Ion-transfer reaction

The distribution of ions in a biphasic system is given by the Nernst equation  

As for a classical redox reaction on a solid electrode it is possible, by controlling the applied potential difference, to determine the ratio between reactants and products, here the partition of the ions between the two phases (/, = a°/af). As for a redox reaction described by the Nernst equation in a better known form, 

If the thermodynamic aspects of ion partitioning are rather well understood, the kinetic aspects of the ion transfer reactions still pose challenging questions. Over the years, as 

In ion transfer reactions the transfer of an ion or proton from the solution to the surface of an electrode is one elementary step. It is often accompanied by either total discharge (e.g., deposition of a metal ion on a metal electrode of the same composition) or partial discharge (e.g., adsorption of halide ions see also below). While for outer sphere electron transfer the reaction coordinate describes the solvent reorganization, the reaction coordinate for ion transfer reactions is associated with the motion of the ion. The rate-determining step in an ion transfer reaction is often the adsorption step of the ion on the electrode, which involves the penetration of the barrier formed by the adsorbed solvent (see, e.g.. Ref. 2 and section 5.2 for a discussion). 

Contrary to outer sphere electron transfer reactions, the validity of the Butler-Volmer law for ion transfer reactions is doubtful. Conway and coworkers [225] have collected data for a number of proton and ion transfer reactions and find a pronounced dependence of the transfer coefficient on temperature in all cases. These findings were supported by experiments conducted in liquid and frozen aqueous electrolytes over a large temperature range [226, 227]. On the other hand, Tsionskii et al. [228] have claimed that any apparent dependence of the transfer coefficient on temperature is caused by double layer effects, a statement which is difficult to validate because double layer corrections, in particular their temperature dependence, depend on an exact knowledge of the distribution of the electrostatic potential at the interface, which is not available experimentally. Here, computer simulations may be helpful in the future. Theoretical treatments of ion transfer reactions are few they are generally based on variants of electron transfer theory, which is surprising in view of the different nature of the elementary act [229]. 

In the remainder of this subsection the results of recent studies of the diabatic free 

For distances smaller than about 4 A from the surface, decreases for the neutral atom. Once the atom has penetrated the compact surface layer and dislocated some of the adsorbed water molecules, the system is stabilized by pushing the atom towards the surface. The behavior is an example of hydrophobic interactions where the insoluble 1° atom is pushed out of the aqueous phase. As it cannot be pushed into the solid phase, the contact adsorbed geometry is the more favorable arrangement. In order to keep the model simple, for the ion and the atom has been combined with the same ab initio interaction energy obtained for I -Pt9 clusters 

To obtain the total free energy curves experienced by the ion and by the atom the interaction energy of the particles with the metal are added. Since the MD simulations yield only free energy differences relative to the bulk state, the relative energies of the two bulk levels have to be obtained by going through a cycle that decomposes the reaction 1 — 1° + e into a series of steps  

Electrical current flows at the oxide-electrolyte interface at a rate fixed by electrochemical kinetics charge can be transferred across this interface either by ions or electrons. While electrons are transferred between the electrode and the electrolyte only if acceptor/donor redox species are available in solution, solvated ions can be transferred to or from the solution, 

The free energy barrier for the flow of ionic charge across the oxide electrode-electrolyte interface has an electrical contribution and consequently the reaction rate can be formally described by Butler-Volmer-type equations [38]. The cation current density corresponding to the process 

Here a is the effective cationic transfer coefficient, X is the complexing agent, and r and r are reaction orders the reaction order in cations is assumed to be one. The subscript indices in front of the chemical symbols and activity terms (a) denote electrolyte (3) and oxide (2), while (1) is reserved for the underlying metal. Another cationic reaction at the oxide solu-tion interface is the transfer of hydrogen ions 

The anion current density corresponding to transfer of oxygen ions between the oxide electrode and the aqueous solution is 

Dissolution deposition Tafel plots of the partial ionic current densities as a function of the oxide electrode potential are schematically represented in Fig. 2 [31,32], 

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Despite this, they are good solvents for chloride-ion transfer reactions, and solvo-acid-solvo-base reactions (p. 827) can be followed conductimetri-cally, voltametrically or by use of coloured indicators. As expected from their constitution, the trihalides of As and Sb are only feeble electron-pair donors (p. 198) but they have marked acceptor properties, particularly towards halide ions (p. 564) and amines. 

Fluoride ion transfer reactions have not been established for FBr03 and may be unlikely, (see p. 879). 

Intercalation chemistry electron/ion transfer reactions. R. Schollhom, Comments Inorg. Chem.,... 

The pulsed source method, despite several limitations, appears to be a very useful technique for studying ion-molecule reactions at thermal energies. Although the studies to have date been limited primarily to simple hydrogen transfer reactions, the technique should also prove useful for studying charge transfer and hydride ion transfer reactions at thermal energies. 

SchoUhom R (1984) Electron/ion-transfer reactions of soUds with different lattice dimen-sionaUty. Pure Appl Chem 56 1739-1752... 

Studies of the polarized IBTILE provide a fundamental knowledge that makes it possible to explain phenomena occurring at the membranes of ion-selective electrodes. In addition, the rates of ion transfer and assisted ion transfer reactions are proportional to concentrations, which is a basis of an amperometric ion-selective (sensitive) electrode. 

The field of electrochemical ion transfer reactions (EITRs) is relatively recent compared with that of electron transfer reactions, and the application of molecular dynamics simulations to study this phenomenon dates from the 1990s. The simulations may shed light on various aspects of the EITR. One of the key questions on this problem is if EITR can be interpreted in the same grounds as those employed to understand electron transfer reactions (ETRs). Eor example, let us consider the electrochemical oxidation reaction of iodine ... 

In this chapter, we wiU review electrochemical electron transfer theory on metal electrodes, starting from the theories of Marcus [1956] and Hush [1958] and ending with the catalysis of bond-breaking reactions. On this route, we will explore the relation to ion transfer reactions, and also cover the earlier models for noncatalytic bond breaking. Obviously, this will be a tour de force, and many interesting side-issues win be left unexplored. However, we hope that the unifying view that we present, based on a framework of model Hamiltonians, will clarify the various aspects of this most important class of electrochemical reactions. 

In a simple ion transfer reaction, the distance of the reactant to the surface changes, and it becomes quite strong when it is actually in contact with the metal. Thus, a full description requires a good treatment of the interaction both with the solvent and with the metal. Nevertheless, the energy of activation is mainly determined by the partial... 

Pecina O, Schmickler W. 1998. On the dynamics of electrochemical ion-transfer reactions. J Electroanal Chem 450 303-311. 

Schmickler W. 1995. A unified model for electrochemical electron and ion transfer reactions. Chem Phys Lett 152-160. 

Impedance Spectroscopy, 21-22 Inner sphere electron transfer, 47-48 Ion transfer reaction, 39-40... 

Charge transfer reactions at ITIES include both ET reactions and ion transfer (IT) reactions. One question that may be addressed by nonlinear optics is the problem of the surface excess concentration during the IT reaction. Preliminary experiments have been reported for the IT reaction of sodium assisted by the crown ether ligand 4-nitro-benzo-15-crown-5 [104]. In the absence of sodium, the adsorption from the organic phase and the reorientation of the neutral crown ether at the interface has been observed. In the presence of the sodium ion, the problem is complicated by the complex formation between the crown ether and sodium. The SH response observed as a function of the applied potential clearly exhibited features related to the different steps in the mechanisms of the assisted ion transfer reaction although a clear relationship is difficult to establish as the ion transfer itself may be convoluted with monolayer rearrangements like reorientation. 

Therefore the lattice-gas model has proved most useful for the study of those processes in which the ionic double layer plays a major role, and there are quite a few. So it has been used to investigate the interfacial capacity, electron and ion-transfer reactions, and even such complex processes as ion pairing and assisted ion transfer. Because of its simplicity we carmot expect this model to give quantitative results for particular systems, but it is ideally suited to qualitative investigations such as the prediction of trends and orders of magnitude for various effects. 

In the following we will review recent applications of the lattice-gas model to liquid-liquid interfaces. We will start by presenting the basics of the model and various ways of treating its statistical mechanics. Then we will present model calculations for interfacial properties and for electron- and ion-transfer reactions. It is one of the virtues of the lattice-gas model that it is sufficiently flexible to serve as a framework for practically all processes at these interfaces. 

In the two bulk phases the potential of mean force is constant, but it may vary near the interface. The difference in the bulk values of the chemical part is the free energy of transfer of the ion, which in our model is —2mu (we assume u < 0). Let us consider the situation in which the ion-transfer reaction is in equilibrium, and the concentration of the transferring ion is the same in both phases the system is then at the standard equilibrium potential 0oo- In Ihis case the potential of mean force is the same in the bulk of both phases the chemical and the electrostatic parts must balance ... 

Unfortunately, at the present time the experimental results for ion-transfer reactions are contradictory, so that it is not possible to verify the predictions of this model. Also, this model is only valid if the rate is determined by the ion-transfer step, and not by transport, and if the concentration of the supporting electrolyte is sufficiently low so that the extension of the space-charge regions is less than the width X of the region where the two solvents mix. These conditions are not always fulfilled in experiments. 

As we have noted in Section IV.D, there are no reliable experimental data for ion-transfer reactions, so that a comparison with experiment is not possible at this time. 

Unfortunately the development of models is hindered by a lack of reliable experimental data. For example, the rates of ion-transfer reactions measured at different times and by different groups vary widely. Also, it has been suggested that the high interfacial capacities that are measured in certain systems are an experimental artifact [13]. While this is frustrating for the researcher who wants to decide between competing models, it can also be viewed as a sign that the electrochemistry of liquid-liquid interfaces is an active field, where fundamental issues are just being explored. 

Heterogeneous ET reactions at polarizable liquid-liquid interfaces have been mainly approached from current potential relationships. In this respect, a rather important issue is to minimize the contribution of ion-transfer reactions to the current responses associated with the ET step. This requirement has been recognized by several authors [43,62,67-72]. Firstly, reactants and products should remain in their respective phases within the potential range where the ET process takes place. In addition to redox stability, the supporting electrolytes should also provide an appropriate potential window for the redox reaction. According to Eqs. (2) and (3), the redox potentials of the species involved in the ET should match in a way that the formal electron-transfer potential occurs within the potential window established by the transfer of the ionic species present at the liquid-liquid junction. The results shown in Figs. 1 and 2 provide an example of voltammetric ET responses when the above conditions are fulfilled. A difference of approximately 150 mV is observed between Ao et A" (.+. ... 

Most electrochemical studies at the micro-ITIES were focused on ion transfer processes. Simple ion transfer reactions at the micropipette are characterized by an asymmetrical diffusion field. The transfer of ions out of the pipette (ejection) is controlled by essentially linear diffusion inside its narrow shaft, whereas the transfer into the pipette (injection) produces a spherical diffusion field in the external solution. In contrast, the diffusion field at a microhole-supported ITIES is approximately symmetrical. Thus, the theoretical descriptions for these two types of micro-ITIES are somewhat different. 

FIG. 13 Schematic illustration of the SECM feedback mode based on a simple ion-transfer reaction. Cations are transferred from the top (organic) phase into the aqueous solution inside the pipette tip. Positive feedback is due to IT from the bottom (aqueous) layer into the organic phase. Electroneutrality in the bottom layer is maintained by reverse transfer of the common ion across the ITIES beyond the close proximity of the pipette where its concentration is depleted. (Reprinted with permission from Ref. 30. Copyright 1998 American Chemical Society.)... 

In this chapter, a novel interpretation of the membrane transport process elucidated based on a voltammetric concept and method is presented, and the important role of charge transfer reactions at aqueous-membrane interfaces in the membrane transport is emphasized [10,17,18]. Then, three respiration mimetic charge (ion or electron) transfer reactions observed by the present authors at the interface between an aqueous solution and an organic solution in the absence of any enzymes or proteins are introduced, and selective ion transfer reactions coupled with the electron transfer reactions are discussed [19-23]. The reaction processes of the charge transfer reactions and the energetic relations... 

The results given here suggest that even the ion transfer through a BLM is controlled mainly by the ion transfer reactions at two aqueous-membrane interfaces in analogy with... 

F. Redox Reaction Between O2 in W and CQH2 in DCE at the interface Coupied with ion Transfer Reactions... 

The redox reaction between O2 in W and CQEI2 in DCE controlled by (or coupled with) an ion transfer reaction at the interface was investigated by shaking W with DCE for 4h. 

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