Chemical Equilibria In Solution Research Articles

Unlocking Phase Diagrams for Molybdenum and Tungsten Nanoclusters and Prediction of their Formation Constants.

Understanding and controlling aqueous speciation of metal oxides are key for the discovery and development of novel materials, and challenge both experimental and computational approaches. Here we present a computational method, called POMSimulator, which is able to predict speciation phase diagrams (Conc. vs pH) for multispecies chemical equilibria in solution, and which we apply to molybdenum and tungsten isopolyoxoanions (IPAs). Starting from the MO4 monomers, and considering dimers, trimers, and larger species, the chemical reaction networks involved in the formation of [H32Mo36O128]8- and [W12O42]12- are sampled in an automatic manner. This information is used for setting up ∼105 speciation models, and from there, we generate the speciation phase diagrams, which show an insightful picture of the behavior of IPAs in aqueous solution. Furthermore, we predict the values of 107 formation constants for a diversity of molybdenum and tungsten molecular oxides. Among these species, we could include several pentagonal-shaped species and very reactive tungsten intermediates as well. Last but not least, the calibration employed for correcting the density functional theory (DFT) Gibbs energies is remarkably similar for both metals, which suggests that a general rule might exist for correcting computed free energies for other metals.

Leaching of chalcopyrite: An emphasis on effect of copper and iron ions

Many researchers found that the Fe2+ together with less amount of Cu2+ can accelerate the leaching of chalcopyrite. In this work, the leaching of chalcopyrite with Cu2+ was investigated. The leaching residuals were examined by Raman spectroscopy. Based on the leaching experiments, the chemical equilibrium in solution was calculated using Visual MINTEQ. The results showed that the Fe in chalcopyrite lattice was replaced by Cu2+; therefore, the chalcopyrite transformed into covellite. Furthermore, the formation of chalcocite occurred when Fe2+ and Fe3+ were added to the solution containing Cu2+. The copper extraction increased with a decrease of the initial redox potential (or the ratio of Fe3+/Fe2+).

NMR spectroscopic study of chemical equilibria in solutions of formaldehyde, water, and butynediol

Liquid mixtures of formaldehyde, water, and butynediol are complex reacting multicomponent systems in which formaldehyde forms oligomers both with water and butynediol. ‐ and ‐NMR spectra of these mixtures are elucidated. The species distribution of the oligomers is quantitatively determined by ‐NMR spectroscopy. The measurements cover temperatures from 293 to 366 K, overall formaldehyde mass fractions from to , and overall butynediol mass fractions from to . A mole fraction‐based and an activity‐based model of the chemical equilibrium in the studied system are developed and chemical equilibrium constants are reported. © 2017 American Institute of Chemical Engineers AIChE J, 63: 4442–4450, 2017

Effects of Unequal Diffusion Coefficients and Coupled Chemical Equilibria on Square Wave Voltammetry at Disc and Hemispherical Microelectrodes

Square wave voltammetry (SWV) at disc and hemispherical microelectrodes is studied when the electroactive species show different diffusivities and/or take part in chemical equilibria in solution, under both transient and steady state conditions. Despite the use of microelectrodes, experiments in pulse techniques such as SWV frequently correspond to transient conditions since the pulse duration are typically very short (<100ms, f>5Hz). As will be shown, under such conditions the case of unequal diffusion coefficients gives rise to a complex behaviour of the SWV peak, the features of which deviate from the theory and criteria already established under the assumption of equal diffusion coefficients. Also, the theoretical treatment is notably more difficult and no analytical solution has been deduced for multipulse techniques.The effects of coupled chemical equilibria and, particularly, of unequal diffusion coefficients on SWV will be described in depth as a function of the electrode size and shape and they will be compared with those observed in differential double pulse voltammetry (DDPV). The main discrepancies in the peak height, width and position with respect to the case of equal diffusion coefficients will be studied. Also, appropriate methodologies and experimental conditions will be discussed for the determination of formal potentials and equilibrium constants.

Reverse Pulse Voltammetry at Spherical and Disc Microelectrodes: Characterization of Homogeneous Chemical Equilibria and Their Impact on the Species Diffusivities

The value of the electrochemical technique Reverse Pulse Voltammetry (RPV) for the characterization of chemical equilibria in solution is theoretically investigated. For this, the RPV response of the square scheme is studied at spherical and disc electrodes of any size assuming that the chemical reactions are at equilibrium. When the effective diffusion coefficients of the oxidized and reduced species are the same, analytical solutions are reported for both electrode geometries. Otherwise, a rigorous analytical solution is derived for spherical electrodes whereas the case of microdiscs is addressed by numerical simulations.The theory for the square scheme enables the study of the RPV response in very different situations, including the cases where the electrochemical and/or chemical transformations alter significantly the species diffusivity such as in electron transfer processes in room temperature ionic liquids or in the association of the electroactive species with (bio) macromolecules and nanoparticles. It is found that, when medium-size (disc or spherical) microelectrodes are employed, the RPV method enables the simultaneous determination of the effective diffusion coefficients of the oxidized and reduced species as well as the apparent formal potential. These magnitudes make it possible a sound physicochemical characterization of the system.

Macroscopic Thermodynamics of Interfaces at Water Pore Scales: Effects on Water-Rock Interactions and Mass Transfer

The possible role of liquid-air and solid-liquid interfacial curvature on water-rock mass transfer in natural settings is here studied by macroscopic thermodynamics that includes the Young-Laplace interphase pressure gradient. Capillary water in non-saturated soils (liquid-air concave curvature toward air), and curved solid as a function of pore sizes (solid-liquid concave curvature towards infilling solution) exemplify what can be expected in terms of solubility. Gas solubilities in capillary water increase with curvature, promoting gas uptake and so modifying chemical equilibria in solutions (e.g., redox and/or acido-basic) accordingly. Applied to solid solubility, the macroscopic thermodynamics of interface treatment highlights the versatility offered by crystalline shaping with respect to the capillary degree (if any) of the surrounding aqueous solution. Additionally, the solid curvature associated with decreasing pore sizes makes the solid less soluble in the infilling bulk solution.

Local autocatalytic co-precipitation phenomena in self-assembled silica–carbonate materials

Under certain conditions, precipitation of barium carbonate in alkaline silica-rich environments affords unusual polycrystalline aggregates exhibiting curved shapes and hierarchical structuring, very much reminiscent of biogenic mineral tissues. The formation of these so-called silica biomorphs is thought to rely on a coupling of chemical equilibria in solution, which drives concerted co-mineralization and self-assembly of components. In the present work, we have studied the effect of fluid motion on morphogenesis by conducting syntheses in media stirred at different rates and exposed to an ultrasonic field, respectively. The traced growth behavior is discussed on the basis of statistical analyses of the occurring morphologies as well as in terms of expected changes in the supersaturation of the system. It is shown that the observed complex architectures arise as a consequence of an autocatalytic precipitation cycle, during which evolving aggregates synthesize and organize their constituents on their own upon growth. These processes occur only at a local scale within a certain active region that appears to extend over some microns beyond the growth front. Our findings provide concrete experimental evidence supporting the proposed formation mechanism and suggest that growth of silica biomorphs is independent of mass transport from the bulk, unless forced convection becomes strong enough to affect the active region and hence interfere with autocatalysis.

THE INFLUENCE OF THE SOLVENT ON THE THERMODYNAMICS OF ION ASSOCIATION

Abstract. Some approaches which allow to divide thermodynamic functions of the ion associationprocess in two components have been developed. The first component belongs to the process, the second oneis caused by the temperature dependence of the dielectric permittivity of the solvent. The theory is confirmedby numerous examples of the ion association process of different electrolytes in the binary mixed solvents.Keywords: covalent part of the constant of ionic association, electrostatic part of the constant of ionicassociation, enthalpy of the chemical equilibria in solution, enthropy of the chemical equilibria in solution,ionic association, ionic equilibrias, the equilibrium constant.

Chemical Bonding and the Equilibrium Composition of Grignard Reagents in Ethereal Solutions

A thorough analysis of the electronic structure and thermodynamic aspects of Grignard reagents and its associated equilibrium composition in ethereal solutions is performed. Considering methylmagnesium halides containing fluorine, chlorine, and bromine, we studied the neutral, charged, and radical species associated with their chemical equilibrium in solution. The ethereal solvents considered, tetrahydrofuran (THF) and ethyl ether (Et(2)O), were modeled using the polarizable continuum model (PCM) and also by explicit coordination to the Mg atoms in a cluster. The chemical bonding of the species that constitute the Grignard reagent is analyzed in detail with generalized valence bond (GVB) wave functions. Equilibrium constants were calculated with the DFT/M06 functional and GVB wave functions, yielding similar results. According to our calculations and existing kinetic and electrochemical evidence, the species R(•), R(-), (•)MgX, and RMgX(2)(-) must be present in low concentration in the equilibrium. We conclude that depending on the halogen, a different route must be followed to produce the relevant equilibrium species in each case. Chloride and bromide must preferably follow a "radical-based" pathway, and fluoride must follow a "carbanionic-based" pathway. These different mechanisms are contrasted against the available experimental results and are proven to be consistent with the existing thermodynamic data on the Grignard reagent equilibria.

Introduction of Electrochemical Potential to General Education for Chemical Equilibria in Solutions

Introduction of Electrochemical Potential to General Education for Chemical Equilibria in Solutions

Application of the GRAAL model to leaching experiments with SON68 nuclear glass in initially pure water

Based on a review of the current state of knowledge concerning the aqueous alteration of SON68 nuclear glass we have proposed a mechanistic model, GRAAL (Glass Reactivity with Allowance for the Alteration Layer) [P. Frugier, S. Gin, Y. Minet, T. Chave, B. Bonin, N. Godon, J.E. Lartigue, P. Jollivet, A. Ayral, L. De Windt, G. Santarini, J. Nucl. Mater. 380 (2008) 8]. This article describes how the GRAAL model hypotheses are solved using a calculation code coupling chemistry and transport. The geochemical solution of this model combines three major phenomena: chemical equilibria in solution, water and ion transport by convection or diffusion, and element diffusion through the passivating reactive interphase. The model results are compared with experimental data for SON68 glass leached in initially pure water both in a closed system and in renewed media. The comparison shows the model very satisfactorily accounts for variations in the pH and the element concentrations in solution as a function of time, the glass surface area in contact with solution, and the solution renewal rate. This success is due to the fact that the diffusion of elements through the alteration gel is taken into account in the model. This mechanism cannot be disregarded under most experimental conditions — if only to predict the solution pH — and must therefore be an integral part of the geochemical model.

FactSage and ChemApp: Two tools for the prediction of multiphase chemical equilibria in solutions

Abstract The integrated thermochemical databank system FactSage enables the user to calculate equilibria and thermodynamic properties for a wide variety of multicomponent, multiphase systems and reactions. The Gibbs energy models and databases for condensed, gaseous, and aqueous solutions presently implemented in FactSage are of interest to chemical and physical metallurgy, chemical and corrosion engineering, inorganic and solution chemistry, geochemistry, environmental science, etc. Models for the aqueous phase include the Pitzer and Helgeson formalisms, but data and subroutines for concentrated aqueous solutions have also been provided by OLI Systems. ChemApp is a programmer's library for thermochemistry that also incorporates the Gibbs energy minimizer of FactSage. Applications of ChemApp include, for example, the handling of repetitive complex equilibrium calculations in application-specific programs and its linking to third-party process simulation packages. In this work, user-defined aqueous solution models in FactSage and ChemApp have been applied to the hydrometallurgical processing of aluminum ores in both caustic and acidic leach solutions.

Chelating reagents for flotation

The paper provides a review of the studies performed, starting from the 1970 up to today by Italian National Council’s Mineral Processing Institute, to develop application for the use of chelating reagents in the flotation of metallic minerals. A summary is given of the various phases through which the studies progressed, starting by employing commercial reagents and then moving on to the synthesis of new long-chain chelate collectors for the flotation of sulphide and oxidized Pb, Zn, and Cu minerals. In particular a description is presented of the thermodynamic and structural criteria to be followed in designing and synthesizing chelate-type collectors with optimal structure for a given metallic mineral. A theoretical method has been developed for choosing chelating groups that exert selective action on one metallic mineral rather than others. The theoretical approach is based on thermodynamic data of the chemical equilibria in solution. Moreover, the flotation results obtained in the flotation of oxidized and mixed Zn–Pb ores, by using chelating reagents with a mixed aliphatic–aromatic structure, provided interesting pointers for designing the non-polar portion. The structural criteria identified via these studies were interpreted on the basis of chemical and steric considerations. Recently, the thermodynamic approach was adopted to identify selective reagents for the removal of minor elements and for the separation of copper from zinc in the xanthate flotation of complex sulphide ores.

Electronic and steric effects of triarylphosphines on the synthesis, structure and spectroscopical properties of mononuclear rhodium(I)–chloride complexes

A systematic study of the reaction between the dinuclear complex [{Rh(μ-Cl)(η 4-COD)} 2], (COD = 1,5-cyclooctadiene) with 14 different triarylphosphines is presented. When two equivalents of the phosphine are employed, the main product is the mononuclear complex [RhCl(η 4-COD)(PR 3)], with R = 4-(OCH 3)C 6H 4 ( 1), 4-(CH 3)C 6H 4 ( 2), C 6H 5 ( 3), 4-FC 6H 4 ( 4), 4-(CF 3)C 6H 4 ( 5), 4-ClC 6H 4 ( 6), 3-(OCH 3)C 6H 4 ( 7), 3-(CH 3)C 6H 4 ( 8), 3-ClC 6H 4 ( 9), 2-(OCH 3)C 6H 4 ( 10), 2-(CH 3)C 6H 4 ( 11) and R 3 = (C 6H 5) 2(C 6F 5) ( 12). No mononuclear complex could be isolated with the electron poor phosphines P(C 6H 5)(C 6F 5) 2 and P(C 6F 5) 3. A chemical equilibrium in solution was observed between the dinuclear and mononuclear species, with the formation of the mononuclear being disfavoured by the use of bulky and electron poor phosphines. The mononuclear complex 11, with the extremely bulky phosphine P(2-CH 3C 6H 5) 3, was undetected in solution by NMR, however could be crystallized and its molecular structure determined by X-ray diffraction and compared with the previously reported structures for 3 and 4. For complexes with isosteric phosphines ( 1– 6), an inverse relationship between the coupling constant 1 J Rh–P and the electronic parameter ( χ) of the phosphine was observed.

Modelling of electrolyte mixtures with application to chemical equilibria in mixtures—prototypes of blood's plasma and calcification of soft tissues

Modelling of the biologic solutions has a great importance for basic physicochemical backgrounds of the living processes, mechanism of the diseases and drugs action, etc. The modelling of the chemical equilibria in solution that served as a prototype of the blood plasma with application to calcification of the tissues is performed. The concentrations of molecular–ionic forms containing calcium and hydrogen cations and phosphate anions in the range of ionized-calcium and total phosphorus concentrations from 0.5 to 3.0 mM and at the solution pH of 7.1–7.8 were calculated. The activities of the ionized species were described in approach to Debye–Hückel's theory. The full set of the equilibria taking into consideration dissociation of the water, phosphoric acid, formation of both inert and ionic calcium phosphates was considered. The states of calcium hydrophosphate dihydrate CaHPO 4·2H 2O (CHPD), calcium phosphate Ca 3(PO 4) 2 (CP), calcium hydrophosphate-phosphate dihydrate Ca 4H(PO 4) 3·2H 2O (CHPPD) and hydroxyapatite Ca 3OH(PO 4) 3 (HA) with respect to the boundary of the region in which they crystallise were determined. A criterion has been introduced to characterize the degree of salt supersaturation with respect to crystallisation, which is based on the concentration distance between the states of a salt in solution and at the boundary of its crystallisation. This criterion is used to provide a quantitative characteristic of the supersaturation of the phosphates and their tendency to crystallise in blood's plasma. It was established that the most soluble of the phosphates, CaHPO 4·2H 2O, is undersaturated and the other phosphates are supersaturated with respect to crystallisation. Thus, this phosphate does not take part in the calcification, and this is the source for ionized calcium in a blood plasma from the soft tissues. The role of the other phosphates in calcification of the soft tissues is decreased in the series HA>CP>CHPPD. The dependencies of the supersaturation of the solution on the pH and on the concentrations of calcium and phosphorus in a mixture are discussed.

Comparison of thermodynamic characteristics of chemical equilibria in solutions calculated from the equilibrium constant polyterms in different concentration scales

Comparison of thermodynamic characteristics of chemical equilibria in solutions calculated from the equilibrium constant polyterms in different concentration scales

Physics of isodesmic chemical equilibria in solution

Exactly solvable one-dimensional lattice-gas model mixtures are used to develop a physics of isodesmic chemical equilibria. Potential distribution theory is used to solve directly for the equation of state and to obtain the cluster statistics needed to discuss self-assembly. A mapping of three-dimensional amphiphilic discotic solutions onto one-dimensional models is proposed and is found to explain the remarkable nature of previous computer simulation data. Here, at fixed pressure, the low concentration regime involves an extreme concentration dependence to solute aggregation, associated with a maximum in the equilibrium constant. This behavior is a class of colloidal phenomena, driven by solvent-solvent attractive interactions. In addition, the exact physics of isodesmic chemical equilibria is used to investigate a variety of conceptual issues concerning the phenomenology of self-assembly. One finds that ${\overline{\ensuremath{\mu}}}_{n}\phantom{\rule{0ex}{0ex}}=\phantom{\rule{0ex}{0ex}}n{\ensuremath{\mu}}_{A}$ is an exact consequence of statistical mechanics and that it is even possible to give a precise meaning to the chemical potential of an aggregate, that, for example, defines what is meant physically by the identity ${\overline{\ensuremath{\mu}}}_{1}\phantom{\rule{0ex}{0ex}}=\phantom{\rule{0ex}{0ex}}{\ensuremath{\mu}}_{A}$. The nonuniqueness of the choice of cluster definition is considered in the context of solvent-excluded clusters; an explicit example appropriate to amphiphilic systems. Finally, the mapping to three-dimensional discotic solutions is extended to inhomogeneous phenomena whereby the disks prefer to adsorb flat onto a surface. This mapping implies that the sticky solvent regime is associated with an overwhelming driving force for chains to attach by one end to a solutelike wall.

Mean-field theory of solutions: Chemical potential and solution structure in the superposition approximation

A simple expression for the chemical potential of a solute in solution is presented under the superposition approximation. In this expression, only the knowledge of the solute–solvent pair correlation function within the solute–solvent interaction range is required to evaluate the chemical potential. The renormalization of the solute–solvent interaction is shown to be related to the indirect part of the solute–solvent potential of mean force. An inconsistency involved in the superposition approximation is then pointed out, and its origin is identified. The shift in a chemical equilibrium in solution induced by insertion of a solute is also investigated under the superposition approximation. It is shown that a correlation function characterizing the effect of a single solvent molecule on the chemical equilibrium plays an important role in determining the shift in the chemical equilibrium. Numerical results are presented for the chemical potential of hard sphere fluid.

Using Quality Circles in General Chemistry

A second semester class in General Chemistry used a student Quality Circle to suggest methods to improve instruction in chemical equilibrium in solution. Composition and procedures for the Circle are described. This Quality Circle prepared and evaluated a flow-chart used by students to solve assigned equilibrium problems. This study utilized control and experimental groups and provides both qualitative and quantitative assessment.

Electrochemical impedance of solutions of polysulfides in liquid ammonia: experimental evidence for the Gerischer impedance

Abstract It is shown that the electrochemical impedances measured in solutions of polysulfides in liquid ammonia (lithium polysulfides Li2SnNH3, ammonium polysulfides (NH4)2S4NH3, and mixed tetrasulfides (NH4)xLi2 − xS4NH3) measured at the equilibrium potential on a stationary gold disk electrode can be analyzed in terms of the Gerischer impedance, i.e. the impedance predicted by Gerischer when the electronic charge transfer is coupled to one (or several) chemical equilibria in solution. Our experimental data are believed to be the first unambiguous experimental evidence for this type of impedance. The comparison of experimental impedances obtained for gold and platinum electrodes confirms that we are dealing with Gerischer impedances. In a given solution, several Gerischer impedances can be observed in different frequency ranges. The theoretical analysis leading to this impedance profile is recalled, and the experimental conditions to obtain valuable data are given. Finally, the data analysis is detailed. The experimental data are weighted by the modulus of the impedance. The high frequency Gerischer impedance originates from rearrangement of the S2−3 polysulfide involving the ammonium ion. The possible origins of the low frequency Gerischer impedances are discussed.