Primary kinetic electrolyte effect

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 rate constants of chemical reactions the yield and the selectivity of a reaction, as well as the conditions for refining or recycling of products can be optimized by the choice of appropriate solvents. Discussion in this section is restricted to reaction mechanisms involving electrolytes or single ions. The role of electrolyte solutions in primary and secondary kinetic salt effects is not considered. For this problem see Refs. s. 

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)... 

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. 

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. 

The use of Ce(IV) to probe the kinetics of outer-sphere electron transfer reactions has centered on compounds of the title metal ions whose primary coordination spheres (in the reduced state at least) are substitution inert on the time scale of electron transfer reactions. For example, Cyfert et al. (1980) investigated the effects of the variation of ionic strength on the rate of Fe(phen) oxidation by Ce(IV) in 0.125 M H2SO4 with different supporting electrolytes (table 9). Variation in the concentrations of NaCl or NaClO in the range of O.l-l.O M changed the rate constants from 1.08 to 0.81 X 10 s and 1.03 to 0.56 x 10 M s, respectively (25°C). The varia-... 

Thus, the reaction order in the reactant itself is useful to evaluate. If the reactant is likely to be adsorbed significantly, then the reaction order must be interpreted, as in other types of heterogeneous reactions, with due attention to the type of adsorption isotherm and the extent of coverage. Since the kinetics of electrode reactions depend in a primary way on the potential (see Section 4), the reaction order must be specified and evaluated with respect to a constant electrode potential. Usually a supporting (electrochemically inactive) electrolyte is used so that effects of ion distribution and potential in the double layer remain approximately constant as other quantities are varied in the experimental analysis, viz., pH, reactant concentration. 

Primary salt effects arise in the actual kinetic step of the reaction and appear in their simplest form when all the electrolytes present in the solution are completely dissociated, so that there are no equilibria which... 

We will consider only the influence of activation overpotential or overvoltage on secondary current distribution. It is useful to regard the slope of the polarization curve dE /di (if any effect of concentration overpotential can be ignored) as a polarization resistance R. This represents the slowness of charge transfer across the interface and is based on the electrode kinetics of the reaction. If acts in series with R, the resistance of the electrolyte, we can distinguish between two situations. If R R, then the kinetics of charge transfer and not electrolyte resistance determine the current distribution, i.e., secondary current distribution dominates. Conversely, if R R, primary current distribution dominates. Secondary current distributions tend to smooth out the severe nonlinear variations of current associated with primary distributions and they eliminate infinite currents associated with electrode edges. 

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