Ion Exchange Selectivity Coefficients Research Articles

Ion Exchange Selectivity Coefficients of Cd(II) Ion on Ca Saturated Kaolinite and Bentonite

Ion Exchange Selectivity Coefficients of Cd(II) Ion on Ca Saturated Kaolinite and Bentonite

Ion Exchange Selectivity Coefficients of Cu(II) and Zn(II) Ions on kaolinite and bentonite

Ion Exchange Selectivity Coefficients of Cu(II) and Zn(II) Ions on kaolinite and bentonite

Study of ion exchange selectivity of organic ions by using 31P NMR spectroscopy

Ion exchange distribution of the phenylphosphoric acid has been studied on various types of chloride form strong base resins as a function of pH at 25 °C, at 0.1 M ionic strength. Equilibrium measurements were supplemented by the study of pH dependence of the 31P NMR spectra of the resin-phase phenylphosphate species. Equations were derived to describe the pH dependence of the overall distribution coefficient and the chemical shift of the resin-phase solute species. Experimental data were evaluated by using these model equations and the values of the individual distribution coefficients, ion exchange selectivity coefficients and the resin-phase 31P chemical shifts of the mono- and divalent ions have been calculated. Comparison of distribution data of the individual species corroborated the significance of the role of hydrophobic interaction in the selectivity of organic ion exchange processes. A well-defined correlation between the ion exchange selectivity and the resin-phase 31P NMR chemical shift data has been pointed out.

Optimisation of the separation of anions by ion chromatography–capillary electrophoresis using indirect UV detection

The separation of a complex mixture of inorganic and organic anions by ion chromatography–capillary electrophoresis using a cationic polymer added to the background electrolyte and indirect UV detection has been studied. The addition of unmodified polymer to an electrolyte suitable for indirect detection resulted in the appearance of a system peak due to the counter-anion on the polymer and while the position of the analytes relative to this system peak could be changed, this was found to be an unacceptable approach for mixtures of large numbers of analytes. Although conversion of the polymer to replace the counter-ion with the indirect UV detection probe ion simplified the system, this approach restricted the flexibility of the system because the probe and polymer concentration were necessarily linked. This limitation could be overcome by selecting the appropriate type of probe ion, with probes having a low ion-exchange selectivity coefficient providing greater retention of analytes than probes with a high ion-exchange selectivity coefficient. Three electrolyte systems with different probes (benzoate, chromate and phthalate) were modelled using a previously derived migration equation and this was used to optimise the electrolyte composition to enable the separation of a mixture of 24 inorganic and organic anions within 7 min. The electrolyte composition was then optimised for the analysis of anions in Bayer liquor with the final separation selectivity being substantially improved for selected key analytes.

On-capillary ion-exchange preconcentration of inorganic anions in open-tubular capillary electrochromatogrpahy with elution using transient-isotachophoretic gradient. 2. Characterization of the isotachophoretic gradient.

Diffuse transient-isotachophoretic boundaries can be used as an elution gradient of increasing eluotropic strength to elute inorganic anions that have been preconcentrated on an open-tubular ion-exchange stationary phase prior to electrophoretic separation. The generation and characteristics of these gradients for elution after preconcentration have been investigated. The gradients are generated by placing a low-mobility, weak ion-exchange competing anion in the capillary (weak electrolyte, WE), and a high-mobility, strong ion-exchange competing anion in the electrolyte vials (strong electrolyte, SE). Application of voltage establishes a diffuse boundary with the composition changing from the weak anion to the strong anion. Comparison of elution gradients generated with different electrolyte systems was accomplished by comparing the eluotropic strength (a function of eluent concentration, ion-exchange selectivity coefficient, and charge) and the shape of the profile as it changes from WE to SE. The ion-exchange selectivity coefficient of the SE competing anion is important in establishing a sharp change in elution strength. A large difference in mobility between the WE and SE competing anions gives an SE with a higher final eluotropic strength, but the slope of the gradient is shallower. This results in a reduction in the efficiency of analyte focusing. To ensure maximum focusing efficiency, the WE and SE electrolytes should be selected such that the SE has the highest possible eluotropic strength for a given concentration of WE. The SE competing anion should also have a sufficiently low electrophoretic mobility to ensure focusing for the maximum number of analytes, and the mobility difference between the WE and SE competing anions should be as small as possible.

Competitive substitution of hexammine cobalt(III) for Na+ and K+ ions in oriented DNA fibers.

Competition of the trivalent cation, Co(NH3)(3+)(6), with K+ and Na+ ions in binding to DNA was studied by equilibrating oriented DNA fibers with ethanol/water solutions (65 and 52% v/v EtOH), containing different combinations and concentrations of KCl and NaCl and constant concentration (0.8 mM) of Co(NH3)(6)Cl(3). The degree of Co(NH3)(3+)(6) binding to DNA does not depend significantly on the ethanol concentration or on the kind of univalent cation (Na+ or K+). The ion exchange selectivity coefficient of monovalent-trivalent ion competition, D(1)(c3), increases with the concentration of Me+, C(o)(+), and the monotonic dependence of log D(1)(c3) vs log C(o)(+) has an inflection between 100 and 300 mM that is caused by a structural transformation of DNA from A- to B-form. The ion exchange experimental data are compared with results of grand canonical Monte Carlo (GCMC) simulations of systems of parallel and hexagonally ordered, discretely charged polyions with density and spatial distribution of the charged groups modeling B- and A-forms of DNA. The GCMC method for discretely charged models of the DNA polyion produces a quantitative agreement with experimental data on trivalent-monovalent ion competition in dependence on DNA structural state and salt concentration. Based on this and previous studies it is concluded that the affinity of DNA for the cations decreases in the order Co(NH3)(3+)(6) >> Ca2+ > Mg2+ >> Na+ approximately K+ > Li+. DNA does not exhibit selectivity for Na+ or K+ in ethanol/water solutions either in the absence or in the presence of Co(NH3)(3+)(6), Ca2+, and Mg2+.

Indirect spectrophotometric detection of inorganic anions in ion-exchange capillary electrochromatography.

The application of indirect spectrophotometric detection was investigated for a capillary electrochromatographic system in which an anion-exchange stationary phase (in the form of aminated latex particles) was coated onto the wall of a fused-silica capillary. The study has focused on the choice of the type and concentration of the absorbing coion (probe) added to the background electrolyte and the role of this species in manipulating the ion-exchange contributions to the separation with a view to controlling the selectivity of the separation. Common inorganic anions were used as analytes and nitrate, p-toluenesulfonate, nicotinate, and chromate were investigated as probes. It was found that most of these probes produced only a limited range of separation selectivities when their concentration was varied over the practically accessible range. p-Toluenesulfonate provided the greatest variation in selectivity, but peak distortion due to electromigration dispersion was evident for the faster ions. When variation of the separation selectivity - from predominantly electrophoretic in nature to predominantly ion-exchange in nature - was desired, this was best achieved by varying the type of probe rather than its concentration. For example, the nitrate probe provided predominantly electrophoretic separations with good peak shapes and high efficiencies. A comprehensive list of probes, ranked in order of ion-exchange selectivity coefficients determined by ion chromatography, was compiled and this proved to be a useful tool to assist in the selection of a probe for a desired separation selectivity. The limits of detection for the analytes and probes studied ranged from 20-55 micromol for the chromate system to 230-600 micromol for the nicotinate system, with nitrate and p-toluenesulfonate giving intermediate values.

Analysis of the Kinetics of Hydrolysis of p-Nitrophenyl Alkanoates in Colloidal Polymer Dispersions by an Ion-Exchange Model

Rates of hydrolyses of p-nitrophenyl acetate, hexanoate, and octanoate in borate buffer solutions at 30 °C are 2.3−16.5 times faster in the presence of 1.2 mg mL-1 quaternary ammonium ion exchange latex particles than those obtained in water alone. The latexes were constructed by emulsion copolymerization of styrene, butyl methacrylate, or 2-ethylhexyl methacrylate with 25 wt % vinylbenzyl chloride (VBC), 1% divinylbenzene, and 1% styrylmethyl(trimethylammonium chloride) followed by quaternization of the VBC units with either trimethylamine or tributylamine. Analysis of the kinetics as a function of particle concentration, pH, and buffer concentration using an ion-exchange model provided partition coefficients of the p-nitrophenyl esters, intraparticle second-order rate constants, and ion-exchange selectivity coefficients. The major contributors to the enhanced rates are the partition coefficient favoring absorption of the p-nitrophenyl ester into the latex by a factor as large as 90 000 and intraparticle hydroxide concentrations up to 10 times higher than those obtained in the external water. The intraparticle second-order rate constants differ little from those in water.

Contaminant Transport at a Waste Residue Deposit: 2. Geochemical Transport Modeling

Contaminant transport in an aquifer at an incinerator waste residue deposit in Denmark is simulated. A two‐dimensional, geochemical transport code is developed for this purpose and tested by comparison to results from another code. The code is applied to a column experiment and to the field site. The ongoing geochemical processes and any unknown geochemical parameters are obtained through simulation of the column experiment that used soil and leachate from the field site. The application of the code to the field site, which has been monitored for more than 15 years, use this geochemical information along with the flow and nonreactive transport parameters obtained by the inverse modeling procedure described in the first paper [Sonnenborg et al., this issue] of this two‐paper series. The simulation results of the site model are compared with several measured component breakthroughs at monitoring wells. Contamination was first controlled by transport, and later by transport and ion exchange. In both the column and field site simulations the code is used to identify the controlling transport processes, physical or geochemical (ion exchange and mineral precipitation), and to estimate the involved parameters (primarily ion exchange selectivity coefficients).

Synthesis of some new cation-exchanger resins

Polystyrene–PEG crosslinked block copolymers were prepared from styrene copolymerization by either poly(ethylene glycol dimethymethacrylate) (PEG-DM) or macronomer initiators (MIM). Mw values of PEG of PEG-DM were 400, 600, 1000, 1500, 3000, 10,000, and 35,000, and of MIM, 400 and 1500. Swelling in H2O or CHCl3 of the sulfonated and unsulfonated block copolymers were determined under comparable conditions and found to be vary significantly. The ion-exchange capacity and selectivity coefficients of these ionexchange resins were investigated. The capacities of the obtained ion exchangers were varied between 0.4 and 2.9 meq/g. © 1996 John Wiley & Sons, Inc.

Ion-exchange voltammetry of tris(2,2′-bipyridyl) ruthenium(II), iron(II), osmium(II) and tris(2,2′-bipyrazyl) ruthenium(II) in acetonitrile solutions at poly(ester-sulphonate) coated electrodes

Glassy carbon electrodes modified with the poly(ester-sulphonate) ionomer Kodak AQ 55 D are used and characterized in acetonitrile solutions. The potential window accessible in acetonitrile + tetrabutylammonium hexafluorophosphate extends from −1.750 V to +1.900 V vs. an aqueous KCl saturated Ag/vbAgCl reference electrode. The electrochemical behaviour of [Ru(bpy) 3] 2+, [Fe(bpy) 3] 2+, [Os(bpy) 3] 2+ and [Ru(bpz) 3] 2+ (where bpy = 2,2′bipyridyl and bpz = 2,2′bipyrazyl) is studied at the modified electrode. All the electroactive cations are preconcentrated efficiently in the polymeric coating. In the case of bpy complexes both a one-electron oxidation as well as two one-electron steps of the stepwise reduction of the starting complex are detected at the modified electrode. In the case of [Ru(bpz) 3] 2+, three consecutive reduction steps can be observed, while no oxidation process is observed in the direct positive scan before reaching the upper limit of the potential range accessible at the modified electrode. The relative order in the ion-exchange selectivity coefficients in acetonitrile solutions for the cations studied is evaluated by comparison with the electrochemical behaviour observed at unmodified electrodes. The possibility of gaining information on preconcentration effects for ion-exchange voltammetry in non-aqueous solvents from the Gibbs energies of transfer from water to hydrophobic media is also discussed.

Potential shifts at electrodes coated with ion-exchange polymeric films

Voltammetry at electrodes modified with ion-exchange polymers, named “ion exchange voltammetry”, has been recently developed for characterizing and determining quantitatively ionic electroactive analytes preconcentrated at the electrode surface. Like for other voltammetric techniques, characterization is based on the position of the response on the potential scale, but an appreciable difference is frequently observed between the formal half-wave potential for redox couples incorporated within ion-exchange polymeric films and those for the same redox couples in solution as measured at bare electrodes. Such a difference has been rationalized here by a generalized equation, inferred from a suitable elaboration of the Nernst equation, whose validity has been tested by a thorough investigation performed at glassy carbon electrodes modified with either cationic (Nafion) or anionic (Tosflex) polymeric films. With this purpose, the effect of both charge and concentration of the analyte and of the loading counterion, this last introduced as the cation or anion of the supporting electrolyte, of the ion-exchange selectivity coefficients of the redox partners and of their stoichiometric coefficients, as well as of the number of electrons involved in the charge transfer has been evaluated. The results obtained agree quite well with theoretical expectations and indicate that the potential shifts found are mainly conditioned by both charge and concentration of the counterion from the supporting electrolyte and by the ratio of the ion-exchange equilibrium constants for the two redox partners involved. Other parameters considered have no influence on the potential shift or lead to negligible effects, provided that the quantities of the redox partners incorporated within the ion-exchange coating represents less than 5% of the film capacity. Again in agreement with theoretical expectations, positive shifts are found for increasing supporting electrolyte concentrations when cationic redox species incorporated within cationic films are involved, while the opposite effect is found for anionic redox species incorporated within anionic films.

Effect of the sample ionic strength on the preconcentration attained in ion exchange voltammetry

A solid state cell proposed elsewhere, enabling the extension of ion exchange voltammetry to trace determination of small ionic species with low ion exchange selectivity coefficients, dissolved in electrolyte-free media, has been adopted to verify whether the degrees of preconcentration achieved with such a voltammetric technique actually increase with the decrease of the sample ionic strength—in agreement with thermodynamic expectations—or whether they are otherwise conditioned by kinetic complications. With this purpose, a series of experiments has been conducted by accumulating either monovalent or divalent cationic analytes (Tl + and Pb 2+) at glassy carbon electrodes modified with Nafion® films loaded with either monovalent or divalent cations (Na + and Ca 2+) from aqueous solutions containing different concentrations of both the analyte and the loading ion introduced as the cation of the supporting electrolyte. These preconcentrated ions were then assayed by inserting the modified electrodes in the solid state cell mentioned. The results obtained indicate that the expected inverse proportional relation between the degree of preconcentration achieved and the ionic strength of the sample analyzed is found only at fairly high concentrations of supporting electrolyte, the values of which depend on both the charge and concentration of the analyte as well as on the charge of the counterion from the supporting electrolyte. In contrast, at lower ionic strengths, the degree of preconcentration is kinetically controlled. This finding is discussed in terms of interdiffusion effects as well as of the decrease in both diffusion coefficients in the polymer and equilibrium constants, owing to cross-linking changes involved in the ion exchange processes. Also the outcome of such kinetic control on analytical determinations of trace ionic species in poorly conducting samples is considered.

Application of the Högfeldt three-parameter model to ion-exchange data in concentrated media

The Högfeldt three-parameter model is applied to ion-exchange data in concentrated media. The dependence of the parameters on the water activity of the solution is shown and the possibility of using the model to predict ion-exchange selectivity coefficients is investigated.

Solid-state cell for the voltammetric determination of trace electroactive ionic species preconcentrated from high-resistive media at electrodes modified by ion-exchange coatings

A solid-state cell is described that allows the extension of ion-exchange voltammetry to the trace determination of small ionic species characterized by low ion-exchange selectivity coefficients, provided that they are dissolved in electrolyte-free media. It consists of a wetted ion-exchange membrane (Nafion), one face of which contacts both the counter and reference electrode, while the other surface is contacted by the electrode modified with the ion-exchange polymeric film used in the preconcentration step. Such a solid-state cell permits voltammetric assays in low-conductive samples of ionic analytes accumulated in the ion-exchange film during the preliminary collection step because when the electrode is held at appropriate potentials they are reduced or oxidized, coupled with an ionic migration through the membrane to maintain electroneutrality. Concurrently, the water that is wetting the Nation membrane reacts at the auxiliary electrode, thus restoring the ionic content of this solid polymer electrolyte interposed between the working and counter electrodes. This device obviates the need to transfer the modified electrode, after preconcentration from low-conductive solutions, into a medium containing an added supporting electrolyte to perform the voltammetric assay of the accumulated analytes, thus avoiding their extensive release owing to the occurrence of significant ion-exchange competition. A suitable reference electrode is adopted which minimizes ion contamination of the solid-state cell. The performance of this device was checked on Pb 2+, Fe 3+, [Fe(CN) 6] 3− and I − ions preconcentrated from electrolyte-free aqueous solutions. Its application to the determination of Pb 2+ in mineral waters by ion-exchange voltammetry is also reported.

Ion exchange in concentrated media. Correlations for variation of selectivity coefficients with medium

An attempt to relate the different values of the ion-exchange selectivity coefficient in different media is made by considering the two phases as ionic solutions where the activity coefficients of species can be evaluated by means of Mikulin's equation. The selectivity coefficients of the NH 4 +/H +, Na +/H +, and Li +/H + exchanges in a sulfonated cation exchanger and of the NO 3 −/Cl − exchange in a quaternary ammonium anion exchanger have been measured under different conditions characterized by the degree of crosslinking of the ion exchanger, the water activity of the solution, the concentration ratio of two counterions, and the concentration ratio of a third possible counterion. The water activity of the solution is an essential parameter to be controlled, particularly in dilute or moderately concentrated solutions. A representation of the experimental results on the ion-exchange selectivity coefficient is given in terms of two parameters allowing the selectivity coefficient to be predicted even in very concentrated ionic media.

A study of ion exchange selectivity coefficient by cyclic voltammetric measurement

AbstractAn equation to express ion exchange selectivity coefficient was derived and used for calculating that of PPY film with the results obtained by cyclic voltammetric measurement. PPY film was synthesized by electrochemical method in aqueous solution using K4Fe(CN)6 as supporting electrolyte, and the anions were doped into the film. Ion exchange behaviour of doped Fe (CN)63‐/4‐ in the PPY film with Cl−, NO3− or F− ions in solution has been studied, and the corresponding ion exchange selectivity coefficients were determined.

CONDUCTING POLYMERS AND DOPING-DEDOPING BEHAVIOUR

Conducting polymer, as polypyrrole (PPy), has been prepared eletrochemically in aqueous solution containing Fe(CN) 6 3-/4- as supporting electrolyte. The redox couple (Fe(CN) 6 3-/4- in the PPy film acts like a probe to monitor the doping-dedoping behaviour of anions and cations from/to the electrolyte solution. A novel method for measuring the ion exchange selectivity coefficient of a conducting polymer film by cyclic voltammetry is proposed

Electrochemistry of Yb 3+ and Eu 3+ at Nafion modified electrodes

The influence of the electrodeposition of a thin mercury film on the voltammetric behaviour of Yb 3+ and Eu 3+ incorporated by ion-exchange in Nation ® coated glassy carbon electrodes is examined. A significant role of the nature of the electrode material is observed only in the case of Yb 3+ and the improvement in the reversibility and reproducibility of the voltammograms observed after mercury deposition is attributed to the higher overpotential required for hydrogen evolution on mercury. No such effect is observed in fact in the case of Eu 3+, which is reduced at far less negative potential values than Yb 3+ and, in this case, reversible and reproducible voltammetric patterns are obtained at both modified electrodes. The detailed analysis of the ion-exchange process demonstrates that mercury deposition does not cause deterioration of the ion-exchange properties of the Nation coating, while the nature of the supporting electrolyte cations examined (Li +, Na +, K +, Rb +, NH + 4, tetraethylammonium) proves to influence significantly both the extent and rate of incorporation. The ion-exchange distribution and selectivity coefficients of Nation for the two rare earth cations studied are also reported together with the relevant apparent diffusion coefficients.

Influence of Hydrophobicity on the Ion Exchange Selectivity Coefficients for Aromatic Amines

Hydrophobic effects could play an important role in determining the selectivity of organic ions for ion-exchange resins in aqueous solutions. We used the octanol–water partition coefficient (P) and the chromatographic capacity factor (k′) as indices of hydrophobicity of a series of primary and secondary amines, and examined their relationships with the amine selectivity coefficient (K) in binding to the Amberlite IRP-69 ion-exchange resin. Good correlations were found between log K versus log P and log K versus log k′, but the relationship appears to be dependent on the degree of substitution at the amino nitrogen. These relationships may be useful for the estimation of selectivity coefficients of various amine drug candidates when they are considered for incorporation with ion-exchange resins in potential controlled-release systems.