Acid-base properties of aquatic fulvic acid

The acid-base properties of aquatic fulvic acid, concentrated from the Göta River, were studied by means of potentiometri titrations. The forward and reverse titrations exhibited hysteresis, which decreased as the fulvic acid underwent repeated titration. A simple model based on six titratable groups, within the pH range 2.5–10.4, could explain the acid-base properties. Electrostatic polymeric effects and heterogeneity of the fulvic acid (i.e., a distribution of constants rather than a set of distinct constants) were not included in the model. It was found that the three most acidic groups were carboxylic.     

Speciation of uranyl ions in fulvic acid and humic acid: a DFT exploration

The speciation of uranyl ions in fulvic acid (FA) and humic acid (HA), based on models of larger sizes, is systematically studied using density functional theory (DFT). Four uranyl binding sites are suggested for FA and based on their energetics, the preferential binding sites are proposed. The computed binding sites include two chelating types, one through the carboxylate group and one via the hydroxo group. A systematic way to attain the possible structure for Stevenson's HA model is carried out using a combined molecular dynamics (MD) and quantum chemical approach. Calculated structures and energetics reveal many interesting features such as conformational flexibility of HA and binding of hydrophobic molecules in agreement with the experimental suggestions. Five potential binding sites are proposed for uranyl binding to HA and the calculated geometries correlate nicely with the experimental observations. Our binding energy calculations reveal that apart from uranyl binding at the carboxylate functional group, binding at other functional groups such as those involving quinone and hydroxo sites are also possible. Finally, based on our cluster calculations the strength of uranyl binding to HAs and FAs is largely influenced by neighbouring groups via hydrogen bonding interactions.     

The binding of strontium and europium by an aquatic fulvic acid--ion exchange distribution and ultrafiltration studies

The complexation of an aquatic fulvic acid, FA, with Sr(2+) and Eu(3+) was studied at 0.10 and O1.O1M NaClO(4) using trace levels of metal ([Sr(2+)] = 10(-9)M and [Eu(3+)] = 10(-11)M) and a constant FA concentration (0.12 g/l) by an ultrafiltration technique (UF) and an ion exchange distribution method (IEDS). The overall complex formation function, beta(OV) for the two metals was calculated and its dependence on pH, ionic strength and method was investigated. The absolute value of log beta(OV), the pH dependence and the influence of the ionic strength on the complexation differed depending on the metal ion and experimental technique employed. By considering the functional group heterogeneity of the FA molecule, it was possible to predict the most predominantly bound site (keto-enol) and resolve the complex formation function for this site and EU(3+) (IEDS: 9.43 +/- 0.29 l/eq at 0.10M and 10.58 +/- 0.72 l/eq at 0.01M; UF: 7.19 +/- 1.51 l/eq at 0.01M and 6.88 +/- 0.91 l/eq at 0.01M). The results are discussed in the light of possible intrinsic problems of the two experimental methods.     

Spectrophotometric determination of rosmarinic acid in plant cell cultures by complexation with Fe2+ ions

A spectrophotometric method for determining rosmarinic acid (α-O-caffeoyl-3,4-dihydroxyphenyllactic acid) in unpurified methanolic extracts from Lavandula cell cultures is reported. It is based on a complexation reaction of rosmarinic acid with Fe²⁺ to give a blue-dark product with λ_max=572 nm and ?₅₇₃=3.82×10³ l mol^?1 cm^?1. The stoichiometry of the reaction and the optimal conditions for colour development are checked. The sensitivity and accuracy of this spectrophotometric method are compared with UV-spectral and HPLC methods for determining rosmarinic acid in plant cell cultures; similar results are obtained.     

The effects of pH and ionic strength on fulvic acid uptake by chitosan hydrogel beads

The objective of this work is to investigate the effects of pH and ionic strength on the adsorption capacity for fulvic acid (FA) by chitosan hydrogel beads. The results indicated that the sorption amount increased with decreasing pH and increasing ionic strength concentration. The sorption isotherms were well described by using non-linear Langmuir, Freundlich and Redliche–Peterson equation. The adsorption kinetics of FA onto chitosan hydrogel beads could be described by pseudo-second-order rate model. The extent of FA removal in the presence of other ions decreases in the order Ca²⁺ > Mg²⁺ > Na⁺ ≈ K⁺ and Cl⁻ > NO₃⁻ > CO₃²⁻. FTIR along with XPS analyses revealed the amine groups on the beads were involved in the sorption of FA and the organic complex between the protonated amino groups and FA was formed after FA uptake. Sorption mechanisms including electrostatic interaction and surface complexation were found to be involved in the complex sorption of FA on the chitosan hydrogel beads.     

Isolation and seasonal effects on characteristics of fulvic acid isolated from an Australian floodplain river and billabong

Fulvic acids from an Australian floodplain river and billabong were isolated using DEAE and DAX-8 resins, and characterised with the use of size exclusion chromatography and solid-state CP-MAS (13)C NMR spectroscopy. Differences between the two resin isolates were evident. Fulvic acids isolated using DEAE-cellulose had higher apparent M(n) and M(w) values, while the DAX-8 resin showed a slight preference for aliphatic components. Fulvic acids from the river and billabong had the same functional groups present, however, the river fulvic acids had higher apparent M(n) (number average molecular weight) and M(w) values (weight average molecular weight), and were more polydisperse than the billabong fulvic acid. There were no significant changes in the characteristics of the fulvic acid isolated from the river at four sampling times: summer, autumn, winter and spring. In contrast, fulvic acids isolated from a billabong displayed seasonal variation in molecular weights. This work emphasizes the importance in ecological studies of isolation procedure for the operationally defined fulvic acids.     

Determination of metal ions by on-line complexation and ion-pair chromatography

An ion-pair chromatographic method utilizing on-line complexation and ion-pair formation in a post-column reactor was developed for the determination of copper, palladium, cobalt and iron in mixtures. The system features a reversed-phase column and a second eluent line feeding the ligand reagent, connected after the column via a T-piece, to a mixer and through that to a knitted tubing reactor. The ion-pair former was added to the eluent before the column and the ligand after it. The separation was studied using a binary eluent system containing cetyltrimethylammonium bromide (CTMABr) or tetradecyltrimethylammonium bromide (TDTMABr) in water-methanol (99:1, v/v) as ion-pair former and methanol. In addition, water-methanol (99:1, v/v) containing 1-nitroso-2-naphthol-6-sulphonate (126NNS) as ligand was added to the eluent, through the T-piece, after the column. Mixing of the two eluents took place in the mixer. Methanol was used both isocratically and in gradient addition. Selective UV-VIS detection of the metal-126NNS ion pairs was at wavelengths 230, 260, 310 and 400 nm and their identification was effected in wavelength range 190–600 nm. The metal complex formation in the aqueous methanol eluent evidently governed the retention of the ion pairs, while the selectivity of the method was provided by the different rates of reaction of the metal, the ligand and the ion-pair former in the mixer-reactor system. The detector response for copper, palladium, cobalt and iron was linear up to concentrations of 10 μM. In spiked water-methanol samples the detection limits for these metals ranged from 1·10⁻³ to 1 mg/l. When the on-line complexation and ion-pair formation method was tested with nickel, mercury and zinc, the results proved that these ion pairs were unstable. Because of the insufficient reproducibility of the absorption intensities of these metal ion pairs, their qualitative study could be performed only in the pH range 7–8. The method was successfully applied to real samples after removal of the organic material.     

Effect of fulvic acid on neodymium uptake by goethite

Experimental studies of the interaction of aqueous neodymium (Nd), Suwannee River fulvic acid (FA), and solid phase goethite were conducted. Results from blank systems (individual Nd and FA), binary systems (Nd-goethite, FA-goethite, and Nd-FA), and ternary systems (Nd-FA-goethite) at 0.1 mol/kg and 25 °C are reported.In the binary Nd-goethite system a classic sorption edge is observed, whereby virtually all Nd is removed from solution above the goethite point of zero charge (PZC). Similarly, the binary FA-goethite system exhibits strong FA sorption; However in this system near complete removal of FA from solution is observed below the goethite PZC. In the binary Nd-FA system, both aqueous Nd and FA feature a sharp decrease in concentration at ca. pH 9.Various experiments in the ternary system were conducted. For all concentrations, FA enhanced Nd sorption below the goethite PZC, attributed to the formation of a Type B ternary surface complex (mineral-ligand-metal ion). Notably, the 100 ppm FA ternary system showed anomalously high dissolved Nd in solution above the PZC (i.e., Nd sorption suppression) and a concomitant increase in goethite dissolution (∼9 ppm total Fe³⁺ observed above circa pH 9.5).Our results suggest that Nd-FA complexation plays a key role in Nd uptake by goethite, and that this process is largely governed by pH: Whereas at pHs below the goethite PZC, Nd-FA complexation facilitates Nd sorption, above the PZC, and particularly at elevated FA concentrations, the formation of aqueous Nd-FA complexes suppresses Nd removal. Moreover, under these conditions, goethite dissolution may also play a role in mitigating Nd uptake by goethite.     

Rare earth element complexation by fluoride ions in aqueous solution

Rare earth fluoride stability constants for Ce, Eu, Gd, Tb and Yb at 25°C have been determined by examining the influence of fluoride ions on the distribution of rare earths between tributyl phosphate (TBP) and 0.68M NaClO₄. Our results indicate that rare earth mono and difluoro complexation constants show a steady increase as a function of atomic number from La to Tb but remain relatively constant after Dy. This behavior is similar to that which has been observed for dicarboxylic acids. Stepwise stability constant ratios, K₂/K₁, obtained in our work (where K₁=[MF²⁺][M³⁺]^–1[F^–]^–1 and K₂=[MF ₂ ⁺ ]^–1[MF²⁺]^–1[F^–]^–1) indicated that, for all rare earths, K₂/K₁=0.09±0.03.     

Electrochemical determination of the anionic particle charge of aquatic fulvic acids

Summary The fulvic acid (FA) fractions of humic substances from river water and bog lakes and of humic-like substances from landfill leachates were isolated by the XAD-method and titrated by a cationic polyelectrolyte. The isoelectric point of net charge of the FA samples was calculated by the streaming current detection (SCD)-technique and a new parameter of charge density (aPC) was calculated from the titer volume. The aPC-values of the FA are low for humic like substances (0.22–1.76 eq/mg DOC) and high (2.41–5.60 eq/mg DOC) for humic substances. Polarographically determined Cu(II)-complexation capacities (CuCC) of humic-like substances are in good linear relationship to their aPC-values. Even though SCD-signals only supply qualitative information, a useful fingerprint of the FA sample is given by the shape of the titration curve.

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Elektrochemische Bestimmung der anionischen Partikelladung aquatischer Fulvinsäuren

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Dedicated to Prof. Dr. G. Tölg on the occasion of his 60th birthday     

Highly selective complexation of metal ions by the self-tuning tetraazacalixpyridine macrocycles

By means of UV-vis and ¹H NMR titrations and X-ray crystallography, complexation of tetramethylazacalix[4]pyridine L1 and tetramethylazacalix[2]arene[2]pyridine L2 with metal ions was studied. While no interaction was observed with alkali and alkaline earth metal ions, both ligands have been found to act as powerful and selective macrocyclic hosts to complex a number of transition and heavy metal ions. Due to the intrinsic nature of the bridging nitrogen atoms that can adopt different electronic configurations and form varied degrees of conjugations with their adjacent pyridine rings, tetramethylazacalix[4]pyridine L1 regulated its conformation and cavity structure to best fit the guest metal ion species, yielding a 1:1 square planar L1-Mⁿ⁺ complex with binding constants log K_1:1 ranging from 2.7(1) to 8.2(8).     

Gas-phase complexation of polyethers with halide ions

Gas-phase complexes of halide anions with a variety of crown ethers and acyclic analogs are formed by ion-molecule reactions in the chemical ionization source of a triple-quadrupole mass spectrometer. The ether complexes of iodide, bromide, and chloride dissociate on collisional activation by cleavage of the halide-ether electrostatic hydrogen bonds, resulting in the formation of bare halide anions. By contrast, the fluoride complexes dissociate by loss of HF, which may occur in conjunction or sequentially with losses of ethylene oxide units. This dissociation behavior is similar to that observed for collisionally activated dissociation of [M ? H]^? ions of the crown ethers and suggests that the fluoride ion is capable of promoting an intramolecular proton abstraction within the [M+F]^? complex. This type of dissociation chemistry is only observed for the fluoride ion complexes, and the fluoride ion is the most basic of all the halides. The kinetic method was used to establish orders of relative halide binding strengths, and the trends for the chloride and bromide affinities were 12-crown-4 < triethylene glycol dimethyl ether < 15-crown-5 < tetraethylene glycol dimethyl ether < 18-crown-6 < 21-crown-7 < tetraethylene glycol < pentaethylene glycol < 1,4,7,10,13-pentathiacyclopentadecane.     

Calculation of all equilibrium concentrations in a system of competing complexation

An iterative method and a computer program are presented for calculating equilibrium concentrations of all species in a multicomponent system of many competing complexation reactions. The initial data required are: pH, total concentration of each reactant, stability constant of each complex, and pKa values. Convergence of the iterations is proved. As an example a system of 7 reactants and 27 complexes is given. A second example for the use of this program, including so-called "ligand buffering", is also shown.     

Determination of copper(I) and copper(II) ions after complexation with bicinchoninic acid by CE

A facile, sensitive, and selective method was developed for the simultaneous separation and determination of copper(I) [Cu(+)] and copper(II) [Cu(2+)] ions using CE with direct UV detection. The copper ions were complexed with a 1.5 mM bicinchoninic acid disodium salt solution at pH 8.7 prior to analysis. Acetate buffer (2 mM) was used as the CE running buffer. Parameters affecting CE separation such as sample pH, applied voltage, concentration of complexing agent, nature of the buffer solution, and interferences by other metal ions, were evaluated. The LODs for Cu(+) and Cu(2+) were 3.0 and 2.5 microg/mL (S/N = 3), respectively. The developed method allows the simultaneous determination of Cu(+) and Cu(2+) in less than 5 min with RSDs of between 5.3 and 9.5% for migration time and between 3.4 and 9.7% for peak areas, respectively. At optimum conditions, the percentage recoveries of Cu(+) and Cu(2+) were found to be 99.4 and 99.5%.     

Spectrophotometric determination of rosmarinic acid in plant cell cultures by complexation with Fe2+ ions

A spectrophotometric method for determining rosmarinic acid (-O-caffeoyl-3,4-dihydroxyphenyllactic acid) in unpurified methanolic extracts from Lavandula cell cultures is reported. It is based on a complexation reaction of rosmarinic acid with Fe²⁺ to give a blue-dark product with _max=572 nm and ₅₇₃=3.82×10³ l mol^–1 cm^–1. The stoichiometry of the reaction and the optimal conditions for colour development are checked. The sensitivity and accuracy of this spectrophotometric method are compared with UV-spectral and HPLC methods for determining rosmarinic acid in plant cell cultures; similar results are obtained.     

The potentiometric study of maleic acid complexation with the alkaline-earth metal ions in aqueous solutions

The potentiometric titration was used to study the process of complex formation of maleic acid (H₂L) with the alkaline-earth metal ions at the ionic strength 0.1 and T = 298 K. The following logarithmic stability constants of maleic acid complexes were obtained: logβ_MgL = 1.79 ± 0.11, logβ_CaL = 1.91 ± 0.05, logβ_SrL = 1.89 ± 0.09, logβ_BaL = 1.94 ± 0.10.     

Purification of soluble fulvic acid low concentrations by a diafiltration technique

The purification of soluble fulvic acids (FA) based on a diafiltration technique is carried out on antarctic water and snow samples, characterised by low humic compound content (0.1-0.8 mg/l). Results are compared with those of FA purified with the dialysis technique that, together with ion-exchange resins, performs an often used purification process. With the adopted procedure it is possible to minimise the high content of inorganic salts deriving in part from the isolation method XAD-8 adopted, as shown by FTIR, elemental and thermogravimetric analysis.     

Cooperative effects in the complexation of anions and Lewis bases by a heteronuclear bifunctional Lewis acid

As shown by the complexation of electron-rich substrates to the heteronuclear bifunctional Lewis acid bis(mu-1,8-naphthalenediyl)(mu-chloride)methyltin-chlorogallium (1), the primary Lewis acidic site of this molecule is the triorganotin chloride moiety whose electron deficiency is enhanced through partial abstraction of its chloride ligand by the neighbouring gallium centre.     

Degradative analysis of aquatic fulvic acid: CuO oxidation versus pyrolysis after tetramethylammonium hydroxide treatments in air and helium atmospheres

The structural composition of Nordic aquatic reference fulvic acid was investigated using chemical and thermal degradation methods: alkaline CuO oxidation and analytical pyrolysis after tetramethylammonium hydroxide (TMAH) pretreatment. Off-line procedures of the TMAH treatments were carried out under both air and helium atmospheres, with the aim of clarifying the effect of oxygen. Irrespective of the fact that the chemical and thermal degradation methods gave qualitatively quite similar basic products (mainly phenols and phenolic acids together with aromatic and aliphatic carboxylic acids), they also revealed their unique selectivity and efficiency for releasing different kinds of structural constituents. The results verify the formation of additional carboxyl functionality in the CuO oxidation. However, some similar oxidative reactions also appeared to take place during the pretreatment procedures of strongly basic TMAH, especially under an air atmosphere. The use of inert and protective atmosphere during the TMAH pretreatment is therefore recommended for producing more relevant structural information about the complex composition of humic substances.