Application of the Schulze–Hardy rule to haematite and haematite/humate colloid stability

The applicability of the Schulze–Hardy rule to haematite and haematite/humate mixtures has been investigated by measuring the rate of fast and slow coagulation after the addition of increasing concentrations of mono, di and trivalent ions. Haematite colloids follow the rule for mono, di and trivalent ions. Haematite colloids coated with humate generally follow the rule for mono and di valent ions with the exception of copper ions. Trivalent ions do not follow the Schulze–Hardy rule because of their relatively strong complexation with humate. The size of the counter ion has an effect on the de-stabilising ability of ions of the same charge.     

Ions in a binary asymmetric dipolar mixture: mole fraction dependent Born energy of solvation and partial solvent polarization structure

Mean spherical approximation (MSA) for electrolyte solution has been extended to investigate the role of partial solvent polarization densities around an ion in a completely asymmetric binary dipolar mixture. The differences in solvent diameters, dipole moments, and ionic size are incorporated systematically within the MSA framework in the present theory for the first time. In addition to the contributions due to difference in dipole moments, the solvent-solvent and ion-solvent size ratios are found to significantly affect the nonideality in binary dipolar mixtures. Subsequently, the theory is used to investigate the role of ion-solvent and solvent-solvent size ratios in determining the nonideality in Born free energy of solvation of a unipositive rigid ion in alcohol-water and dimethyl sulfoxide-acetonitrile mixtures, where the solvent components are represented only by their molecular diameters and dipole moments. Nonideality in Born free energy of solvation in such simplified mixtures is found to be stronger for smaller ions. The slope of the nonideality for smaller alkali metal ions in methanol-water mixture is found to be opposite to that for larger ion, such as quaternary tertiary butyl ammonium ion. For ethanol-water mixtures, the slopes are in the same direction for all the ions studied here. These results are in qualitative agreement with experiments, which is surprising as the present MSA approach does not include the hydrogen bonding and hydrophobic interactions present in the real mixtures. The calculated partial polarization densities around a unipositive ion also show the characteristic deviation from ideality and reveal the microscopic origin of the ion and solvent size dependent preferential solvation. Also, the excess free energy of mixing (in the absence of any ion) for these binary mixtures has been calculated and a good agreement between theory and experiment has been found.     

Negative rejection of ions in pressure-driven membrane processes

Negative rejections of ions in pressure-driven membrane processes can be caused by several distinct mechanisms. In a number of cases, in a final count, the phenomenon is brought about by increased concentration of an ion in the membrane phase. In the case of charged membranes, the increased concentration has to be accompanied by a weakening of electric field of filtration potential, which normally retards counter-ions and prevents the increased concentrations from manifesting themselves in negative rejections. This occurs in charge-mosaic membranes due to the so-called current circulation phenomenon or in electrolyte mixtures due to the presence of more mobile counter-ions. Negative rejections can also occur for ions whose concentration is decreased in the membrane phase. This occurs in electrolyte mixtures due to the acceleration of such ions by the electric field of diffusion potential arising because of strong rejections of other mixture components. This phenomenon is most pronounced for single-charge ions in the presence of predominant amounts of ions of higher charge of the same sign. All those mechanisms are considered within the scope of a common theoretical framework. An attempt is made of a tentative classification of mechanisms of negative rejections. An overview of available literature data is provided and it is shown that in a number of cases the published information is not sufficiently detailed for a reliable identification of the mechanisms. It is concluded that the studies of negative rejections could be a valuable membrane characterization tool but they need to be more systematic and targeted to fulfil this role.     

Self-diffusion coefficients of ions in the presence of charged obstacles

The self-diffusion coefficient of ions of the charge- and size-symmetric +1:-1 (or +2:-2) electrolyte was studied in the presence of ionic obstacles (matrix) representing disordered media. For this purpose the Brownian dynamics method was used, complemented with the replica Ornstein-Zernike theory for the partly-quenched systems. The matrix was prepared by a rapid quench of the size-symmetric +1:-1 (in few cases also of +2:-2) electrolyte solution being in equilibrium at (temperature, relative permittivity) T0, epsilon. Within the matrix the charge- and size-symmetric (+1:-1 or +2:-2) electrolyte at T1, epsilon1 was distributed. This component was fully mobile (annealed) and in thermodynamic equilibrium with the matrix. In this study a special attention was paid to the self-diffusion of the annealed ions. The ratio D/D degrees, where D degrees is the self-diffusion coefficient of ions at infinite dilution, has been studied for various model parameters varying the concentration of all species in the system. The presence of charged obstacles decreases the self-diffusion of the annealed electrolyte; the D/D degrees values are lower in the partly-quenched mixtures than in the fully annealed electrolyte of the same concentration. In the investigated range of concentrations and solvent dielectric constants, the D/D degrees values first increased with the increased concentration of annealed electrolyte present and then decreased. An increase of the strength of the Coulomb interaction between annealed ions, or between annealed and quenched charges, yielded a decrease of the self-diffusion. In the range of concentrations investigated in this work, the decrease is mainly due to the Coulomb interaction with the matrix, since the presence of neutral obstacles did not modify the diffusion properties with respect to the situation without obstacles.     

Density Prediction for Aqueous Multicomponent Solutions Obeying Isopiestic Relation and Isopycnotic Mixing Rule

The density predictive method based on isopiestic relation has been revised. The accuracy of density prediction based on the isopiestic mixing rule has been compared with that based on the isopycnotic mixing rule on ten aqueous ternary electrolyte systems, one quaternary electrolyte system, and one aqueous ternary nonelectrolyte system at 25°C. Of the ten ternary electrolyte systems, five systems show better predictive accuracy by the isopycnotic method, two by isopiestic relation, and three systems show comparable results. For the aqueous quaternary electrolyte and ternary nonelectrolyte systems, the isopycnotic method gives better predictive accuracy. The overall average error of density prediction based on isopiestic mixing rule is 0.00031, while that based on isopycnotic mixing rule is 0.00017.     

A new surface structural approach to ion adsorption: tracing the location of electrolyte ions

Electrolyte ions differ in size leading to the possibility that the distance of closest approach to a charged surface differs for different ions. So far, ions bound as outersphere complexes have been treated as point charges present at one or two electrostatic plane(s). However, in a multicomponent system, each electrolyte ion may have its own distance of approach and corresponding electrostatic plane with an ion-specific capacitance. It is preferable to make the capacitance of the compact part of the double layer a general characteristic of the solid-solution interface. A new surface structural approach is presented that may account for variation in size of electrolyte ions. In this approach, the location of the charge of the outersphere surface complexes is described using the concept of charge distribution in which the ion charge is allowed to be distributed over two electrostatic planes. It was shown that the concept can successfully describe the pH dependent proton binding and the shift in the isoelectric point (IEP) in the presence of variety of monovalent electrolyte ions, including Li(+), Na(+), K(+), Cs(+), Cl(-), NO(-)(3), and ClO(-)(4) with a common set of parameters. The new concept also sheds more light on the degree of hydration of the ions when present as outersphere complexes. Interpretation of the charge distribution values obtained shows that Cl(-) ions are located relatively close to the surface. The large alkali ions K(+), Cs(+), and Rb(+) are at the largest distance. Li(+), Na(+), NO(-)(3), and ClO(-)(4) are present at intermediate positions.     

Semi-ideal solution theory. 2. Extension to conductivity of mixed electrolyte solutions

Conductivities were measured for the ternary systems NaCl-LaCl(3)-H(2)O and KCl-CdCl(2)-H(2)O and their binary subsystems NaCl-H(2)O, KCl-H(2)O, CdCl(2)-H(2)O, and LaCl(3)-H(2)O at 298.15 K. The semi-ideal solution theory for thermodynamic properties of aqueous solutions of electrolyte mixtures was used together with the Eyring absolute rate theory to study conductivity of mixed electrolyte solutions. A novel simple equation for prediction of the conductivity of mixed electrolyte solutions in terms of the data of their binary solutions was established. The measured conductivities and those reported in literature were used to test the newly established equation and the generalized Young's rule for conductivity of mixed electrolyte solutions. The comparison results show that the deviation of a ternary solution from the new conductivity equation is closely related to its isopiestic behavior and that the deviations are often within experimental uncertainty if the examined system obeys the linear isopiestic relation. While larger deviations are found in the system with large ion pairing effect, the predictions can be considerably improved by using the parameters calculated from its isopiestic results. These results imply that the previous formulation of the thermodynamic properties of aqueous solutions of electrolyte mixtures has a counterpart for transport properties.     

Modified McKay–Perring Equation and Its Two Particular Solutions

Zdanovskii’s rule is the simplest isopiestic molality relation of mixed electrolyte aqueous solutions and the McKay–Perring equation is a differentio-integral equation particularly suitable for calculating solute activity coefficients from isopiestic measurements. However, they have two unsolved problems, which have puzzled solution chemists for several decades: (1) Zdanovskii’s rule has been verified by precise isopiestic measurements. But, several scientists suggested that the rule contradicts the Debye–Hückel limiting law for extremely dilute unsymmetrical mixtures. (2) In the McKay–Perring equation, a solute activity coefficient is multiplied by a solute composition variable. Different scientists have suggested that the composition variable may be the total ionic strength, common ion concentration, total ionic concentration, or an additive function with arbitrary proportionality constants. But, the different choices of the composition variable may lead to different activity coefficient results. Here, I derive a modified McKay–Perring equation in which the composition variable has the exclusive physical meaning of total ionic concentration for mixed electrolyte solutions (or of total solute particle concentration for the mixed solutions containing nonelectrolyte solutes). I also demonstrate that Zdanovskii’s rule is consistent with the Debye–Hückel limiting law for extremely dilute unsymmetrical mixtures. I derive two particular solutions of the modified McKay–Perring equation: one for the systems obeying Zdanovskii’s rule and another for the systems obeying a limiting linear concentration rule. These theoretical results have been verified with literature experiments and model calculations.     

A Molecular Dynamics study of short-chain polyelectrolytes in explicit water: Toward the origin of ion-specific effects

We present all atom explicit water Molecular Dynamics simulation results for the structure of short (six monomer units) aliphatic ionenes in mixtures with a low-molecular weight electrolyte. The SPC/E model was used to describe water. Long-range effects were taken into account via the Ewald summation procedure. The results indicate that strongly hydrated ions such as F⁻ cannot approach the nitrogen atom on the oligoion. In contrast, weakly hydrated I⁻ ions accumulate close to the oligoion. These findings are consistent with the thermodynamic data and ongoing measurements of the dielectric relaxation in ionene solutions.     

等压法测定Li2SO4-MgSO4-H2O体系的渗透和活度系数

在25 ℃下, 离子强度范围分别为0.2—8.7、0.6—12.7和1.4—13.5 mol·kg~(-1)时, 测定了纯Li_2SO_4、MgSO_4及Li_2SO_4-MgSO_4混盐水溶液的渗透系数. 计算了Pitzer方程和Scatchard方程的离子作用参数. 用上述两种方程及由热力学关系式直接推导出的Mckay-Perring方程计算并比较了Li_2SO_4和MgSO_4在混合溶液中的平均活度系数, 三者在实验误差范围内一致. 利用获得的Pitzer参数计算该体系的溶解度与文献值基本一致.     

Self‐concentration effects in preparative SEC of mineral electrolytes using nanoporous neutral polymeric sorbents

Separation of mineral electrolytes according to size exclusion mechanism using neutral nanoporous polystyrene sorbents and carbonaceous materials has been examined in detail on hand of HCl and CaCl₂ solutions, taken separately and in the mixture. Phase distribution coefficients of the above electrolytes have been measured under static conditions at different concentrations. The k‐values and their dependence on the concentration were correlated with the elution curves of the components from a chromatographic column. Both the static and dynamic data suggest a strong dependence of the hydration number and effective size of ions on the concentration. Self‐concentration of HCl in small pores and that of CaCl₂ outside of small pores exhibits itself in both static and chromatographic experiments and results from the relocation of the components largely within the volume of the initial mixture. Additional apparent “retention” of HCl in the porous volume of the sorbent in concentrated electrolyte mixtures was shown to be caused by the competition between all ions of the system for hydration water. Increased separation selectivity and enhanced self‐concentration effects for more concentrated mixtures point to preparative and industrial perspectives of size exclusion chromatography (SEC) of complex electrolyte mixtures on nanoporous neutral sorbents.     

Modelling of pressure-driven membrane separation of electrolytes.: `High temperature' approximation

A model of pressure-driven membrane process of electrolyte separation is presented. The electric field potential assumed as being known, exact solution for permeate composition is readily obtained. All species are assumed to have the same convection velocity. Local electroneutrality condition is not used. The electric potential has been taken into account under high temperature approximation, thus reducing the problem to algebraic equation in exp(Ψ), where Ψ is dimensionless flow potential, and making it possible to calculate concentrations of ions in permeate. Negative retention is shown to be possible for one-component electrolyte solution. For electrolyte mixtures, concentration of ion with high charge is shown to “govern” the membrane selectivity in respect to low-charge ions. Results obtained are in qualitative accordance with the earlier experimental data on membrane separation of reaction mixtures in homogeneous catalysis.     

流动注射萃取火焰原子吸收光谱法测定硫酸镍电解液中微量铅

本文采用流动注射在线萃取火焰原子吸收光谱法测定硫酸电解液中微量铅，考察了水相介质，共存离子，萃取装置，相比等因素的影响。分析含铅１．４ｍｇ／Ｌ和０．１６ｍｇ／Ｌ的试样，分析值的相对标准偏差（ｎ＝１０）分别为３．９％和１２．３％，方法检出限为０．０２４ｍｇ／Ｌ。     

On the validity of the correlation – Belda equation for some physical and chemical properties in 1,4-dioxane + water mixtures

Knowledge and prediction of physicochemical properties of binary mixtures is of great importance in understanding intermolecular interactions. The literature shows that most such systems exhibit non-linear behaviour. In this frame, a correlating equation was recently proposed by Belda and tested with success on 50 binary systems for density, viscosity, surface tension and refractive index. In order to assess the validity of the proposed equation, it has been applied to 1,4-dioxane + water mixtures at 298.15?K for nine physicochemical properties. These mixtures exhibit strong interactions. The results were fitted and compared with the Redlich–Kister (R–K) equation of the same number of parameters. The equation seems to offer better results for some properties than those of R–K with two free parameters.     

Development of capillary electrophoresis for the determination of metal ions using mixed partial and complete complexation techniques

A new capillary electrophoretic (CE) method was developed for the selective and sensitive determination of common metal ions. The proposed method is based on conventional CE separation of metal cations followed by complete complexation of separated analytes with 1,10-phenanthroline using the zone-passing technique. This approach combines both partial and complete complexation modes and, thus, enables rapid, selective, efficient separation together with sensitive direct UV detection of metal species. The optimal conditions for the separation and derivatization reaction were established by varying type of electrolyte, electrolyte pH, introduction time and concentration of 1,10-phenanthroline. The optimized separations were carried out in 50 mmol l(-1) glycolic acid electrolyte (pH 6.0 with imidazole) using direct UV detection at 254 nm. Five common metal cations (Fe2+, Co2+, Ni2+, Cu2+ and Zn2+) were separated in less than 4 min. The proposed system was applied to the determination of Fe(II) and Zn(II) in snow samples. The recovery tests established for snow samples were within the range 100+/-12%.     

Electrosorption of ions from aqueous solutions by nanostructured carbon aerogel

Electrosorption is generally defined as potential-induced adsorption on the surface of charged electrodes. After polarization of the electrodes, ions are removed from the electrolyte solution by the imposed electric field and adsorbed onto the surface of the electrodes. Experimental and modeling studies were conducted using two types of carbon aerogel composites of different surface areas to provide a better understanding on the mechanisms of electrosorption. The experimental results revealed that no significant specific adsorption of F- ions occurred, while strong specific adsorption was observed for NO3- and Cu2+ ions. In addition, although the two types of carbon aerogel electrodes had different surface areas, their capacities were found to be very similar because of the electrical double-layer overlapping effect in micropores. An electrical double-layer model developed in our previous work (16), in which the electrical double-layer overlapping correction is included, is expanded in the present work by considering the effect of the specific adsorption on the electrosorption process. Modeling results were compared with experimental data obtained under various conditions. When the overlapping effect and specific adsorption were considered, the model provided results that were in good agreement with experimental data.     

Modification of PSRK mixing rules and results for vapor–liquid equilibria, enthalpy of mixing and activity coefficients at infinite dilution

The predictive Soave–Redlich–Kwong (PSRK) equation of state (EOS) is a well-established method for the prediction of thermodynamic properties required in process simulation. But there are still some problems to be solved, e.g. the reliability for strong asymmetric mixtures of components which are very different in size. The following modifications are introduced in the PSRK mixing rules: the Flory–Huggins term in the mixing rule for the EOS parameter a, and the combinatorial part in the UNIFAC model are skipped simultaneously; a nonlinear mixing rule for the EOS parameterb, instead of the linear mixing rule, is proposed. With these two modifications better results are obtained for vapor–liquid equilibria and activity coefficients at infinite dilution for alkane–alkane systems, specially for asymmetric systems. In order to obtain better results for enthalpy of mixing, temperature-dependent parameters are used. Group interaction parameters have been fitted for several groups, and the results are compared with the Modified UNIFAC (Dortmund), and the PSRK methods.     

Simple regression model using an optode for the simultaneous determination of zinc and cadmium mixtures in aqueous samples.

An optical-fiber chemical sensor (optode) is described for the simultaneous determination of Zn2+ and Cd2+ mixtures using 2-(5-bromo-2-pyridylazo)-5-(diethylamino)phenol immobilized onto XAD-4 (Br-PADAP/XAD-4). A simultaneous analysis was achieved by using a single-wavelength kinetic approach coupled with a regression model. Here, the regression model was derived for the slope response of Br-PADAP/XAD-4 to binary mixtures of the metal ions, Zn2+ and Cd2+, at pH 8. The model was used then to predict the concentration of one metal ion while the other was kept constant. This was found to be reasonably successful for Zn2+ and Cd2+, with deviations of 3% and 10%, respectively. The sensor was also found to provide excellent sensitivity for both metal ions with excellent LODs (< 0.1 ppm) and very short analysis times (< 30 s).     

Debye-Hückel theory for mixtures of rigid rodlike ions and salt

Like-charged surfaces are able to attract each other if they are embedded in an electrolyte solution of multivalent rodlike ions, even if the rods are long. To reproduce this ability the Poisson-Boltzmann model has recently been extended so as to account for the rodlike structure of the mobile ions. Our model properly accounts for intraionic correlations but still neglects correlations between different rodlike ions. For sufficiently long rods, the model shows excellent agreement with Monte Carlo simulations and exhibits two minima - a depletion and a bridging minimum - in the interaction free energy. In the present work, we generalize the Poisson-Boltzmann model to systems with polydisperse rod lengths and arbitrary charge distributions along the rods, including the presence of salt. On the level of the linearized Debye-Hu?ckel model we derive a general criterion for whether an electrolyte with given distribution of rodlike ions is able to mediate attraction between like-charged surfaces. We numerically analyze two special cases, namely the influence of salt on symmetric and asymmetric mixtures of monodisperse rodlike ions. The symmetric mixture is characterized by the presence of both negatively and positively charged (but otherwise identical) rodlike ions. For the asymmetric mixture, the system contains rodlike ions of only one type. We demonstrate that the addition of salt retains the depletion minimum but tends to eliminate the bridging minimum.