Primary kinetic electrolyte

Although these effects are often collectively referred to as salt effects, lUPAC regards that term as too restrictive. If the effect observed is due solely to the influence of ionic strength on the activity coefficients of reactants and transition states, then the effect is referred to as a primary kinetic electrolyte effect or a primary salt effect. If the observed effect arises from the influence of ionic strength on pre-equilibrium concentrations of ionic species prior to any rate-determining step, then the effect is termed a secondary kinetic electrolyte effect or a secondary salt effect. An example of such a phenomenon would be the influence of ionic strength on the dissociation of weak acids and bases. See Ionic Strength... 

A kinetic electrolyte effect ascribable solely to the influence of the ionic strength on activity coefficients of ionic reactants and transition states is called a primary kinetic electrolyte effect. A kinetic electrolyte effect arising from the influence of the ionic strength of the solution upon the pre-equilibrium concentration of an ionic species that is involved in a subsequent rate-limiting step of a reaction is called a secondary kinetic electrolyte effect. A common case encountered in practice is the effect on the concentration of a hydrogen ion (acting as catalyst) produced from the ionization of a weak acid in a buffer solution. 

Quantitative relations of the primary kinetic salt effect are well explained by the Debye-Hiickel theory of strong electrolytes and the transition state theory. 

The primary question is the rate at which the mobile guest species can be added to, or deleted from, the host microstructure. In many situations the critical problem is the transport within a particular phase under the influence of gradients in chemical composition, rather than kinetic phenomena at the electrolyte/electrode interface. In this case, the governing parameter is the chemical diffusion coefficient of the mobile species, which relates to transport in a chemical concentration gradient. 

In screening electrolyte redox systems for use in PEC the primary factor is redox kinetics, provided the thermodynamics is not prohibitive, while consideration of properties such as toxicity and optical transparency is important. Facile redox kinetics provided by fast one-electron outer-sphere redox systems might be well suited to regenerative applications and this is indeed the case for well-behaved couples that have yielded satisfactory results for a variety of semiconductors, especially with organic solvents (e.g., [21]). On the other hand, many efficient systems reported in the literature entail a more complicated behaviour, e.g., the above-mentioned polychalcogenide and polyiodide redox couples actually represent sluggish redox systems involving specific interactions with the semiconductor... 

Three types of methods are used to study solvation in molecular solvents. These are primarily the methods commonly used in studying the structures of molecules. However, optical spectroscopy (IR and Raman) yields results that are difficult to interpret from the point of view of solvation and are thus not often used to measure solvation numbers. NMR is more successful, as the chemical shifts are chiefly affected by solvation. Measurement of solvation-dependent kinetic quantities is often used (<electrolytic mobility, diffusion coefficients, etc). These methods supply data on the region in the immediate vicinity of the ion, i.e. the primary solvation sphere, closely connected to the ion and moving together with it. By means of the third type of methods some static quantities entropy and compressibility as well as some non-thermodynamic quantities such as the dielectric constant) are measured. These methods also pertain to the secondary solvation-sphere, in which the solvent structure is affected by the presence of ions, but the... 

C02 is produced as the primary product and this precludes the use of alkaline electrolytes due to the precipitation of CO - in the pores of the anode and consequent electrode fouling. Acid electrolytes lead to problems of corrosion and slow kinetics for the reduction of 02 at the air cathode. 

In many PEC systems the chemical kinetics for the primary charge transfer process at the interface are not observed at the light intensities of interest for practical devices and the interface can be modeled as a Schottky barrier. This is true because the inherent overpotential, the energy difference between where minority carriers are trapped at the band edge and the location of the appropriate redox potential in the electrolyte, drives the reaction of interest. The Schottky barrier assumption breaks down near zero bias where the effects of interface states or surface recombination become more important.(13)... 

Provided that v > v for most values of h then the form of curve shown in Figure 1 is obtained. When the magnitude of is substantial, say >> 10 kT, a stable dispersion is obtained. The form of the potential energy curve obtained by this approach shows immediately that the stability of a dispersion to electrolyte is kinetic in origin rather than thermodynamic, that is, the lowest free energy state is in the primary minimum and entry into this is prevented by the presence of the large activation energy represented by AV. A more sophisticated and detailed representation of these ideas can be found elsewhere (12,15,16). 

Hunkeler and Bohni used this approach to show that pit growth in A1 foils occurred under ohmic control (24). It was also shown that nitrate and chromate inhibitors, added to the electrolyte after pit initiation, inhibited pit growth kinetics though the effect due to chromate additions was small. Several other inhibitors added to solution increased pit growth kinetics, since their primary influence was in decreasing the solution resistance. 

Most of the work reported before the Second World War was carried out in aqueous electrolyte solutions. Since 1945 the focus has shifted to include the application of nonaqueous solvents. This has allowed for the detection of the primary intermediates, typically radical anions and radical cations, and for the study of their reactions. The theoretical foundations, for the analysis of kinetics and mechanisms by, for instance, cyclic voltammetry and related techniques were mostly published in the 1960s and 1970s. The application of such techniques has resulted in a steadily increasing understanding of the kinetics and mechanisms of organic electrochemical processes. The current trend is to return to water-like conditions reflecting the need to substitute organic solvents with environmentally friendlier electrolyte systems. 

Under the assumption that the concentrations are uniform within the electrolyte, potential is governed by Laplace s equation (5.52). Under these conditions, the passage of current through the system is controlled by the Ohmic resistance to passage of current through the electrolyte and by the resistance associated with reaction kinetics. The primary distribution applies in the limit that the Ohmic resistance dominates and kinetic limitations can be neglected. The solution adjacent to the electrode can then be considered to be an equipotential surface with value o- The boundary condition for insulating surfaces is that the current density is equal to zero. 

The composition of the electrolyte is of primary importance owing to the required high ionic conductivity, compatibility with active and inert cell components, wetting characteristics, effects on the course of electrode reactions, and the cell s cost. The optimized electrolyte mixtures consist of alkali-metal halides. An all Li ion electrolyte would be preferred from the point of view of kinetic and transport considerations however, the decreased specific energy and high cost are then prohibitive. 

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