Nernst-Planck Research Articles

Cs+ removal and optical detection by microporous lanthanide silicate Eu-AV-20 in a fixed-bed column

Microporous silicate Eu-AV-20 has recently proved to be a promising ion exchange material for cesium removal from aqueous solutions, and its potential for Cs+ photoluminescence sensing was additionally demonstrated. In this work, Cs+ removal was performed in a fixed-bed column, and the influence of linear velocity and mass of ion exchanger on the breakthrough curves was analysed. The experimental data were modelled on the basis of Nernst–Planck (NP) equations and with four well-known analytic models. The analytic expressions provided low errors (root mean square deviation, RMSD, between 3.20% and 6.47%); the 2-parameter NP-based model fitted the data quite well (RMSD=6.66% for correlation and 6.54% for prediction), yielding crucial information on both the transport mechanism within the Eu-AV-20 particles, and the intrinsic dynamic behaviour of the fixed-bed ion exchange column. Taking into account that Eu-AV-20 samples loaded with different amounts of Cs+ exhibited distinct photoluminescence spectra, our results reinforce the potential of AV-20 materials for Cs+ sensing, which raises the possibility of online monitoring the ion exchange in a fixed-bed column using an optical fibre and a spectrometer.

Transport Studies of Divalent Ions through Nafion-117 Ion Exchange Membrane in the Presence of Polyacrylate

The effect of binding of divalent ions (Mg2+ and Ba2+) with a polyacrylate anion on the Donnan membrane equilibrium distribution and the transport kinetics have been studied using Nafion-117 membrane. The extent of association of the divalent counterions with polyacrylate has been ascertained from the deviation from Donnan membrane equilibrium condition. The equilibrium concentrations of ions in polyacrylate compartment have been found to be in excess of expected equilibrium concentrations obtained from Donnan membrane equilibrium condition. This has been attributed to higher electrostatic interactions of divalent ions with the polyacrylate backbone compared to the monovalent ions. The strength of binding has been found to depend on the nature of divalent ion. Precipitation has been observed for Ba2+, thereby changing the total salt concentration of the polyacrylate compartment. Also, the strength of counterion binding has been found to depend on salt concentrations of the two compartments. The rate of transport of the ions with polyacrylate in the receiver solution has been calculated based on modified Nernst–Planck (NP) approach. The calculated transport kinetics have been found to agree with the experimental transport kinetics.

Transport Studies of Monovalent Ions through Nafion-117 Ion Exchange Membrane in Presence of Polyacrylate

The results of equilibrium distribution and transport kinetics of monovalent (Li+, Na+, and Cs+) ions through Nafion-117 ion exchange membrane in presence of polyacrylate anion have been presented. For Na+-Li+ system, equilibrium distributions in the presence and absence of polyacrylate have been found to be the same indicating that the Donnan membrane equilibrium is not affected by the presence of polyacrylate. For the Na+-Cs+ system, a favored transport of Cs+ ion to the polyacrylate compartment has been observed, indicating the binding of Cs+ ion with polyacrylate is stronger than other monovalent ions. This is due to the lower hydration radius of Cs+ ions. The deviation from Donnan membrane equilibrium condition has been used to obtain information about the fraction of counter-ions bound to the polyacrylate. The transport profile in the presence of polyacrylate has also been calculated based on the modified Nernst Planck (NP) approach, taking into account the fraction of counter-ions bound to the polyacrylate ion. The Donnan relation has been used to obtain the concentration of ions at the solution/membrane interface. The self-diffusion coefficients of ions and membrane ion exchange capacity have been given as input parameters in the calculations. The calculated time profile has been found to agree well with the experimental time profile.

Multiscale Multiphysics and Multidomain Models I: Basic Theory.

This work extends our earlier two-domain formulation of a differential geometry based multiscale paradigm into a multidomain theory, which endows us the ability to simultaneously accommodate multiphysical descriptions of aqueous chemical, physical and biological systems, such as fuel cells, solar cells, nanofluidics, ion channels, viruses, RNA polymerases, molecular motors and large macromolecular complexes. The essential idea is to make use of the differential geometry theory of surfaces as a natural means to geometrically separate the macroscopic domain of solvent from the microscopic domain of solute, and dynamically couple continuum and discrete descriptions. Our main strategy is to construct energy functionals to put on an equal footing of multiphysics, including polar (i.e., electrostatic) solvation, nonpolar solvation, chemical potential, quantum mechanics, fluid mechanics, molecular mechanics, coarse grained dynamics and elastic dynamics. The variational principle is applied to the energy functionals to derive desirable governing equations, such as multidomain Laplace-Beltrami (LB) equations for macromolecular morphologies, multidomain Poisson-Boltzmann (PB) equation or Poisson equation for electrostatic potential, generalized Nernst-Planck (NP) equations for the dynamics of charged solvent species, generalized Navier-Stokes (NS) equation for fluid dynamics, generalized Newton's equations for molecular dynamics (MD) or coarse-grained dynamics and equation of motion for elastic dynamics. Unlike the classical PB equation, our PB equation is an integral-differential equation due to solvent-solute interactions. To illustrate the proposed formalism, we have explicitly constructed three models, a multidomain solvation model, a multidomain charge transport model and a multidomain chemo-electro-fluid-MD-elastic model. Each solute domain is equipped with distinct surface tension, pressure, dielectric function, and charge density distribution. In addition to long-range Coulombic interactions, various non-electrostatic solvent-solute interactions are considered in the present modeling. We demonstrate the consistency between the non-equilibrium charge transport model and the equilibrium solvation model by showing the systematical reduction of the former to the latter at equilibrium. This paper also offers a brief review of the field.

Numerical Simulation of Electroosmotic Flow in Flat Microchannels with Lattice Boltzmann Method

In the present work, a new Lattice Boltzmann model has been developed to study the electroosmotic flows in a 2-D flat microchannel. The governing equations are presented in the continuum model, while a set of equivalent equations in Lattice Boltzmann model is introduced and solved numerically. In particular, the Poisson and the Nernst–Planck (NP) equations are solved by two new lattice evolution methods. In the analysis of electroosmotic flows, when the convective effects are not negligible or the electric double layers have overlap, the NP equations must be employed to determine the ionic distribution throughout the microchannel. The results of this new model have been validated by available analytical and numerical results. The model has also been used to examine the electroosmotic flows in a heterogeneous flat microchannel. Furthermore, the predictions of the NP model are compared with those of the Poisson Boltzmann (PB) equation to identify the applicability of the PB equations for such flows, which can also serve as validations for the present model.

Selective transport through a model calcium channel studied by Local Equilibrium Monte Carlo simulations coupled to the Nernst–Planck equation

We have recently introduced the Local Equilibrium Monte Carlo (LEMC) technique (Boda, Gillespie, J. Chem. Theor. Comput. 8 (2012) 824-829) in which a non-equilibrium system is divided into small volume elements and separate Grand Canonical Monte Carlo simulations are performed for each using a local intensive parameter, which, as soon as local equilibrium is assumed, can be identified with the local electrochemical potential. The simulation provides the concentration profiles of the steady-state diffuse system, where ions are transported through a membrane from one bulk compartment to the other. The dynamics of the ions is described with the Nernst–Planck (NP) transport equation. The NP equation is coupled to the LEMC simulations via an iteration procedure that ensures that conservation of mass (the continuity equation) is satisfied. We apply the method to a simple calcium channel model and demonstrate its efficiency. The computer experiments are inspired by real electrophysiological experiments for the Ryanodine Receptor calcium channel. The diffusion coefficients in the channel are fitted to results of Dynamic Monte Carlo simulations.

Membrane Transport of Several Ions During Peritoneal Dialysis: Mathematical Modeling

Peritoneal dialysis utilizes a complex mass exchange device created by natural permselective membranes of the visceral and abdominal muscle tissues. In mathematical modeling of solute transport during peritoneal dialysis, each solute is typically considered as a neutral, independent particle. However, such mathematical models cannot predict transport parameters for small ions. Therefore, the impact of the electrostatic interactions between ions on the estimated transport parameters needs to be investigated. In this study, transport of sodium, chloride, and a third ion through a permselective membrane with characteristics of the peritoneal transport barrier was described using two models: a model with the Nernst-Planck (NP) equations for a set of interacting ions and a model with combined diffusive and convective transport of each ion separately (DC). Transport parameters for the NP model were calculated using the pore theory, while the parameters for the DC model were estimated by fitting the model to the predictions from the NP model. Solute concentration profiles in the membrane obtained by computer simulations based on these two models were similar, whereas the transport parameters (diffusive mass transport parameters and sieving coefficients) were generally different. The presence of the third ion could substantially modify the values of diffusive mass parameter for sodium and chloride ions estimated using the DC model compared with those predicted by NP. The extent of this modification depended on the molecular mass and concentration of the third ion, and the rate of volumetric flow. Closed formulas for the transport parameters of the DC model in terms of the NP model parameters, ion concentration profiles in the membrane, and volumetric flow across the membrane were derived. Their reliable approximations, which include only boundary ion concentrations instead of spatial intramembrane concentration profiles, were formulated. The precision of this approximation was demonstrated by numerical simulations of the investigated three-ion system. Our modeling demonstrated that the fitted transport parameters depend not only on ion molecular weight but also on the characteristics and concentration of all other ions in the fluid as well as on the fluid flow rate through the membrane. Therefore, theoretical predictions of ion transport parameters need to take into account multi-ionic character of dialysis and body fluids. The transport parameters estimated using the DC model for one ion may vary with the ionic composition, ion concentrations in the fluids, and volumetric flow and may not reflect the theoretical description of diffusive and convective characteristics of single ion.

Simulation of steady-state diffusion: Driving force ensured by dual control volumes or local equilibrium Monte Carlo

We provide a systematic comparative analysis of various simulation methods for studying steady-state diffusive transport of molecular systems. The methods differ in two respects: (1) the actual method with which the dynamics of the system is handled can be a direct simulation technique [molecular dynamics (MD) and dynamic Monte Carlo (DMC)] or can be an indirect transport equation [the Nernst-Planck (NP) equation], while (2) the driving force of the steady-state transport can be maintained with control cells on the two sides of the transport region [dual control volume (DCV) technique] or it can be maintained in the whole simulation domain with the local equilibrium Monte Carlo (LEMC) technique, where the space is divided into small subvolumes, different chemical potentials are assigned to each, and grand canonical Monte Carlo simulations are performed for them separately. The various combinations of the transport-methods with the driving-force methods have advantages and disadvantages. The MD+DCV and DMC+DCV methods are widely used to study membrane transport. The LEMC method has been introduced with the NP+LEMC technique, which was proved to be a fast, but somewhat empirical method to study diffusion [D. Boda and D. Gillespie, J. Chem. Theor. Comput. 8, 824 (2012)]. In this paper, we introduce the DMC+LEMC method and show that the resulting DMC+LEMC technique has the advantage over the DMC+DCV method that it provides better sampling for the flux, while it has the advantage over the NP+LEMC method that it simulates dynamics directly instead of hiding it in an external adjustable parameter, the diffusion coefficient. The information gained from the DMC+LEMC simulation can be used to construct diffusion coefficient profiles for the NP+LEMC calculations, so a simultaneous application of the two methods is advantageous.

Numerical simulation of flow electrolysers: Effect of obstacles

This study aims at understanding the role the velocity profile can play in affecting the performance of the electorneutral bulk of a continuous flow electrolyser. Numerical simulations have been carried out by solving Navier–Stokes (NS) equations together with the Nernst–Planck (NP) equations. Performance of different geometries in which velocity profile has been altered by putting different types of obstacles has been evaluated. Performance of electrolysers having moving cathode has also been evaluated. Role played by inlet channel has also been investigated. The results obtained for different geometries suggest that it is possible to enhance the performance of the electroneutral bulk by putting obstacles in the flow path but this enhancement may not be very significant. An electrolyser with the cathode moving co-currently with the electrolyte is found to perform better than an electrolyser with cathode moving counter-currently with the electrolyte. Performance of an electrolyser with the inlet channel is found to be better than an electrolyser without inlet channel.

Steady-State Electrodiffusion from the Nernst-Planck Equation Coupled to Local Equilibrium Monte Carlo Simulations.

We propose a procedure to compute the steady-state transport of charged particles based on the Nernst-Planck (NP) equation of electrodiffusion. To close the NP equation and to establish a relation between the concentration and electrochemical potential profiles, we introduce the Local Equilibrium Monte Carlo (LEMC) method. In this method, Grand Canonical Monte Carlo simulations are performed using the electrochemical potential specified for the distinct volume elements. An iteration procedure that self-consistently solves the NP and flux continuity equations with LEMC is shown to converge quickly. This NP+LEMC technique can be used in systems with diffusion of charged or uncharged particles in complex three-dimensional geometries, including systems with low concentrations and small applied voltages that are difficult for other particle simulation techniques.

Variational multiscale models for charge transport.

This work presents a few variational multiscale models for charge transport in complex physical, chemical and biological systems and engineering devices, such as fuel cells, solar cells, battery cells, nanofluidics, transistors and ion channels. An essential ingredient of the present models, introduced in an earlier paper (Bulletin of Mathematical Biology, 72, 1562-1622, 2010), is the use of differential geometry theory of surfaces as a natural means to geometrically separate the macroscopic domain from the microscopic domain, meanwhile, dynamically couple discrete and continuum descriptions. Our main strategy is to construct the total energy functional of a charge transport system to encompass the polar and nonpolar free energies of solvation, and chemical potential related energy. By using the Euler-Lagrange variation, coupled Laplace-Beltrami and Poisson-Nernst-Planck (LB-PNP) equations are derived. The solution of the LB-PNP equations leads to the minimization of the total free energy, and explicit profiles of electrostatic potential and densities of charge species. To further reduce the computational complexity, the Boltzmann distribution obtained from the Poisson-Boltzmann (PB) equation is utilized to represent the densities of certain charge species so as to avoid the computationally expensive solution of some Nernst-Planck (NP) equations. Consequently, the coupled Laplace-Beltrami and Poisson-Boltzmann-Nernst-Planck (LB-PBNP) equations are proposed for charge transport in heterogeneous systems. A major emphasis of the present formulation is the consistency between equilibrium LB-PB theory and non-equilibrium LB-PNP theory at equilibrium. Another major emphasis is the capability of the reduced LB-PBNP model to fully recover the prediction of the LB-PNP model at non-equilibrium settings. To account for the fluid impact on the charge transport, we derive coupled Laplace-Beltrami, Poisson-Nernst-Planck and Navier-Stokes equations from the variational principle for chemo-electro-fluid systems. A number of computational algorithms is developed to implement the proposed new variational multiscale models in an efficient manner. A set of ten protein molecules and a realistic ion channel, Gramicidin A, are employed to confirm the consistency and verify the capability. Extensive numerical experiment is designed to validate the proposed variational multiscale models. A good quantitative agreement between our model prediction and the experimental measurement of current-voltage curves is observed for the Gramicidin A channel transport. This paper also provides a brief review of the field.

Dynamic model of ion and water transport in ionic polymer-metal composites

In the process of electro-mechanical transduction of ionic polymer-metal composites (IPMCs), the transport of ion and water molecule plays an important role. In this paper, the theoretical transport models of IPMCs are critically reviewed, with particular emphasis on the recent developments in the latest decade. The models can be divided into three classes, thermodynamics of irreversible process model, frictional model and Nernst-Planck (NP) equation model. To some extent the three models can be transformed into each other, but their differences are also obvious arising from the various mechanisms that considered in different models. The transport of ion and water molecule in IPMCs is compared with that in membrane electrode assembly and electrodialysis membrane to identify and clarify the fundamental transport mechanisms in IPMCs. And an improved transport model is proposed and simplified for numerical analysis. The model considers the convection effect rather than the diffusion as the major transport mechanism, and both the self-diffusion and the electroosmosis drag are accounted for in the water flux equation.

Polarization Effect of a Dielectric Membrane on the Ionic Current Rectification in a Conical Nanopore

Ionic current rectification (ICR) refers to a phenomenon that an ionic current flowing through a nanopore exhibits a preferential direction. In this paper, we investigate the ICR phenomenon in a conical nanopore embedded within a dielectric membrane using a continuum-based model, which is composed of the Nernst–Planck (NP) equations for the ionic mass transport, the Poisson equation for the electrostatics, and the Navier–Stokes (NS) equations for the flow field. Different from the existing studies, the emphasis of this investigation is placed on the polarization effect of the dielectric membrane on the ionic current and ICR in a conical nanopore. The results show that the polarization effect can influence the ion current and ICR, especially under the conditions of a relatively low κRt, which is the ratio of the nanopore tip radius and the Debye length, and a relatively low surface charge density of the nanopore.

Testing the Applicability of Nernst-Planck Theory in Ion Channels

The question of whether Nernst-Planck (NP) theory, which is a macroscopic method for calculating ion flux, is still valid in microscopic narrow ion channels has been remaining a mystery for some years. Recently, we tested the ability of the NP theory to accurately predict channel currents by combining and comparing the results with those of Brownian dynamics (BD) simulations. The extensive tests for simplified and realistic ion channels indicate that the NP theory is still applicable in narrow ion channels provided that accurate concentrations and potentials can be input into the NP equation properly, as the currents obtained from the combination of BD and NP match well with those obtained directly from BD simulations. Here, we show first results comparing NP calculations and molecular dynamics (MD) simulations that show promising agreement, further confirming the validity of the NP theory at the microscopic scale. This finding opens a door to utilizing the results of microscopic simulations in continuum theory which can provide an efficient way to calculate the ion flux in ion channels, and might stimulate further effort in this direction.

Limiting current density in the electrodialysis of multi-ionic solutions

The aim of this work was the development of an explicit model to estimate the limiting current density in the electrodialysis of dilute multi-ionic solutions. The model assumes that the ionic transport occurs in a film layer adjacent to an ion exchange membrane and it is quantified by a linearized form of the Nernst–Planck (NP) equations together with the electroneutrality requirement at the solution/ion exchange membrane interface. An explicit expression for the limiting current density of a dilute multi-ionic solution was derived, involving a mass-transfer coefficient based on an effective diffusivity of the multi-ionic solution. The model further assumes that the steric exclusion of the ions by the membrane is negligible, and thereby the limiting current density is attained when the concentration of each and every ion is null at the solution/membrane interface. The model predictions for the limiting current density were compared with experimental data of single salt solutions (MgCl 2) and multi-ionic solutions (MgSO 4 + MgCl 2) in a bench-scale electrodialysis unit (EUR2C-7P18, Eurodia, France) for various Reynolds numbers and salts concentrations. The average relative deviations between the model predictions and the experimental data were lower than 13% for MgCl 2 solutions (10, 20 equiv./m 3) and solutions of MgSO 4 + MgCl 2 (5 + 5, 10 + 10 equiv./m 3). The dimensionless limiting current density and the counterions transport numbers predicted by the linearized and non-linear Nernst–Planck equations were also compared for a broad range of dimensionless operating parameters and a fair to good agreement was observed between the two approaches.

A computational study of ion conductance in the KcsA K+ channel using a Nernst–Planck model with explicit resident ions

The biophysical mechanisms underlying the relationship between the structure and function of the KcsA K(+) channel are described. Because of the conciseness of electrodiffusion theory and the computational advantages of a continuum approach, the Nernst-Planck (NP) type models, such as the Goldman-Hodgkin-Katz and Poisson-NP (PNP) models, have been used to describe currents in ion channels. However, the standard PNP (SPNP) model is known to be inapplicable to narrow ion channels because it cannot handle discrete ion properties. To overcome this weakness, the explicit resident ions NP (ERINP) model was formulated, which applies a local explicit model where the continuum model fails. Then, the effects of the ERI Coulomb potential, the ERI induced potential, and the ERI dielectric constant for ion conductance were tested in the ERINP model. The current-voltage (I-V) and current-concentration (I-C) relationships determined in the ERINP model provided biologically significant information that the traditional continuum model could not, explicitly taking into account the effects of resident ions inside the KcsA K(+) channel. In addition, a mathematical analysis of the K(+) ion dynamics established a tight structure-function system with a shallow well, a deep well, and two K(+) ions resident in the selectivity filter. Furthermore, the ERINP model not only reproduced the experimental results with a realistic set of parameters, but it also reduced CPU costs.

Interdiffusion of Exchanging Counterions in Poly(perfluorosulfonic acid) Membrane

The kinetics of forward (Li+/Na+mem <=> Mn+aq) and reverse (Mn+mem <=> Li+/Na+aq) exchanges of metal ions(Mn+ ) Ag+, Cs+, Ba2+, and Eu3+) in the poly(perfluorosulfonic acid) membrane (Nafion-117) equilibrated with a well-stirred aqueous salt solution were experimentally measured using radioactivity tagged counterions. A numerical solution of the Nernst-Planck (N-P) equation for interdiffusion was used to interpret the kinetics of these exchanges. The experimentally measured kinetics of forward and reverse Na+ ion exchanges with Mn+ ions between the membrane and the equilibrating solution were found to be close to that predicted by the N-P equation. The minor differences between the experimental and predicted exchange rate profiles were attributed to change in water content of the poly(perfluorosulfonic acid) membrane in different ionic forms. The kinetics of Li+ ion exchanges with Na+ and Cs+ ions was found to be unusual as the self-diffusion coefficient D(ion)(m) of Li+ ions in the membrane were quite different in the forward and reverse exchanges. The values of D(ion)(m) for Li+ ion, obtained by the N-P equation, were found to be 2 x 10-6 and 0.2 x 10-6 cm2 s-1in Li+mem <=> Na+/Cs+aq and Na+/Cs+mem <=> Li+aq exchanges, respectively. The drastic differences in diffusion mobility of Li+ ions during forward and reverse exchanges was attributed to weak electrostatic interactions of Li+ ions with the fixed exchange sites as indicated by the reported equilibrium constant Kex of the Li+-exchange reaction in Nafion membrane. D(ion)(m) is high in the forward exchange due to minimum retardation in diffusion mobility of Li+ ions by the electrostatic forces in the membrane. In reverse exchange, Li+ diffusion mobility is retarded due to its slow replacement of counterions from the membrane.

Electroosmotic flow driven by oscillating zeta potentials: Comparison of the Poisson–Boltzmann model, the Debye–Hückel model and the Nernst–Planck model

We consider electroosmotic flows (EOF) generated by temporally varying zeta potential which is usually adopted for pumping or mixing of fluids. In this case the dynamics of ions in the electric double layer (EDL) influences the induced electric field and consequently the EOF significantly. Therefore, the appropriate model should be the Nernst–Planck (NP) model for all zeta potentials or the Debye–Hückel (DH) model for low zeta potentials rather than the Poisson–Boltzmann (PB) model which is based on the equilibrium distribution of ions in the EDL. In the present investigation, we compare the predictions from the DH model and the PB model with the exact ones from the NP model for a range of frequency of zeta potential oscillation. It is found that one may adopt the PB model when the frequency is low and the DH model when the zeta potential is low. However, for either high frequency of zeta potential oscillation or large value of zeta potential, one must adopt the NP model to get accurate predictions of EOF.

Comparative study of methods used to estimate ionic diffusion coefficients using migration tests

Ionic diffusion coefficients are estimated rapidly using electromigration tests. In this paper, electromigration tests are accurately simulated by numerically solving the Nernst–Planck (NP) equation (coupled with the electroneutrality condition (EN)) using the finite element method. Numerical simulations are validated against experimental data obtained elsewhere [E. Samson, J. Marchand, K.A. Snyder, Calculation of ionic diffusion coefficients on the basis of migration test results, Materials and Structures/Matériaux et Constructions 36 (257) (2003) 156–165., H. Friedmann, O. Amiri, A. Aït-Mokhtar, A direct method for determining chloride diffusion coefficient by using migration test, Cement and Concrete Research 34 (11) (2004) 1967–1973.]. It is shown that migration due to the non-linear electric potential completely overwhelms diffusion due to concentration gradients. The effects of different applied voltage differences and chloride source concentrations on estimations of chloride diffusion coefficients are explored. We show that the pore fluid within concrete and mortar specimens generally differs from the curing solution, lowering the apparent diffusion coefficient, primarily due to interactions of chloride ions with other ions in the pore fluid. We show that the variation of source chloride concentration strongly affects the estimation of diffusion coefficients in non-steady-state tests; however this effect vanishes under steady-state conditions. Most importantly, a comparison of diffusion coefficients obtained from sophisticated analyses (i.e., NP–EN) and a variety of commonly used simplifying methods to estimate chloride diffusion coefficients allows us to identify those methods and experimental conditions where both approaches deliver good estimates for chloride diffusion coefficients. Finally, we demonstrate why simultaneous use and monitoring of current density and fluxes are recommended for both the non-steady and steady-state migration tests.

Nernst–Planck analysis of propagating reaction-diffusion fronts in the aqueous iodate–arsenous acid system

Propagating fronts can be generated in solution by combining diffusion and chemical reactions with an autocatalytic feedback mechanism. Front propagation is usually analyzed in terms of the rate equations for the chemical reactions and Fick's laws of molecular diffusion. In practice, however, reaction-diffusion fronts are known mainly for aqueous electrolyte solutions. A more accurate description of front propagation in these systems is developed by using Nernst-Planck (NP) transport equations. This treatment includes diffusion fluxes driven by the concentration gradients and, for the ionic species, the migration fluxes driven by the electric field which is generated internally by the diffusion of ions of different mobility. NP equations are used to describe propagating fronts for the iodate oxidation of aqueous arsenous acid. The analysis provides a detailed picture of front structure and propagation, including concentration profiles, reaction rate profiles and velocity profiles for the solution species. After a short induction period, fully-developed fronts reach steady velocities and the profiles across the fronts transformed from laboratory coordinates to the frame of reference moving with the front become time-independent. The velocities of the autocatalytic I(-) ions ahead of the fronts are nearly identical to the steady front velocities. Electric fields generated by ionic diffusion across the fronts reach maximum strengths of about 0.4 V cm(-1), producing ion migration velocities as large as 50% of the front velocities.