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The transfer of trace level concentrations of transition metal ions from aqueous solution to anion exchangers, when the anion of the supporting electrolyte serves as both the counterion of the exchanger and the metal ion complexing ligand, is a phenomenon of considerable interest. In such systems the only species extracted by the exchanger, present in gel or liquid form, is the fully coordinated one even when this species is absent from the solution phase. The other potentially transferable species, electroneutral and negatively charged, that are present in the aqueous medium do not appear in the exchanger phase. Examples of such behavior, available in studies of the distribution of trace level concentration of Zn 2+ ion between Dowex 1-x4 and LiCl, HCl and CsCl, are duplicated by the liquid anion exchanger, 10% by weight methyldioctyl amine hydrochloride in trichloroethylene. The fact that such duplication occurs has been attributed to the comparable hydrophobicity projected by their positively charged organic constituencies. This hydrophobicity limits their water uptake similarly and leads to the elevated Cl − ion concentration levels defined by the Donnan potential developed at equilibrium. At these elevated Cl − ion concentration levels, formation of the tetrahedrally coordinated ZnCl 4 2− complex is highly favored while the water coordinated presence of any ZnCl 2 ·2H 2 O and ZnCl 3 − ·H 2 O that may be transferred to the exchanger phase, leads to their complete rejection. The difference between the Zn(II) distribution patterns obtained with LiCl, HCl and CsCl is also examined.

Gibbs−Donnan-based logic, employed to examine counterion equilibria in charged polymer suspensions, has led to assessment of their water sorption properties. With such information made available, a...

Various approaches to the interpretation of ion-exchange equilibria have been examined. In these approaches, units of concentration, standard states, reference states, and reference functions employed in the two phases either differ or are the same. In the first instance, useful composition dependent corrections for nonideality are necessarily empirical and do not provide a molecular level understanding of arrangements and interactions between the exchanging ions. The second path, accessible through the employment of Gibbs-Donnan-based logic, provides real insight, on the other hand.

A careful investigation of the effect of solution ionic strength on the ion-exchange behavior of zeolites that was conducted by R. Harjula and co-workers demonstrated that mass-action-based formulations failed to anticipate the equilibria encountered once the ionic strength of the solution in contact with the zeolite was reduced to ≤1.0 x 10 -3 M. The results of this research have been analyzed employing Gibbs-Donnan-based concepts in an attempt to understand better the nature of this phenomenon. Complications introduced by zeolite hydrolysis reactions became amenable to analysis through application of the Donnan model. Whereas this led to a more knowledgeable assessment of the hydrolysis phenomenon than the earlier empirical treatment by Harijula and co-workers, failure to anticipate ion-exchange behavior at low ionic strengths still persisted. Reasonable rationalization of this failure, unavailable to the mass-action-based formulation used earlier by Harijula and co-workers, was, however, provided by the Gibbs-Donnan model.

Application of the unified physicochemical approach to the description of the dissociation properties of Laurentide fulvic acid, extracted from peat, has shown that five predominant acidic moieties may account for the observed potentiometric titrations in aqueous medium. Information from non-aqueous titrations and titrations in presence of excess metal ions, e.g. Cu(2+) and Eu(3+), has shown that three of the acidic moieties are carboxylic in nature while the other two are of alcoholic nature. A comprehensive description of the dissociation properties of natural organic acids, as ligands, is advocated as a pre-requisite step towards understanding their interaction with metal ions in the environment.

The dissociation and metal ion binding properties of a soil fulvic acid have been characterized. Information thus gained was used to compensate for salt and site heterogeneity effects in metal ion complexation by the fulvic acid. An earlier computational scheme has been modified by incorporating an additional step which improves the accuracy of metal ion speciation estimates. An algorithm is employed for the prediction of metal ion binding by organic acid constituents of natural waters (once the organic acid is characterized in terms of functional group identity and abundance). The approach discussed here, currently used with a spreadsheet program on a personal computer, is conceptually envisaged to be compatible with computer programs available for ion binding by inorganic ligands in natural waters.

Explication quantitative de la forte sensibilite des proprietes potentiometriques des polyelectrolytes acides faibles a la force ionique du milieu

Potentiometric studies of the neutralization of several fulvic acid sources with standard base in aqueous and nonaqueous media have been conducted. Analysis of the results with a recently developed unified physicochemical model has shown that the protonation behavior of these fulvic acid sources is a reflection of their polyelectrolyte nature and their heterogeneity. It has been possible to ascribe the polyelectrolyte properties observed to a rather inflexible fulvic acid molecule whose variably charged surface is impermeable to simple electrolyte. 42 references, 24 figures, 4 tables.

The sensitivity of the acid dissociation equilibria of alginic acid and chondroitin sulfate to changes in the counterion concentration levels of their aqueous solutions has, on the basis of earlier detailed studies of linear weak-acid polyelectrolytes and their crosslinked gel analogs, been attributed to the development of a counterion-concentrating solvent sheath circumscribing the surface of each charged molecule. A Gibbs-Donnan based approach to the use of this model has, as in the earlier studies, facilitated a meaningful and useful interpretation of this sensitivity. Estimates of solvent uptake by these hypothetical, counterion-concentrating regions of the alginic acid and the chondroitin sulfate, during their neutralization with standard base, have been resolved through use of this approach and yield insight with respect to their conformational properties.

Gibbs-Donnan-based logic is applied for anticipation of the acid dissociation response of poly(acrylic acid) to changes in salt and polyelectrolyte concentration levels during its neutralization with standard base. The experimental data used for this purpose are those compiled by Nagasawa and co-workers in the early 1960s. To facilitate this investigation, a single neutralization experiment is selected to provide the parameters needed for anticipation of the results in the other experiments. Agreement between the experimental and model-based estimates is sufficient to provide strong support for use of the Gibbs-Donnan approach to describe such equilibria

A unified analytical treatment of metal complexation equilibria in weak-acid linear polyelectrolyte and gel polyelectrolyte systems is presented. Complexation equilibria of monovalent (Ag+) and divalent (Ca2+ and Zn2+) ions with carboxymethyldextran, CmDx, and the gel analogues, CM-Sephadex C-25 and C-50 (Pharmacia, Sweden), have been investigated in sodium salt media at 25°C. It was revealed that the formation of monodentate ligand complexes, AgA0, CaA+, and ZnA+ (A−= carboxymethyl group) is predominant. The stability constants of the complexed species, AgA0, CaA+, and ZnA+ have been determined to be 2.7, 3.8, and 4.8 mol−1dm3 , respectively, in good agreement with the stability constants of the respective acetate complexes, the monomer analogues of the CmDx and the cross-linked CmDx complexes.

The molecular weight and aggregation tendency of a reference-soil fulvic acid in Armadale horizon Bh were determined by vapor-pressure osmometry using tetrahydrofuran and water as solvents. With tetrahydrofuran, number-average molecular weight values of 767 ± 34 and 699 ± 8 daltons were obtained from two separate sets of measurements. Two sets of measurements with water also yielded values within this range (754 ± 70 daltons) provided that the fulvic acid concentration in water did not exceed 7 mg ml−1; at higher concentrations (9.1–13.7 mg ml−1) a number-average molecular weight of 956 ± 25 daltons was resolved, providing evidence of molecular aggregation. Extension of these studies to 80% neutralized fulvic acid showed that a sizeable fraction of the sodium counter ion is not osmotically active.

An ultrafiltration method was employed to study Fe(II)/Fe(III) and Cu(II) binding by Armadale Horizon fulvic acid. The results of the Fe—fulvate studies indicate the formation of “aggregates” between Fe and the fulvic acid molecule. The reduction of Fe(III) to Fe(II) by fulvic acid is also observed. Results for Cu(II)—fulvate electrode method. An algorithm developed previously for the prediction of metal ion binding by fulvic acid at different pH and salt concentration levels was tested further by comparing Cu 2+ ion binding predictions with their measured values.

The Oak Ridge National Laboratory was established during the Second World War. Its primary objective, as a part of the Manhattan Project in 1943, was the development, as rapidly as possible, of a feasible process for the production of the fissionable nuclides, Pu240 and U235 at a purity level and rate sufficient for the assembly of the nuclear bomb which was under development concurrently at the laboratory complex established in the University of California Project at Los Alamos, New Mexico (1,2) as another part of the Manhattan Project. In the Oak Ridge, Tennessee complex the Clinton Laboratories, identified as X-10 at the time (1943), were operated by the University of Chicago. It was involved with the development of a chemical process for the separation of slow-neutron fissionable Pu240, produced in the graphite-moderated nuclear reactor built for this purpose, from the components of the fuel elements and the radioactive products formed simultaneously. Research programs to develop a technique for the separation of the slow-neutron fissionable U235 isotope from its much more abundant naturally occurring isotope, U238, were carried out in two other installations in Oak Ridge. The first of these, known as Y-12, was operated by Tennessee Eastman and examined electromagnetic techniques for their separation. Union Carbide was in charge of operations at K-25, the second installation used to examine the feasibility of uranium isotope separation by a gaseous diffusion process. In this process advantage was taken of mass-based differences in the diffusion of U235F6 and U238F6 through porous barriers.

The development and refinement of a two-phase model for describing the potentionmetric properties of crosslinked weak acid polyelectrolyte gels and their linear analogues are discussed. Application of this approach has been facilitated by identifying functional group heterogeneity and separate phase effects as the major complications in the description of ion binding by humic substances. By their separation and quantification, equilibrium properties of these systems have been anticipated. This approach may be described as a "discrete ligand" model with solution chemistry effects taken into consideration. Examples of its applications are enumerated and an attempt is made to compare the approach with others.

The interaction of Cu(II) ion with a humic acid gel was carefully examined as a function of ionic strength and degree of dissociation to test the utility and validity of the fundamental approach developed by the authors for the unambiguous analysis of the protolytic and metal-ion complexation properties of such organic acid substances. Several of the equations developed earlier have been useful for this purpose. Two complexed species, CuS2, where S is believed to be an oxygen atom accessible from ether, aldehyde, ketone, or phenolic groups contained in the three-dimensional array of aromatic rings and their side chains which determine the structure of humic acid, and (RCOOCu+)ν are believed to be the only Cu(II)-bound entities formed. The stability constant of the more strongly complexed CuS2 is approximately 1 × 104; the formation constant of (RCOOCU+)ν is ∼18, a value not too different from the formation constant that has been reported for this particular species when formed by the interaction of Cu(II) with mononuclear carboxylic acid molecules. The sizable variation in pK(HA)νapp of three orders of magnitude, attributed initially to the heterogeneity of humic acid samples, was found to be more likely a consequence of differences in the accessibility of the functional units (RCOOH)ν, (RCOO−)ν, and (RCOOCu+)ν in these condensed systems.

Ferric salts of EDTA, HEDTA, CDTA and NTA were examined using nuclear γ-ray resonance spectroscopy to determine the relative amounts of monomeric and dimeric anion present in the solid, solution, polyelectrolyte and ion exchanger phases as a function of the pH of the equilibrating solutions. In most cases, the dimeric anion became more prevalent as the pH of the equilibrating solution increased. Results were compared with data derived from potentiometric titrations and rationalized in terms of an electrostatically-based interpretation of polyelectrolyte and ion exchanger selectivities. Monomeric and chelated polymeric analogs of IMDA were also examined to compare directly the polyaminocarboxylate ligand in the chelated environment where dimerization is not possible to the free IMDA anion in equilibrating solution at various pH values.

Though numerous publications have appeared relating metal-ion binding to humic substance properties no unified physicochemical description of the equilibria encountered has been available to provide a unique analysis of these systems. A fundamentally sound approach for the achievement of this goal is developed here in an attempt to rectify this situation. In particular the physical chemical basis for the unambiguous analysis of the protolytic and metal-complexation behavior of humic acid gels during potentiometric investigation is detailed. Complication introduced into the analysis by heterogeneity encountered in humic substances is examined briefly as well.

Proprietes physicochimique de gels polymeriques en milieu aqueux, hydrophiles et hydrophobes

The potentiometric properties of the rigidly cross-linked Sephadex gel CM-25 have been studied. The gel was equilibrated in sodium perchlorate at fixed concentration levels (0.010 and 0.0010M) and with variable concentration levels of NaClO(4) (from 0.0010 to 0.10M). The volume of the gel was measured for every equilibrated sample simultaneously with the solution pH and pNa. The pK computed after correction for a Donnan potential term was found to be independent of alpha, the degree of neutralization, and ionic strength; its value of 3.35 +/-0.15 was in excellent agreement with the intrinsic pK measured earlier for the flexible CM-50 Sephadex gel and reported for its linear polyelectrolyte analogue, carboxymethyldextran. An interesting observation from these studies is the efficient screening of the charged surface by the counter-ions in the presence of a three-dimensional charged network.

Summary The development of a model for the prediction of ion-exchange selectivity has been described in some detail. Limitation of this model to moderately concentrated gels (> 0.5 m on a monomer basis) and/or simple salt-containing polyelectrolytes is specified to define the range of its utility. Its successful application in the defined operational range has been fully demonstrated. In addition the method has been successfully employed to facilitate the ultimate objective of this manuscript, the interpretation of data comparing the uptake by complexation of an ion such as Ni2+ with the uptake of the uncomplexed Na+ ion. The great quantity of such data that are reported in the literature has not been amenable to correct assessment. With the predictive quality of selectivity computations with the model presented it is now possible to resolve the complexation equilibria encountered in these studies from such data.

Since linear, weak-acid polyelectrolytes respond to neutral salt just like their crosslinked analogs, their equilibria have been presumed to be as amenable to analysis with the Gibbs-Donnan approach as the gels. To examine this possibility this approach has been used to estimate solvent uptake by carboxymethyldextran and poly(acrylic acid) in the course of their neutralization with standard base. These estimates have been compared with estimates obtained for the same parameter by using the ion-condensation model of Manning.

A zeolite ion exchange technique for measuring mean molal activity coefficient ratios of mixed electrolytes has been further developed. The work was confined to systems of the type[Formula: see text]The 4A Zeolite has been calibrated for this kind of system by fitting the function[Formula: see text]to a fifth degree polynomial, in the range 0.1 ≤ XNa ≤ 0.7.Selectivity coefficient measurements with calibrated zeolite have been used to calculate ratios of mean molal activity coefficients for mixed Na+–K+ salts of model compounds. The model compounds were: p-toluene sulfonate, polystyrene sulfonate (mean molecular weight 400 000), and polystyrene sulfonate (mean molecular weight 30 000 to 40 000). The agreement between these measurements of activity coefficient ratios, and those calculated by Myers and Boyd for Dowex resins, ranges from excellent to fair.

The distribution of the lighter lanthanides between Dowex I-X8 (carbonate form) and K2CO3 solutions has been studied as a function of carbonate concentration in order to elucidate their complexing behaviour in the presence of an inorganic ligand. Elution chromatography was employed using radioactive isotopes of Ce, Pr, Nd, Pm, Sm and Eu to facilitate the investigation. The distribution of tracer 35S in the form of 35SO42- between Dowex I-X8 and K2CO3 solutions was studied to determine the effect on the system of imbibed electrolyte and changing internal and external activity coefficients. Analysis of the data indicates imbibition and activity coefficient corrections are negligible over the concentration range covered, thus giving an experimental basis for the theoretical treatment of the data. Distribution coefficients were measured over the concentration range of 0.37 to 2.60 molal K2CO3. Over this region a logarithmic plot of the distribution coefficient versus K2CO3 molality results in straight lines for all the lanthanides that were studied, indicating the presence of stable complex anions in solution. These trivalent rare earths may be classified into two groups. Group one, consisting of Eu and Sm, is characterized by a slope of —3.0 and group two, consisting of Pr, Nd, and Pm, by a slope of —2.5. Analysis of these results indicates that the complexes of group one and two are of the form [M (OH) (CO3)4]6- and [M(CO3)4]5-, respectively.

The development of a model for the prediction of ion-exchange selectivity has been described in detail. Cation-exchange selectivity in aqueous media is shown to be determined by the difference in interaction of various cations with water. The difference in selectivity is resolvable through the analysis of osmotic coefficient data obtained for the pure ionic forms of the linear polyelectrolyte analogue of the cross-linked cation exchange resin. The utility of this approach has been tested and is shown to be applicable to a weakly dissociated cation-exchange resin as well as to the fully dissociated one. The insight gained from this study is believed to be useful to the analysis of mass transport phenomena in cation-exchange materials.

The arid-dissociation properties of three weakly acidic cation-exchange gels, CM-Sephadex C-50-120, Biogel CM-2 and CM-Biogel A, have been studied. Each of the gels was equilibrated in sodium polystyrene sulphonate (NaPSS) solutions at three different concentration levels (0.1, 0.01 and 0.001M). The volume of the gels was measured as a function of a (their degree of neutralization), and Cp (the NaPSS concentration); pH and pNa were also measured at each α-value. Intrinsic pK values of 3.25 and 4.55 have been found for CM-Sephadex C-50-120 and Biogel CM-2 respectively.

The complex-forming properties of a fulvic acid sample have been studied by an ion-exchange distribution method. The results have been analysed by a novel physicochemical approach which attempts to take into account complications introduced by the polyelectrolyte nature and functional heterogeneity of the fulvic acid molecule. These studies with trace level concentrations of cobalt, zinc and europium show that these metal ions are selectively complexed by the weakly acidic enol grouping of the fulvic acid molecule. The logarithms of the stability constants (β 1 ) for the cobalt, zinc and europium chelates are 6.5, 6.4 and 10.3 respectively.

The distribution of the lighter lanthanides between Dowex I-X8 (carbonate form) and K2CO3 solutions has been studied as a function of carbonate concentration in order to elucidate their complexing behaviour in the presence of an inorganic ligand. Elution chromatography was employed using radioactive isotopes of Ce, Pr, Nd, Pm, Sm and Eu to facilitate the investigation. The distribution of tracer 35S in the form of 35SO42- between Dowex I-X8 and K2CO3 solutions was studied to determine the effect on the system of imbibed electrolyte and changing internal and external activity coefficients. Analysis of the data indicates imbibition and activity coefficient corrections are negligible over the concentration range covered, thus giving an experimental basis for the theoretical treatment of the data. Distribution coefficients were measured over the concentration range of 0.37 to 2.60 molal K2CO3. Over this region a logarithmic plot of the distribution coefficient versus K2CO3 molality results in straight lines for all the lanthanides that were studied, indicating the presence of stable complex anions in solution. These trivalent rare earths may be classified into two groups. Group one, consisting of Eu and Sm, is characterized by a slope of —3.0 and group two, consisting of Pr, Nd, and Pm, by a slope of —2.5. Analysis of these results indicates that the complexes of group one and two are of the form [M (OH) (CO3)4]6- and [M(CO3)4]5-, respectively.