Effect Of Ion Association Research Articles

The aqueous chemistry of the copper-ammonia system and its implications for the sustainable recovery of copper

The dissolution of copper during the leaching of chalcopyrite in ammonia solutions is an attractive alternative to acid sulfate leaching in the treatment of ores with high consumption of acid. Despite considerable research into this complex leaching system, a lack of understanding of the fundamental chemical drivers has delayed the implementation of the ammonia process. In the present study, various ammonium salts solutions (chloride, sulfate, carbonate) have been used to study the effect of ion association on the dissociation constant of the ammonium ion at temperatures of 25 and 35 °C. Experimental and calculated solubilities of Cu 2+ have been obtained under different conditions and plotted in speciation distribution diagrams, in other to assess the accuracy of predictions using available thermodynamic properties. Ion association was found to significantly affect the dissociation constant of the ammonium ion in solutions containing sulfate, chloride and carbonate anions; thus, influencing the free ammonia concentration in solution. Increasing temperature from 25 to 35 °C was found to decrease the dissociation constant of the ammonium ion. These findings highlight the importance of using the correct anionic ligands for the ammonium ions and temperature in order to obtain high dissolution of copper. It has been established that solubility of copper in ammonia solution is affected by the anionic ligands, temperature and addition of chloride ions. The NH 3 ligand forms strong coordination compounds with cupric or cuprous ions depending on the anionic ligand, generating an increase in solubility between pH 8.5 and 10.0. The present study, therefore, identifies important constraints on the role of varying anion associated with ammonia, temperature, pH, and addition of chloride ions and the inter-dependence of these factors in controlling Cu 2+ solubility in ammoniacal systems. The results from the present study provides experimental pKa values and solubility constants of the various ammoniacal systems to provide commercial processing via ammoniacal routes the optimal conditions in which to maximise Cu recovery and maintain free ammonia at levels to minimise volatility and loss. The findings are directly beneficial to future commercial application employing effective ammonium-anion lixiviant strategies in the sustainable recovery of Cu. • Ion association affect pKa of the ammonium ion in solutions containing sulfate, chloride and carbonate anions. • Solubility of copper in ammonia solution is affected by the anionic ligands, temperature, and addition of chloride ions. • The findings are beneficial to commercial application employing ammonium-anion lixiviant in the sustainable recovery of Cu.

Li+ Selectivity of Carboxylate Graphene Nanopores Inspired by Electric Field and Nanoconfinement

The regulation of the ion selectivity by electric field and ion association on the Li+ selectivity of carboxyl functionalized graphene nanopores are investigated by molecular dynamics simulation. Carboxylate graphene nanopores of sub-2nm exhibit excellent Li+ selectivity under the electric field of 1.0Vnm-1 . The results show that ion association inspired by electric field may be a key factor affecting ion selectivity of sub-2nm nanopores. The ion association of Mg2+ and Cl- can be promoted obviously near the nanopores under the electric field of 1.0Vnm-1 . The migrating of Mg2+ can be retarded by stable clusters of Mg2+ and Cl- formed near nanopores. The degree of association of Li+ with Cl- is relatively low and the disassociation of the Li+ cluster is easier so that Li+ can more easily pass through the nanopores. These results gain insight into the effect of ion association inspired by electric field and nanoconfinement of graphene nanopore on Mg2+ /Li+ separation, and provide helpful information for the application of nanoporous materials in extraction of Li+ ion from salt-lake brine.

Dissolution of calcium, phosphate, fluoride and rare earth elements (REEs) from a disc of natural fluorapatite mineral (FAP) in perchloric, hydrochloric, nitric, sulphuric and phosphoric acid solutions: A kinetic model and comparative batch leaching of major and minor elements from FAP and RE-FAP concentrate

Some rare earth phosphate minerals are associated with fluorapatite (FAP) in phosphate ores which make them potential sources of rare earth elements (REEs), to satisfy the increasing demand of REEs, especially for green energy technology and new energy materials. Previous studies have shown the leachability of different elements, which may be categorised as REEs and non-REEs, in different acid solutions with the possibility of selective leaching of non-REEs by phosphoric acid to produce a REE rich leach residue which can be processed separately. This study investigates the dissolution of calcium, phosphate, fluoride and REEs from a rotating disc of a natural FAP mineral sample in perchloric, hydrochloric, nitric, sulphuric and phosphoric acid solutions under various leach conditions including different acid concentrations and temperatures and compares the results with batch leaching of natural FAP mineral and RE-FAP concentrate particles.The dissolution rate of REEs, which occur as minor elements in FAP mineral disc, are lower than that of the major elements: Ca, P and F. Higher dissolution rate of calcium from FAP disc in solutions of HCl, HNO3 and H2SO4 compared to HClO4 at high acid concentrations are due to the influence of Cl−, NO3− and SO42− ions, caused by association with Ca2+ ions in disc leaching. However, the precipitation of CaSO4 retards particle leaching in H2SO4. Despite the lower leaching rates of REEs per unit surface area, due to low abundance of REEs on FAP mineral disc surface, the rates in 0.5 mol L−1 acid solutions follow the order H2SO4, HNO3 > HCl > HClO4 > H3PO4 indicating the beneficial effect of ion-association of the type RE(III)-sulfate/nitrate compared to RE(III)-phosphate precipitation. The higher leaching efficiencies of REEs in H3PO4 during first 10 min also decrease with prolonged leaching due to RE(III)-phosphate precipitation. The leaching efficiencies of Ca, Sr, Fe, Al and Mg are in the range 50–100% in 3.25 mol L−1 HCl, HNO3, HClO4 and H3PO4, compared to REEs, Th and U which vary in the range 20–80%, depending upon the formation of complex species or precipitation. A comparison between elemental composition of natural FAP mineral and a RE-FAP concentrate and their similarities in leaching behaviour in different acids is also presented highlighting the exceptional behaviour of phosphoric acid to selectively leach majority of non-REEs leaving REEs and silica in the leach residue.The kinetic models are used to propose mechanisms which are used to show that the dissolution reaction of calcium is mainly controlled by a chemical phenomenon in strong acids and phosphoric acid at the temperature range 25–50 °C. The experimental reaction orders with respect to H+ concentration in HCl, HNO3, HClO4, H2SO4 and H3PO4 in the range 0.46–0.93 for the dissolution reaction of calcium and/or phosphate are explained on the basis of stoichiometric dissolution and the influence of ion-association of anions with H+ and Ca2+.

Investigation of the Self-Diffusion Coefficients of Trivalent Gd3+ in aqueous solutions: The Effect of Hydrolysis and nitrate ion association

In recent work, we have deduced that berkelium and gadolinium, which have 4f and 5f with half-filled configurations, also display similar diffusion properties. Results, using self-diffusion coefficients to study thermodynamic properties, show very similar behavior, which is understood given their similarities in charge, ionic size, mobility, conductance, and hydration number. In this paper we undertook a study of Gd3+ as an aid for deducing the thermodynamics properties of the Bk3+ .In first part we compare magnitudes of transport (self-diffusion coefficient) of the trivalent aquo ions of Gd III for two concentrations, C1 = 2 10-4 mol.L-1, C2 = 10-3 mol.L-1 , and in perchlorate and nitrate media . In second part, we studied the variation of D with the medium’s pH. Self-diffusion coefficients, D, of the trivalent f-elements aqua ions have been determined in an aqueous electrolyte support of Gd(NO3)3 and Nd(ClO4)3 and at a pH=2.5. (HNO3, HClO4 ) and at 25 °C using the open-end capillary method (O.E.C.M.).The variation of D versus pH shows three zonesWe changed the NO3- counter ion with ClO4- ,and found that measurements must be carried in perchloric acid medium so as to avoid pairing phenomena. Using association constants between Gd3+ and NO3- , and a hydration constant, we estimated self-diffusion coefficients for Gd(NO3)2+ and Gd(OH)2+ .The results justified our choice of working at pH = 2.5 which constitute an optimum condition to avoid hydrolysis and pairing phenomena. Our results were confirmed by mobility measurements using an electro migration technique.

Environment effect on the redox properties of Self-Assembled Monolayers: a theoretical investigation of the nature of the supporting electrolyte

Gibbs free energy calculations have been performed to calculate the change in the simulated redox properties of FcC6S–/C12S–Au and FcC6S–/C4S–Au in three different electrolytes (NaClO4, Na2SO4 and NaPF6). The effect of surface ion association between the ferricinium cation and the electrolyte anions is investigated on the redox properties of SAMs. The microscopic description of the binary SAMs before and after oxidation indicates that the formation of surface ion pairs is strongly correlated with the hydration energy of the corresponding anion. The correlation between the desolvation of the anions and the ion-pairs formation is established. We have shown from the energy contributions that a general understanding of the thermodynamic behavior of these systems should be analyzed in the context of the enthalpy–entropy compensation effect.

Controlled fabrication of nanostructured polypyrrole on ion association template: Tubes, rods and networks

Polypyrrole (PPy) with variable nanostructure such as nanotubes, nanorods and networks has been fabricated by employing an ion association of heparin–methylene blue as new morphology-directing agent. The ion association exhibited both advantages of hard and soft template in template-induced chemical polymerization of pyrrole. By altering the ration of ion association component, PPy could be easily switched in morphology between nanotubes and nanorods. With the coordination effect of ion association and oxidant Fe 3+, PPy networks with high conductivity could also be obtained in static condition. The achieved nanostructured polypyrrole were all doped with heparin, a biochemical material, and had potential exploitation value in electrochemistry and biochemistry.

Excess Volume of Electrolytes in the Mean Spherical Approximation

An expression for the excess partial molar volume of electrolyte solutions was derived in the framework of the mean spherical approximation (MSA). This integral theory, an extension of the Debye−Huckel theory, includes the effect of the ion size in the radial ion−ion distribution function, and it allows the description of excess thermodynamic properties of electrolytes over a wide range of concentration. The general pressure and temperature dependence of the excess volume predicted by the model were analyzed for model electrolytes, and the results for NaCl aqueous solutions at temperatures up to 300 °C were compared with experimental data over a wide range of concentrations (up to 4 mol·kg−1). Crystallographic and adjustable ion diameters were used in the calculations, allowing reasonable predictions of the NaCl(aq) excess volume over different concentration regions. The effect of ion association, excluded volume, and reference system conversion were also analyzed.

Apparent and Standard Partial Molar Volumes of NaCl, NaOH, and HCl in Water and Heavy Water at T = 523 K and 573 K at p = 14 MPa

Apparent molar volumes, Vphi,2, of aqueous NaCl, NaOH, NaOD, HCl, and DCl in water and heavy water were determined at T = 523 and 573 K and p = 14 MPa with a high-temperature platinum vibrating-tube densimeter in the aquamolality range 0.25 </= maq </= 2.5 mol. (55.509 mol solvent)-1. The experimental results have been represented with an extended Debye-Hückel equation to describe the concentration dependence of Vphi,2 and to derive standard partial molar volumes of these electrolytes in light and heavy water, V degrees 2,H and V degrees 2,D, respectively. For NaCl and NaOH, the D2O isotope effect at infinite dilution, [V degrees 2,H - V degrees 2,D], increases from 0.2 and 0.8 cm3 mol-1 to 4.5 and 7.1 cm3 mol-1, respectively, when the temperature is increased from 523 to 573 K. For HCl and DCl, the effect is smaller and the sign is reversed, [V degrees 2,H - V degrees 2,D] = -0.7 cm3 mol-1 at 523 K and -1.4 cm3 mol-1 at 573 K. When the effect of ion association is included, the deuterium isotope effect for HCl becomes positive, [V degrees 2,H - V degrees 2,D] approximately 17 cm3 mol-1 at 573 K, consistent with NaCl and NaOH. Two models are proposed to describe the solvent isotope effect on the infinite dilution limit, one based on the Born equation and the other on the dimensionless Krichevskii parameter. The experimental values of V degrees 2,D also have been used to calculate the first reported values for the pressure dependence of the ionization constant of D2O at temperatures higher than 313 K.

Steady-state voltammetric study of the reduction of decamethylferrocenium in dichloromethane at microelectrodes

The reduction of decamethylferrocenium hexafluorophosphate in a low permittivity solvent, dichloromethane, was studied by steady-state voltammetry at microelectrodes, using tetrabutylammonium hexafluorophosphate as the supporting electrolyte. The limiting current was used to calculate the diffusion coefficient of the decamethyferrocenium cation and of its ion pair with the hexafluorophosphate anion at 298.15 K by employing a recent theory (OCSB [K.B. Oldham, T.J. Cardwell, J.H. Santos, A.M. Bond, J. Electroanal. Chem. 430 (1997) 25]) which takes into account the effect of ion association on the limiting currents. The OCSB theory describes accurately the contribution of migrational processes to the overall Faradaic current when variable quantities of supporting electrolyte are added to the solution.

Modeling the Effects of Ion Association on Direct‐Current Polarization of Solid Polymer Electrolytes

Considerable experimental evidence indicates that ion association occurs in solid polymer electrolytes. This work provides a thorough theoretical analysis of the effect of ion association on the conductivity, general current‐potential behavior, and limiting current density in a solid polymer electrolyte. The model employs dilute solution theory to describe the fluxes of cations, anions, and ion pairs in a motionless continuum but neglects higher order association. The predictions of the model highlight the effects of the relative diffusion coefficients and dimensionless association constant on concentration distributions of simple ions and ion pairs, the limiting current density, and the potential drop required to drive a specified current density. If ion pairs have a diffusivity comparable to those of cations and anions, increasing ion association leads to a continuous decrease in molar conductivity and current density at constant applied potential. If the ion pairs have a diffusivity that is large compared to cations, the situation is quite different. In this case, ion association increases the limiting current density to values that may be several times that found in the case of full dissociation. Furthermore, the model predicts maxima in the molar conductivity and current density at fixed potential drop as the degree of ion association increases. © 2000 The Electrochemical Society. All rights reserved.

Charging process in polypyrrole films: effect of ion association

Abstract Cyclic voltammetric (CV) response of polypyrrole (PPy) films in contact with tetraphenylborate (TPB) acetonitrile (AN) solutions has demonstrated pronounced anomalies compared to the CV for the usual anions. The redox activity of the film is considerably reduced and both the anodic and cathodic waves consist of two peaks whose positions are strongly dependent on the ionic strength of the solution. The diminution of the charging ability has been related to a much lower maximum concentration of the doping anions (TPB) inside the film due to their large size. The observed splitting of the waves into two peaks cannot be attributed to two different electronic processes like the successive generation of `polarons' and then `bipolarons'. On the other hand, the hypothesis that ions inside the film may be in two different forms, `free' and `bound', has allowed us to reproduce all the experimental features, in particular, two separated peaks at lower electrolyte concentrations, their shift in the opposite directions with ±60 mV slope versus logarithm of the ionic strength, independence of the peak heights or shapes of the concentration, merging of the peaks at higher ionic strengths. The same concept has been able to explain an observed separation of the broad peak for polyaniline (PANI) films in contact with mixed perchlorate–polyanion solutions into two narrower peaks upon diminishing the perchlorate content. This conclusion is in conformity with our analysis of data for the charging process of PPy in perchlorate solutions obtained earlier by in situ electrochemical quartz-crystal microbalance studies (EQCM). Experimental plots for mass versus its charge have quite different shapes for films polymerised at high or low potentials. In particular, those curves in the latter case possess opposite slopes at low and high film charges as well as a pronounced hysteresis. These observations have also been interpreted successfully within the framework of the above model of two forms of ions inside the polymer phase. The comparison has also provided evidence that `bound' ions can be exchanged gradually with their `free' form during the cyclic voltammetry process. This process results in a complicated shape for the mass versus charge plots including a crossing of the anodic and cathodic branches as well as a loop within the range of low charges. All these predictions of the model are in conformity with the above experimental results. The theory considers `immobile' ions as associated with neutral sites of the polymer matrix, either due to the formation of a bond, or the ion binding by micro-cavities. An alternative approach assumes the anions to form a complex with positively charged electronic species (like `polarons' or `bipolarons'). However, the latter is unable even to provide a qualitative explanation of the above data, e.g. opposite signs of the slope for low and high charge ranges.

The effect of the ion exchanger site-counterion complex formation on the selectivities of ISEs

Using the model of ideally associated solution, the effect of ion association of the ion exchanger sites with main and foreign counterions on the selectivity of ISEs based on liquid ion exchangers has been considered. Equations which describe the potentiometric selectivity coefficient as a function of ion association constants in the membrane phase and of standard free energies of transfer of the determined and foreign ions from water to the membrane are obtained for the following main cases: (a) the determined and foreign ions are single-charged; (b) the determined ion is double-charged and the foreign ion is single-charged. It is shown that in the case of single-charged main and foreign ions, the ratio of the ion association constants has a great effect on the potentiometric selectivity of membranes, only if the ion exchanger sites produce less strong associates with the determined counterion as compared with the foreign one. Otherwise, this effect is insignificant. The selectivity for double-charged ions should increase, other things being equal, as the first constant of association of these ions with the ion exchanger sites increases. The effect of producing ion triplets of the type I 2 R (±) on the selectivity of ISEs is also considered. Experimental data are presented which illustrate the effect of the nature of the ion exchanger on the potentiometric selectivity. Some procedures employing the factor of ion association for increasing the potentiometric selectivity of liquid ion exchange membranes are considered.

An Investigation by Diffusion Layer Imaging of Coupled Homogeneous Chemical Reactions of the Benzophenone Radical Anion in N,N‐Dimethylformamide

AbstractThe electrochemical reduction of benzophenone (BP) to its primary radical anion BP− with coupled homogeneous chemical reactions was investigated in N,N‐dimethylformamide. Protonation of BP− with benzoic acid and with o‐cresol, respectively, used as proton donors, and disproportionation of BP− in the presence of excess Li+ ions were studied by DLI technique. Rate constants for protonation and disproportionation as well as equilibrium constants for ion association were evaluated. Possible reaction pathways, and the effect of ion association on the kinetic behaviour are discussed.

Conductometric and potentiometric studies of NaI solutions in water + sec-butanol mixtures at 298.15 K : The effect of ion association on the thermodynamic functions of transfer

The electrical conductivity of NaI solutions and the emf of the cell glass electrode (Na)|NaI|AgI, Ag in aqueous sec-butanol mixtures in the complete miscibility range of the alcohol have been measured. The conductivity data are analysed using the Fuoss—Justice equation. The standard dissolution enthalpies of NaI have been recalculated, and the thermodynamic functions Δ H t 0, Δ G t 0 and Δ S t 0 are calculated taking the ion-association processes into account.

Effect of ion association on the properties of ionomers obtained from methyl methacrylate-methacrylic acid copolymer

The solution behaviour of Na, Ba, Zn and Al ionomers of methyl methacrylate-methacrylic acid copolymer (P(MMA-MAA)) with different ion contents has been studied by viscosity and dipole moment measurements. Two solvents with different character are chosen for viscosity measurements: dioxane ( ε = 2.22) and acetone ( ε = 20.7). The expected polyelectrolyte behaviour of the ionomers in acetone is reflected by the viscometric measurements. Mean squared dipole moment 〈 μ 2〉 values are calculated over a temperature range of 20–100°C in dioxane. Effects of neutralization, cation size and cation charge are discussed for each ionomer.

The enthalpy of dilution and apparent molar heat capacity of NaOH(aq) to 523 K and 40 MPa

The molar enthalpy of dilution Δ dil H m of NaOH(aq) was measured from 6.3 to 0.008 mol · kg −1 at temperatures from 298 to 523 K and pressures from 7 to 40 MPa with a differential flow mixing enthalpy calorimeter. Apparent molar heat capacities C p, φ were measured with a flow heat-capacity calorimeter from 4.0 to 0.1 mol · kg −1 from 323 to 523 K at 7 MPa. These results are correlated with available literature values as functions of temperature, pressure, and molality with the ion-interaction thermodynamic treatment. The extended form of the ion-interaction model, including an approximation for the effect of ion association, was needed to represent the enthalpy results quantitatively above 473.15 K; this additional term does not have a large effect on the activity or osmotic coefficients. Standard heat capacities calculated from this work are consistent with literature values for the heat-capacity change on ionization of water.

Steady state current flow in solid binary electrolyte cells: Part 2. The effect of ion association

The steady state current flow following the application of a constant voltage to cells of the form M(s)< M +X −(s)¦M(s) where M +X −(s) is a binary solid electrolyte, such as a polymer electrolyte, containing mobile M + cations, X − anions and MX ion pairs, is discussed. General equations are derived for the effective conductivity of the electrolyte in terms of the diffusion coefficients of the three species and the association constant. A number of special cases which are of particular importance to the study of polymer electrolytes are considered briefly. It is shown (i) that measurements of steady state currents can indicate when significant concentrations of ion pairs are present, and (ii) that this technique may provide an excellent method of evaluating the practical merit of these materials for applications such as power sources or electrochromic devices. It is also noted that transference numbers cannot be obtained from this or the related ac technique when mobile ion pairs are present.

Effect of ion association on transport in polymer electrolytes

Numerous techniques have been employed in an attempt to elucidate details of the conductivity mechanism in polymer electrolytes, and in particular to determine whether cations, anions or both are mobile. Until recently, most of these studies have assumed a ‘strong electrolyte’ model. Here we examine the effect of ion association on the interpretation of different techniques, including Hittorf, concentration cell, centrifugal cell, ratio-tracer, pulsed field gradient n.m.r. and d.c. polarisation methods. The circumstances under which certain of these experiments yield individual ionic transport numbers are described. It is also shown that, in general, measured diffusion coefficients may not be sensibly compared with mobilities from conductivity measurements using the Nernst–Einstein relationship. The d.c. polarisation technique is examined in some detail and the value of the steady-state conductivity in assessing polymer electrolytes for battery applications is described. Finally, some experimental evidence is presented which appears to support the existence of ion association in polymer electrolytes.

Fractionation of rubidium isotopes in cation-exchange chromatography

Ion-exchange chromatography of rubidium was carried out to study the rubidium isotope effect in aqueous ion-exchange systems. The heavier isotope 87Rb was found to be preferentially fractionated into the resin phase. This trend is independent of the type of the resin (strongly acidic vs. weakly acidic), the kind of rubidium salts (rubibidum chloride vs. rubidium lactate) and the operating temperature (25°C vs. 70°C). The effect of ion association in and out of the resin phase on the single-stage separation factor is discussed.

1H N.m.r. Chemical shifts and hydrogen exchange rates oftrans-[Co(en)2XY]n+ complexes in liquid ammonia

1H n.m.r. spectra of coordinated ethylenediamine HN< hydrogens for 23trans-[Co(en)2XY]n+ complexes and1H-2H exchange rates for some selected compounds are reported in the solvent liquid ammonia. Assignments are presented for asymmetric complexes. These are based on the chemical shift order of the symmetric complexes. The assignments are confirmed by independent experimental findings including the effect of specific ion-association and the observed exchange rates. The exchange rates follow the relation kobsd=k0+k1 [N2H4ClO4]−1, illustrative of general base-catalysis; in this case rate-determining proton abstraction by N2H3 and N2H2−, respectively. Chemical shifts and exchange rates in liquid ammonia follow the published order of the aqueous parameters.