Analysis of Trace Metal Humic Acid Interactions Using Counterion Condensation Theory (original) (raw)
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Biophysical Chemistry, 2000
A model, developed within the framework of the counterion condensation theory of linear polyelectrolytes, is presented in this paper to describe the acid᎐base properties of linear polyelectrolytes, consisting of several types of functional ionizable groups. This formalism has been successfully applied to Fluka humic acid under salt-free Ž . conditions, as well as in the presence of supporting simple 1:1 salt KNO at three different concentrations. As part 3 of this approach, the charge density of the humic acid is obtained from the activity coefficient measurements of potassium counterions at different humic acid concentrations at a constant degree of dissociation of the polyelectrolyte. The humic acid average charge density was found to be 0.80" 0.05. Using the present model, we are able to satisfactorily describe the experimental data obtained from acid᎐base potentiometric titrations. Four main functional Ž . Ž 0 . groups making up the polymer are determined through their fractional abundances X and intrinsic p K p K i i values. The fractional abundances remained constant and independent of the ionic strength, indicating that the humic acid constitution does not depend on the concentration of excess salts. The p K 0 values show a small change i with ionic strength, which can be explained by the polyelectrolytic behavior of the solution. ᮊ
Electroanalysis, 2003
A set of titrations of different amounts of humic acid with cadmium are performed at pH 6.0 and fixed ionic strength (0.1 M KNO 3 ). Both reverse pulse polarograms, which minimize the electrodic adsorption effects on the limiting current, and potentiometric measurements with an ion selective electrode are recorded along the titrations. The free metal concentration is determined from the polarographic measurement by means of an iterative numerical method recently developed. The resulting binding curve is found to agree very well with the one obtained through the potentiometric data obtained along the same titration. After application of the Donnan model to correct for polyelectrolytic effects, different complexation isotherms have been used to fit the specific binding. Complexation parameters for the Cd/HA system obtained with a Langmuir-Freundlich and Gaussian isotherms are briefly discussed.
Voltammetric Analysis of Heterogeneity in Metal Ion Binding by Humics
Environmental Science & Technology, 2001
The complexation of Cd, Pb, and Cu by fulvic acids at a fixed pH and ionic strength is studied by means of different voltammetric techniques at any metal-to-ligand ratio. When using Reverse Pulse Polarography (RPP) the complex species are electrochemically labile and not subject to significant electrodic adsorption. RPP titrations of fulvic acid with metal ions are interpreted on the basis of a recently proposed analytical expression for limiting currents valid for fully labile heterogeneous complexation. The voltammetric data are transformed into the corresponding binding curve, i.e., the fraction of occupied sites vs free metal concentration. Finally, the competition between metal ions and protons in their interaction with the fulvic binding sites as well as the concomitant polyelectrolytic effects are analyzed in terms of the NICCA-Donnan model. The results show that voltammetric techniques can be applied to the studies of heterogeneous complex systems in a broad range of metal-to-ligand ratios.
Complexation of heavy metals by humic acids: analysis of voltammetric data by polyelectrolyte theory
Colloids and Surfaces A-physicochemical and Engineering Aspects, 2002
The complexation of heavy metals by humic acids has been studied by voltammetry for a number of different cases, including (a) different metal ions; (b) various concentrations of the supporting salt; (c) different total metal concentrations and (d) different charge densities of the humic acid. The voltammetric speciation data are compared with predictions of a theoretical model which, besides polyelectrolytic interactions, takes into account: (i) chemical binding of metal ions; (ii) ionic strength effects; (iii) entropic effects; and (iv) competitive interactions between counterions of different valences. All experimentally obtained speciation data agree well with theoretical predictions by considering a single value of the intrinsic free energy of binding, which appeared to be the same for the Cd and Zn metal ions. This means that under the presently applied experimental conditions the interactions between the humate ion and the metal ions is similar. It is furthermore shown that with the present experimental-theoretical procedure a very consistent and precise behavior of the stability constant of the complex is reached in the wide range of the physico-chemical solution variables reported in this work.
Biophysical Chemistry, 1995
Voltammetric speciation data for the potassium/zinc/polymethacrylate system, recently obtained for various charge densities of the polyelectrolyte (Díaz-Cruz et al., Anal. Chim. Acts, 264 (1992) 163) and for different concentrations of monovalent counterions (van den Hoop and van Leeuwen, Anal. Chim. Acts, 273 (1993) 275), are compared with theoretical predictions computed according to a new thermodynamic model developed by Paoletti et al. (Biophys. Chem., 41 (1991) 73) and recently extended by Benegas and Paoletti (in preparation). The model allows: (i) the simultaneous condensation of both monovalent and divalent counterions and (ii) can account for a certain specific affinity of the polyelectrolyte for one type of the counterion over the other. For various charge densities of the polyelectrolyte, experimentally obtained speciation data for the K/Zn/PhU system agree wel1 with theoretical predictions by considering an extra reduced molar affinity energy of -4RT for the Znz+ polyelectrolyte binding. The agreement between experimental and theoretical values for the distribution of Zn*+ ions over the free and bound state becomes less perfect for relatively high concentrations of monovalent counterions. chemical and/or physical-chemical farms in natura1 0301-4622/95/$09.50 0 1995 Elsevier Science B.V. Al1 rights reserved SSDI 0301-4622(94)00125-1
Potentiometric titrations have been performed for poly(acrylic acid) (PAA) and poly(methacrylic acid) (PMA) in solution in the presence of different metals (Ca, Mg, Zn, and Cu) by titrating with KOH without additional salt. For the Me/PAA system, experimentally obtained apparent dissociation constants (pK a) appear to decrease in the initial part of the titration curve, which was found to be most pronounced in case of Cu. This was also found for the Cu/PMA system. For all systems studied, an increase in pK a was observed at higher degrees of dissociation. Analytical expressions for the change in apparent pK a upon ionization have been derived on the basis of counterion condensation theory taking into account both electrostatic and chemical binding of counterions of different valences. Taking also into account the effects of the flexibility of the polymer, the agreement between calculated and experimentally obtained data is certainly satisfying for the larger part of the titration curve. The agreement is completely lost by considering only electrostatic interactions between metal ions and the polyanions. An increase in chemical binding as indicated by a more negative value of the intrinsic reduced free energy of binding (g b) is observed in the order Mg ≈ Ca < Zn < Cu for both polyelectrolytes. The present approach allows us to quantify the contributions of the individual physicochemical processes related to the overall distribution of counterions around charged macromolecules. Introduction Knowledge of association phenomena of metal ions with charged macromolecules is of importance for the understanding of their physicochemical behavior in environmental and biological systems. From an experimental point of view, information is usually obtained with respect to the oVerall distribution of counterions around the charged macromolecules without having the ability to distinguish between different fractions which might be attributed to various underlying interaction processes, like electrostatic and chemical binding. Recently, we have presented an extension of the counterion condensation theory, which takes into account a number of association phenomena of counterions around linear charged polyelectrolytes. 1 The contribution of the territorial and site-specific association processes to the overall distribution has been discussed and illustrated from the theory for a wide range of different chemical conditions, including polyelectrolyte charge density, concentrations of polymer and monovalent and divalent counterions, etc. The proton dissociation of weak polyacids in solutions with its own counterions and with added 1:1 simple salt is one of the physicochemical processes largely studied theoretically and used experimentally. 2-7 However, potentiometric analysis for the case of polyelectrolyte solutions containing a mixture of counterions of different valence has received little attention. 8-13 Ishikawa 9 and Zhang and Nilsson 11 have used the solution of the Poisson-Boltzmann equation to interpret their results, assuming only electrostatic interactions between the polyelec-trolyte and the low molecular ions in the solution. On the other hand, and on the basis of potentiometric experiments, chemical binding of Cu to polymethacrylate polyion was proposed as part
Talanta, 2005
Studies on the complexation of copper(II) by phenolic acids, as ligand models of humic substances were done by potentiometry. The acids under study were: 3,4-dihydroxyhydrocinnamic acid or hydrocaffeic acid (1), 3,4-dihydroxyphenylacetic acid (2) and 3,4-dihydroxybenzoic acid or protocatechuic acid (3). Acidity constants of the ligands and the formation constants of metal-ligand complexes were evaluated by computer programs. The carboxylic group of the phenolic acids has different pK a1 values, being the dissociation constants intrinsically related with the distance between the function and the aromatic nucleus. The results obtained allow concluding that acidity constants of the catechol moiety of the compounds are similar with pK a2 and pK a3 values between 9.47-9.41 and 11.55-11.70. The complexation properties of the three ligands towards copper(II) ion are quite similar, being the species found not different either in nature or stability. Although the model ligands have some structural differences no significant differences were found in their complexation properties towards copper(II). So, it can be postulated that complexation process is intrinsically related with the presence of a catechol group.
Agronomy
A common technique for the potentiometric analysis of complex protolytic systems is mathematical data processing. This approach allows for transforming the experimental titration curve into a function of the ionogenic group distribution of the titrated object by the values of the pK (−lgKa) acid dissociation. This distribution function is called the pK spectrum, and the approach itself is called pK spectroscopy. In this case, solutions are found not in the form of discrete dissociation constants but as a distribution function of the concentrations of ionogenic groups over a continuous spectrum of dissociation constants. The purpose of this work is to test the possibility of using the pK spectroscopy method for assessing the acid–base properties of humic substances, water-soluble fraction, and soils. The results of testing the pK spectroscopy method showed that this method is expressive and more informative than the traditional method of continuous potentiometric titration. The use o...
Polyelectrolytic aspects of the titration curve. The semi-flexible model
Biophysical Chemistry, 1991
Extended random coil; Polypeptide; Polyelectrolyte; Counterion condensation theory; Titration curve An analysis is given of the theoretical approach to the quantitative description of proton dissociation curves for weak polyacids. The basic model of the counterion condensation theory has been used, with the modification reported in the preceding study (S. Paoletti, A. Ceshro, C. Arce Samper and J.C. Benegas, Biophys. Chem. 34 (1989) 301). In this paper we demonstrate the effect of relaxing the hypothesis of a rigid conformation on the polyelectrolytic properties of weak polyacids. As an application of the present approach. a description is given of the titration curves for two weak polyacids, poly(oI.-glutamic acid) (PDLGA) and poly(L-aspartic acid) (PLAA).
Journal of Colloid and Interface Science, 1998
namic description remains an insurmountable task. Such is The chemical implications of the fitting parameters employed the case for humic substances. by two empirical models on the characterization of complexation Humic substances are a heterogeneous mixture of polyreactions by dissolved humic substances were evaluated on various acidic compounds that result from the degradation of plant polyelectrolyte systems. Potentiometric titrations of polyacrylic and animal residues. The individual components of such acid (PAA), and different mixtures of PAA and polyvinylsulfonic polydisperse mixtures may differ widely in functional group acid, were conducted at different ionic strengths. The Scatchard composition, molecular structure, and configuration (2, 3). Quasiparticle model (QM) and the Gaussian distribution model They undergo self-association or association reactions with (GM) were utilized to describe the experimental data. Both models other components of the mixture as a response to changes provided an adequate representation of the results. However, the QM required three different classes of functional groups (five fit-in the composition of the system (e.g., pH, ionic strength) ting parameters), whereas the GM required only one type of bind-(4-6). These factors greatly complicate efforts to charactering site (two fitting parameters) to achieve that goal. Based on ize the reactivity of their functional groups. the chemical nature of the polymers, it was concluded that the A vast number of chemical equilibrium models have been GM offered a more realistic description of the results. The effects proposed to describe such reactions (7-9). The effectiveof electrostatics were also evaluated. A successful description of the ness of each model is generally ascribed to its ability to experimental data suggested that such effects could be adequately reproduce different data sets. Such an approach requires an accounted for with the model employed. However, geometry optioptimization procedure where one or more fitting parameters mization calculations indicated that the fitting parameters held are varied until the experimental data are appropriately delittle relation to the physical reality of the systems, thus invalidatscribed. The use of fitting parameters is generally justified ing any assumptions based on the results of the fitting exercise.