A new model for the activity coefficients of individual ions in aqueous electrolyte solutions

In this study, the individual ion activity coefficient in an aqueous solution is modeled with a new model. This model contains two physical significant ionic parameters regarding the ionic solvation and the closest distance of approach between ions in a solution. The present model was evaluated by the estimation of the individual activity coefficients of the ions of thirty electrolytes in aqueous solutions. The results showed that this model suitably predicts the individual ion activity coefficients in aqueous two-electrolyte solutions consisting of the binary pairs of electrolytes of NaCl, KCl, KBr and CaCl₂ in a temperature range from 298.15 to 243.15 K. The results by this model were compared to the literature values. The average absolute relative deviations of vapor pressures showed acceptable agreement between experimental data and the results of this model.     

Selectivity coefficient changes for liquid-membrane electrodes

The effects of the activity levels of the measured ion and interfering ions, and of the detection limit of the electrode on the values of selectivity coefficients for liquid-membrane ion-selective electrodes are discussed. The coefficients were determined by the mixed-ion solution method. Depending on the activity of the interfering ion, the activity of the main ion for which the selectivity coefficient is determined may differ. The best conditions of measurement are those which involve the largest contribution from the term containing the selectivity coefficient in the Nikolsky equation; the measurements are then most precise, and the values of the selectivity coefficients describe the electrode behaviour most consistently. When the limit of detection of the electrode is comparable with the other terms, it must be taken into account in calculations. Under the optimal conditions, selectivity coefficients were calculated for the Orion calcium and divalent cation electrodes, with calcium as the main ion and alkali metal ions as interfering ions.     

离子选择性微电极用于原位测量离子扩散系数

扩散系数是描述物质扩散过程的重要参数,而用膜池法、放射性或荧光示踪法、分子动力学模拟等现有方法无法原位进行生物体系中离子扩散系数的实时测量。 本文利用离子选择性微电极响应迅速、高选择性、高灵敏度、高空间分辨率、对样品无污染等优势,通过分析单个植物细胞原生质体在培养液中破裂时所形成的离子浓度脉冲信号,建立了相应的点源扩散模型,推导出了描述离子浓度随时间变化的理论公式,并通过该公式对实验测得的脉冲信号进行拟合,得到了离子的扩散系数,从而建立了一种用离子选择性微电极原位测定离子扩散系数的新方法,并将其应用于芦荟细胞原生质体破裂时离子扩散系数的测定,得到了Ca²⁺、Na⁺和K⁺的扩散系数分别为(6.51±0.12)×10^-6、(2.93±0.15)×10^-5和(3.03±0.35)×10^-5 cm²/s。 对比发现,拟合得到的Ca²⁺、Na⁺和K⁺扩散系数均略高于已报道的数值(纯水中),这一现象的产生可能是因为原生质体是在低渗液中吸水膨胀,细胞膜内压力升高产生内外压力差,该压力差会加速细胞破裂时离子的扩散。 这一方法对生物体系无干扰,较好地解决了生物体系中离子扩散系数原位实时测量的难题。     

Activity coefficients of ions in sodium halide solutions: Critical remarks

The fundamentals of an experimental method proposed by Zhuo et al. [1], to determine activity coefficients of sodium and halide ions in sodium halide solutions, are critically examined. It is shown that this method relies on a key hypothesis, which proves to be incorrect, about the liquid junction potential, whose value is assumed not to change when the concentration of the sample solution is changed. The direct consequence of this assumption is that results that are interpreted as the activity coefficients of sodium and halide ions are, instead, conventional values, which only depend on the mean activity coefficients and transport numbers, and have no connection with the activity coefficients of the respective ions.     

Thermodynamic description of superequivalent sorption by ion exchangers: Theoretical background

A thermodynamic approach to describing ion exchange and superequivalent absorption that occur simultaneously is proposed, based on the stoichiometry of both processes. This approach allows us to calculate the activity coefficients of components of the sorbent phase and the thermodynamic constants of ion exchange and superequivalent absorption.     

On the Rodil–Vera method for determining ion activity coefficients

The basic principles of the method that Rodil and Vera described [E. Rodil, J.H. Vera, Fluid Phase Equilib. 205 (2003) 115–132] to calculate the liquid junction potential and to deduce ion activity coefficients from potentiometric data are critically discussed. It is shown that their procedure is based on an inconsistent loop, and the ion activity coefficients it yields are only an artefact of arbitrary assumptions, with no relationship to the real values, which remain unknown. To provide evidence of this fact, an identical procedure is applied to virtual data referring to a simulated potentiometric experiment with a hypothetical electrolyte whose ion activity coefficients are known; the procedure proves to be unable to recover these activity coefficients. The failure is irremediable and affects all activity coefficients of single ions, which have been reported by Vera and co-workers in the numerous papers they have published so far, whose conclusions lack any scientific support.     

Activities of Individual Ions From Infinite Dilution to Saturated Solutions

A new procedure, which provides a closer approximation for the junction potentials than the Henderson equation, is tested to reduce new emf data for the chloride ion in CsCl solutions and previously measured data for individual ions in aqueous solutions of KCl, NaCl, and NaBr. The liquid junction potential is calculated from numerical integration of its basic equation without assuming constant mobility or using concentrations instead of activities. The mean ionic activity coefficients of the salts, obtained from the activity coefficients of the individual ions, show good agreement with values reported in the literature. The activity coefficients of the individual chloride ion at 25°C in aqueous solutions of CsCl up to 3 molal and in KCl solutions were measured using a chloride ion-selective electrode. It has been confirmed that the activity of the chloride ion is equal to the activity of the cation in CsCl solutions and, contrary to the prediction of hydration theory, it is higher than the activity of the cation in aqueous KCl solutions. The New Hydration Theory has been developed to overcome the shortcomings of the older hydration theory and has been used to smooth the experimental activity coefficients of the individual ions in aqueous solutions and to extrapolate them up to the saturated solution.     

Osmotic and activity coefficients of perrhenic acid and perrhenate salts

Osmotic and activity coefficients are reported for the aqueous solution of perrhenic acid and for its lithium, sodium and tetramethylguanidinium slats at 25°C. These coefficients are similar in order of magnitude but smaller than the coefficients of the corresponding perchorates. Evidence is submitted for the ion pairing of the perrhenate ion with both hydronium and tetramethylguanidinium ions in fairly dilute aqueous solutions. The association with hydronium ion decreases in more concentrated solutions.     

Electron beam potential depression as an ion trap in Fourier transform ion cyclotron resonance mass spectrometry

Trapping of ions in the electron beam of a FTICR mass spectrometer is investigated and a simple model describing the confinement process is presented. Detection of resistive-wall destabilization of the magnetron motion of ions in the trapped-ion cell is used to determine conditions for ion trapping within and escape from the electron beam. The model predicts a potential well that is dependent on electron beam current, energy, and dimension in defining its capacity for low energy ions. Plots of ion retention time versus ion number are consistent with a model in which ions are initially trapped in the electron beam but with increasing ion formation will eventually overcome the potential depression in the electron beam and escape into magnetron orbits. Based upon this model, expressions are derived for ion retention time which are then fit to the experimental data. The model is used to estimate ion number, initial magnetron radius and ion cloud shape and density. One example in which electron trapping is important in the FTICR experiment is in the efficient transfer of ions between dual trapped-ion cells. Ion transfer within the potential depression of the electron beam environment is shown to be virtually 100% efficient over a 10 ms interval whereas all ions are lost to collisions with the conductance limit after 2 ms when transferring without the confining aid of the electron beam. Several analytical applications of electron traps in the ICR cell are now being investigated.     

Activity coefficients of concentrated strong and weak electrolytes by a hydration equilibrium and group contribution model

A new speciation-based group contribution model for activity coefficients is proposed to estimate the equilibrium properties of aqueous solutions containing electrolytes. The chemical part of the model accounts for the hydration equilibrium of water and ions with the formation of ion n-water complexes in a single stage process; the hydration number n and the hydration equilibrium constant K are the two independent parameters in this part. The physical part of the model is the UNIFAC group contribution model for short-range interactions. Each ion is considered as a group. Long-range interactions are accounted for by a Pitzer contribution (Debye–Hückel theory). The model is compared with experimental data at 25 °C including water activity, osmotic coefficients, activity coefficients, and pH of binary diluted and concentrated electrolyte solutions (up to 20 mol kg⁻¹ for NaOH, 16 mol kg⁻¹ for HCl, etc.).     

Experimental evaluation of single ion activities

When an ion-exchange membrane separates two electrolyte solutions having two different co-ions and a common counterion a bi-ionic potential (bi-co-ionic potential) appears across the membrane. If the membrane is ideally permselective for counterions and the activities of counterions on both sides of the membrane are equal to each other, the membrane potential Δψ becomes zero. We selected KCl as a reference electrolyte because of a symmetrical electrolyte in aqueous solutions. Then, the mean activity of ions in KCl may be assumed to be equal to each single ion activity at low molalities. Single ion activity in a test electrolyte containing K⁺ ion or Cl⁻ ion was estimated from the molality of KCl at Δψ=0 by interpolating bi-co-ionic potential to zero for KCl/membrane/LiCl, NaCl, C₆H₅COOK, or p-CH₂CHC₆H₄SO₃K systems. The values of single ion activity coefficients estimated in this work were fairly different not only from the mean activity coefficients of ions but also from the single ion activity coefficients estimated by Debye–Hückel formula.     

How ions affect the structure of water

We model ion solvation in water. We use the MB model of water, a simple two-dimensional statistical mechanical model in which waters are represented as Lennard-Jones disks having Gaussian hydrogen-bonding arms. We introduce a charge dipole into MB waters. We perform (NPT) Monte Carlo simulations to explore how water molecules are organized around ions and around nonpolar solutes in salt solutions. The model gives good qualitative agreement with experiments, including Jones-Dole viscosity B coefficients, Samoilov and Hirata ion hydration activation energies, ion solvation thermodynamics, and Setschenow coefficients for Hofmeister series ions, which describe the salt concentration dependence of the solubilities of hydrophobic solutes. The two main ideas captured here are (1) that charge densities govern the interactions of ions with water, and (2) that a balance of forces determines water structure: electrostatics (water's dipole interacting with ions) and hydrogen bonding (water interacting with neighboring waters). Small ions (kosmotropes) have high charge densities so they cause strong electrostatic ordering of nearby waters, breaking hydrogen bonds. In contrast, large ions (chaotropes) have low charge densities, and surrounding water molecules are largely hydrogen bonded.     

Molecular dynamics simulation of solution structure and dynamics of aqueous sodium chloride solutions from dilute to supersaturated concentration

Constant-temperature and constant-pressure (NpT) molecular dynamics simulations were performed to study the effects of salt concentration ranging from dilute to supersaturated concentrations on solution structure and dynamical properties of aqueous sodium chloride solutions at 298 K. The rigid SPC/E model was used for water molecules, and sodium and chloride ions were modeled as charged Lennard–Jones particles. Na⁺–Cl⁻ radial distribution functions showed the presence of contact ion pairs and solvent separated ion pairs. The coordination numbers of Na⁺–Cl⁻ ion pairs increased with salt concentration up to saturated concentration, although the number of contact ion pairs was almost constant in supersaturated regions. The tracer diffusion coefficients of both ions decreased with salt concentration up to saturated concentration, while that of sodium ion was almost constant in supersaturated regions. The tracer diffusion coefficients of both ions were therefore quite close to each other. The constant number of the contact ion pairs and the almost equality of the tracer diffusion coefficients of both ions would lead to the formation of clusters in supersaturated solutions.     

Activity coefficients of sodium,potassium, and nitrate ions in aqueous solutions of NaNO3, KNO3, and NaNO3+KNO3 at 25°C

Ion selective electrodes have been used to measure the activity coefficients at 25°C of individual ions in aqueous solutions of NaNO₃ up to 3.5 molal, KNO₃ up to 3.5 molal and mixtures of NaNO₃ and KNO₃ up to 2.4 molal total nitrate ion concentration. The experimental results confirm that the activity coefficient of anion and cation in aqueous single electrolyte solutions of NaNO₃ and KNO₃ were different from each other over the whole range of concentrations studied. These effects are attributed to the ion-ion and ion-solvent interactions. The results also show that the activity coefficients of nitrate ions in the presence of sodium and potassium counterions do not depend significantly on the nature of the counterions present in the solution. The experimental data obtained in this study were correlated by a model proposed previously.     

A three-variable model for the explanation of the "supercatalytic" effect of hydrogen ion in the chlorite-tetrathionate reaction

It has been shown that not only the slow direct but also the indirect (HOCl-catalyzed) reaction between chlorite and tetrathionate ions is second order with respect to hydrogen ion. Since the direct reaction was found to be orders of magnitude slower than the parallel HOCl-catalyzed pathway, a three-variable model is derived from the previously published five-step model taking into account the experimentally determined H+ concentration dependence of its rate coefficients by neglecting the direct reaction. The new three-variable model indicates that the "supercatalytic" effect of the hydrogen ion in the HOCl-catalyzed pathway arises from the pH dependence of the individual reactions of the five-step model. The new three-variable model also accounts for the continuous change of the stoichiometric ratio of the reactants and provides a simple kinetic law for involving it in the partial differential equation systems widely used in the study of spatiotemporal behavior of the chlorite-tetrathionate reaction.     

Colloid chemical characteristics of nanoporous glasses with different compositions in solutions of simple and organic electrolytes. 3. Calculation of electrochemical characteristics of membranes in terms of a homogeneous model

The electrosurface characteristics of nanoporous glass membranes–ion concentrations in pores with taking into account the specificity of counterions, electrokinetically mobile charge, the convective component of pore solution electrical conductivity, electroosmotic mobility of a liquid in the field of streaming potential and ion mobilities in pore space–were calculated within the homogeneous model. The effects of the type of counterion (sodium, potassium, ammonium, tetramethylammonium, and tetraethylammonium ions), solution concentration, glass composition, and pore size on the equilibrium and transport characteristics of membrane systems have been analyzed. A method for the determining of electrolyte activity coefficients in the membranes has been proposed.     

计算离子液体溶液汽液相平衡的分子热力学模型

用平均球近似理论、微扰理论和UNIFAC基团贡献方法分别考虑离子之间的长程静电作用、离子与溶剂之间的中程静电作用以及所有粒子之间的短程作用,本文提出了一种新的分子热力学模型,可用于离子液体溶液中溶剂活度系数的计算.通过对含烷基咪唑磷酸酯类离子液体与水、甲醇或乙醇组成的9个二元体系的饱和蒸汽压数据进行关联,获得了相关的模型参数,即溶剂的分子直径和基团之间的交互作用能参数.溶剂活度系数及饱和蒸汽压的计算结果与实验值的平均偏差为1.40%,符合良好,因此本模型可望用于含离子液体体系汽液相平衡的预测.     

298.15K下果糖水溶液中卤化钠单个离子活度系数

利用离子选择性电极和甘汞电极分别测定了NaX+果糖+水体系中的单个离子活度系数和离子平均活度系数.结果表明:基于Debye-Hückel扩展方程和Pitzer方程求得的单个离子活度系数彼此一致;由单个离子加合求得的与直接测定的平均离子活度系数也很一致;随果糖含量的增大,单个离子活度系数减小;在相同混合溶剂(果糖+水)和相等电解质浓度条件下,对于不同的NaX,Na+的活度系数大小顺序为NaFNaClNaBr.基于离子间的相互作用对结果进行了讨论.