappearance, respectively) after 10 and 60 min, respectively. The order of disappearance, gamma-HCH > alpha-HCH > delta-HCH > beta-HCH, coincided with a decreasing order of the axially positioned Cl atoms of these isomers (considering their thermodynamically most stable configuration). This study is the first description of the rapid degradation of delta- and beta-HCH under abiotic conditions, and the results demonstrate the effect of the reducing potential on the reductive dehalogenation of HCH isomers.

Optical band gap tuning and study of strain in CdS thin films

In this work we have taken advantage of the control of strains at the CdS films–substrate interfaces to obtain films with tunable energy band gap. We found that on decreasing the amount of strain at the layer–substrate interface, the energy band gap of the films also decreases. Thus, we could obtain CdS films with energy band gap between 2.26 and 2.5eV. The CdS films were deposited on glass slide substrates by the chemical bath deposition method using an ammonia-free cadmium–sodium citrate system. The amount of strain at the CdS–glass substrate interface was controlled by the amount of cadmium ions in the chemical deposition process. The higher the concentration of cadmium ions, the higher the amount of strain at the layer–substrate interfaces. The properties of the CdS films were determined by X-ray diffraction and optical transmission spectroscopy measurements.

Iron uptake by Escherichia coli cultured with antibodies from cows immunized with high-affinity ferric receptors.

The synergistic effects of immunoglobulin G (IgG) from cows vaccinated with ferric citrate receptor (FecA) and IgG from cows vaccinated with ferric enterobactin receptor (FepA) were measured in an in vitro iron uptake assay. Serum was isolated and pooled within treatment from five cows each vaccinated with FepA or FecA or not vaccinated. Immunoglobulin G was isolated by ammonium sulfate precipitation and protein G affinity chromatography. Six Escherichia coli isolates from bovine intramammary infections were cultured in an iron-depleted medium to induce high-affinity iron acquisition systems and, in iron-depleted conditions, to specifically induce the expression of FecA. The bacterial cells were mixed with either 3 or 6 mg/mL of purified IgG and 55Fe. The radioactivity of 55Fe taken up by the bacterial cells was measured by a liquid scintillation counter after 5-, 10-, and 15-min incubations at 37 degrees C. The combination of anti-FecA IgG and anti-FepA IgG reduced 55Fe uptake compared with either anti-FecA or anti-FepA alone. Iron uptake was reduced more by anti-FecA IgG than by anti-FepA IgG when the ferric citrate system was induced. Reduction of iron uptake did not differ between anti-FepA alone and anti-FecA alone when citrate was absent from the medium.

Chemically deposited CdS films in an ammonia-free cadmium–sodium citrate system

In this work, we report the properties of chemically deposited CdS thin films in a cadmium–sodium citrate system. This chemical bath deposition process does not employ ammonia. We deposited four series of films at different cadmium content in the chemical bath process and determined their properties. The obtained information can be very useful for the optimization of the deposition process in order to reduce the amount of toxic chemical waste, mainly Cd-containing waste. The structural and optical properties of the CdS films were determined from X-ray diffraction, optical transmission and reflection spectroscopy and scanning electron microscopy measurements. We found that the properties of the films are very sensitive to the amount of cadmium in the deposition process. The process allows the deposition of good quality CdS thin films using 1.12, 0.84 and 0.76 mg of cadmium per milliliter of reaction solution.

Identification of Al(III) species in a solution containing equimolar concentrations of Al(III) and citric acid using varying-temperature 27Al NMR spectrometry.

27Al NMR spectra of an equimolar (10 mM) Al(III)-citrate system were obtained at different temperatures. The linewidths of the signals decreased in the spectra recorded at elevated temperatures, which enabled us to unequivocally identify the resonance lines. From the spectrum recorded at 65 approximately C, a mononuclear Al(III)-citrate complex was identified at a solution pH of 3.0 in addition to trinuclear Al(III)-citrate complex, which dominated at pH 4.0.

Potentiometric and UV–Vis Spectroscopy Studies of Citrate with the Hexaquo Fe3 + and Cr3 + Metal Ions

We are reporting the interaction and behavior of citrate with the hexaqo Fe(NO3)3, and Cr(NO3)3 (Fe3 + and Cr3 + will be used as shorthand notation for the two metal ions) using ultraviolet and visible spectroscopy (UV–Vis). The identity of the proposed metal complexes was confirmed by using the potentiometric titration technique. With the Fe3 +/citric acid system, it appeared that the simple 1:1 complex and the bis‐complex predominate. From the UV–Vis we observed the presence of two isosbestic points, one at 320 nm above the physiological pH, and another at 350 nm in the acidic pH range of 2.10 to 3.73. The appearance of an isosbestic point at 350 nm in the acidic region is novel. These isosbestic points indicate the presence of major species at a certain pH range. In contrast to the Fe3 +/citrate system, the Cr3 +/citrate systems showed no isosbestic points. To our surprise, it appeared that the Cr3 +/citric acid system and the Cr3 +/sodium citrate system had different UV–Vis patterns. The perturbation in the 4A2g→4T1g electronic transition is dramatically different when comparing the two systems. It also appeared that there is a minor difference in the 4A2g→4T2g electronic transition between the two systems.

Partition behavior and partial purification of hexokinase in aqueous two-phase polyethylene glycol/citrate systems.

This study dealt with the partition behavior and partial purification of hexokinase (HK) from baker's yeast by liquid-liquid extraction using aqueous two-phase polyethylene glycol (PEG)/citrate systems. First, we investigated the effect of agitation type (vortex and 8 rpm rotation) on the stability of the system, and then the effects of sodium citrate concentration, PEG concentration, and molar mass of PEG on the partition coefficient of this enzyme by using a 25 factorial experimental design. The results of this factorial experiment showed the possibility of a partial purification of HK by using two extraction steps, since the enzyme preferentially migrated to the top phase and the total proteins (mainly contaminants) remained in the bottom phase. The purification factor (PurTOP) of the enzyme in the top phase was 1.87, and the partition coefficient of the total proteins (KProt) was 0.47.

Separation of vanadium isotopes by ion-exchange chromatography

Cation-exchange displacement chromatography of VO 2+ was carried out for studying vanadium isotope effects in carboxylate ligand-exchange systems. The heavier isotope 51V was enriched in the carboxylate complex solution. The isotope separation coefficients ϵ(= α−1) for 50V/ 51V were 2.2·10 −4 and 2.4·10 −4 for citrate and lactate systems at 298 K, respectively. These values are much larger than those obtained in a previous study on the malate system. The existence of binuclear complexes of VO 2+ may create the conditions for larger isotope fractionation. From the viewpoint of the process development of isotope separation, the heights equivalent to a theoretical plate of these processes were analyzed and found to be very small in each system due to the homogeneous, small and highly porous resin used. Citrate may be better than the other tested systems for the vanadium isotope separation.

Effect of Added Complexing Anions on Cation Exchange of Cu(II) and Zn(II) with a Strong-Acid Resin

Ion exchange is widely used for the recovery and separation of metals from process and waste streams in chemical process industries. The effect of added complexing anions such as EDTA, NTA, and citrate on the cation exchange of Cu 2+ and Zn 2+ with strong-acid Purolite NRW100 resin was studied at 298 K, with the metals in solution in excess over the complexing anions. Experiments were performed as a function of solution pH (1.0-6.0), concentration of metals (4.5-22.5 mol/m 3 ), and concentration ratio of complexing anions to metals (0-1). It was shown that the rate of metal exchange decreased when the complexing anions were present. The addition of EDTA and NTA decreased the amount of metal exchange throughout the entire pH range studied, but it was the case for the citrate system only at high pH (>4). In the absence of complexing anions, the Langmuir equation could be used to correlate the exchange isotherms at a given pH on a macroscopic basis. However, a proposed chemical model that takes into account possible solid-liquid exchange reactions and solution chemistry could be used to describe the metal exchange equilibria in the presence of complexing anions.

Serum citrate levels, haptoglobin haplotypes and transferrin receptor (CD71) in patients with HIV-1 infection.

The progression of HIV-1 infection towards its more advanced stages is accompanied by changes in iron metabolism and increased body iron stores. Given the ability of HIV to alter iron metabolism, we studied the principal (transferrin system) and alternative (citrate system) iron pathways in a group of 65 HIV-infected patients (symptomatic stage B1-B3) and in a group of 36 healthy seronegative individuals. We determined serum citrate levels, haptoglobin (Hp) haplotypes, expression of transferrin receptor (CD71) on cell lines infected with HIV-1 as well as iron markers including blood iron, transferrin and ferritin. Our data showed decreased serum citrate levels in the HIV-infected patients compared to controls (92.9 +/- 22.4 microM/l vs 126.2 +/- 29.2 microM/L; p < 0.01). In particular, the serum citrate levels negatively correlated with HIV-1 RNA copy number (mean: 2.53 +/- 1.88 x 10(5)/ml, r(s) = 0.70, p < 0.01) and positively correlated with CD4+ T-lymphocyte count (mean: 241 +/- 168/ml, r(s) = 0.64, p > 0.05). Accordingly, blood iron, transferrin and red cell concentrations were lower in HIV-infected patients compared to the controls, whereas serum ferritin levels were higher in HIV-infected patients. Moreover, the Hp haplotype distribution showed significant differences only in the group of HIV-infected patients (p = 0.02; chi2 test). Our results show that iron metabolism is altered in patients with HIV-1 infection. The alternative pathway (citrate system) is particularly affected, since when citrate levels are low, both aconitase activity and HIV-1 replication need iron.

Systematic synthesis, structural characterization, and reactivity studies of vanadium(V)–citrate anions [VO 2(C 6H 6O 7)] 2 2− , isolated from aqueous solutions in the presence of different cations

The aqueous chemistry of vanadium with physiologically relevant ligands constitutes a subject of burgeoning research, extending from bacterial metalloenzymic functions to human-health physiology. Vanadium, in the form of VCl 3 and V 2O 5, reacted expediently with citric acid, in a 1:2 molar ratio in water at pH∼4, and, in the presence of various cations, afforded crystalline materials bearing the general formula (Cat) 2[V 2O 4(C 6H 6O 7) 2]· nH 2O (A) (Cat +=Na +, NH 4 + , n=2; Me 4N +, K +, n=4). Exploration of the reactivity of A toward H 2O 2 yielded the peroxo-containing complexes (Cat) 2[V 2O 2(O 2) 2(C 6H 6O 7) 2]·2H 2O (B) (Cat +=K +, NH 4 + ). Both classes of compounds were characterized analytically and spectroscopically. The X-ray structures of complexes A and B emphasize the exceptional stability of the dimeric rhombic unit V V 2O 2, which is retained upon H 2O 2 reaction, and the preserved mode of coordination of the citrate ligand as a doubly deprotonated moiety. In these complexes, typical six and eight coordination numbers were observed for the Na + and K + counter-ions, respectively. The variety of synthetic approaches leading to A, along with the stepwise and direct assembly and isolation of peroxo-compounds (B), denotes the significance of reaction pathways and intermediates in vanadium(III–V)–citrate synthetic chemistry. Hence, a systematic investigation of reactivity modes in aqueous vanadium–citrate systems emerges as a crucial tool for the establishment of chemical interconnectivity among low MW complex species, potentially participating in the intricate biodistribution of that metal ion in biological fluids.

Uranium(VI) complexation with citric, humic and fulvic acids

The binding of uranium(VI) by Suwannee River humic and fulvic acids was studied at pH values of 4.0 and 5.0 in 0.10 M NaClO4using an ion-exchange technique. Few data sets currently exist for metal binding to different molecular weight fractions from the same source. The complexation of U(VI) by citric acid was also studied under the same experimental conditions in order to "calibrate" the experimental and modeling approaches. For the citric acid system, the experimental results were analyzed using Schubert´s ion-exchange method, which indicated the formation of only a 1:1 uranyl-citrate complex. Close agreement was found for the values of log β1,1(6.69±0.03 at I = 0.10) determined from nonlinear regression of data collected at pH values of 4.0 and 5.0. This value represents a more direct measurement of the binding constant for the 1:1 uranyl-citrate complex than do other existing literature values derived from experimental data requiring the simultaneous consideration of 1:1 and 2:2 species.Both humic and fulvic acids were demonstrated to strongly bind U(VI), with humic acid forming slightly stronger complexes and exhibiting greater pH dependence. Analyses of the data for the humic and fulvic acid systems using the Schubert´s equation previously applied to the citrate system result in an apparent nonintegral number of ligands binding the uranyl ion. Schubert´s method is only appropriate for interpreting mononuclear complexes with integral moles of binding ligands. Thus, a more elaborate binding model was required and the data were interpreted assuming either: (1) a mixture of 1:1 and 1:2 uranyl-ligand complexes or (2) a limited number of high affinity sites forming a 1:1 complex. While both of these modeling approaches are shown to provide excellent fits to the data, the second is deemed more appropriate given the large size of humic and fulvic acid molecules as well as previous results obtained with other metal cations, such as Cu(II).

Complexation of iron(III) and iron(II) by citrate. Implications for iron speciation in blood plasma

Estimates of the concentrations and identity of the predominant complexes of iron with the low-molecular-mass ligands in vivo are important to improve current understanding of the metabolism of this trace element. These estimates require a knowledge of the stability of the iron–citrate complexes. Previous studies on the equilibrium properties of the Fe(III)–citrate and Fe(II)–citrate are in disagreement. Accordingly, in this work, glass electrode potentiometric titrations have been used to re-determine the formation constants of both the Fe(III)– and Fe(II)–citrate systems at 25 °C in 1.00 M (Na)Cl and the reliability of these constants has been evaluated by comparing the measured and predicted redox potentials of the ternary Fe(III)–Fe(II)–citrate system. The formation constants obtained in this way were used in computer simulation models of the low-molecular-mass iron fraction in blood plasma. Redox equilibria of iron are thus included in large models of blood plasma for the first time. The results of these calculations show the predominance of Fe(II)–carbonate complexes and a significant amount of aquated Fe(II) in human blood plasma.