Chemical surface exchange coefficient

As for bulk diffusion, depending on the experimental method used, various surfece exchange coefficients have to be distinguished. For chemical relaxation experiment, for example, the resolution of Eq. (8.12) with appropriate boundary conditions gives the mathematical definition of the chemical surface exchange coefficient kchem. which is viewed as the coefficient of proportionality between... 

As for diffusion coefficients, the chemical surface exchange coefficient iTchem and the tracer surface exchange coefficient k are linked together by the thermodynamic factor according to the following relation ... 

S. Diethelm, A. Closset, J. Van Herle, K. Nisancioglu, Determination of chemical diffusion and surface exchange coefficients of oxygen by electrochemical impedance spectroscopy, J. Electrochem. Soc., 2002,149,11, pp. E424-E432. 

Electrical conductivity relaxation (ECR) method. ECR method is rather simple, it does not require expensive equipment and provides high precision in determination of the oxygen chemical diffusion and surface exchange coefficients in mixed ionic-electronic conductors [64-66],... 

In the expressions for the gas exchange coefficient employed previously, it is evident that the air-water gas exchange flux density is proportional to the difference between a chemical concentration in the water (Cw) and the corresponding equilibrium concentration (Cw H) in air. Consequently, the difference between actual and equilibrium concentration in the water tends to decay exponentially, as expected for any first-order process. In many situations, exponential decay may provide a useful model of a volatile chemical concentration in a surface water. A classic example is degassing of a dissolved gas from a stream if the gas is present at concentration C0 upstream, atmospheric concentration of the gas is negligible, and flow is steady and uniform along the stream, then the gas concentration in the stream is given by... 

For solving heterogeneous models it is necessary to take into account the effect of such processes as dissolution and mineral formation, surface complex formation and ion exchange. That is why beside stability constants of complexes in water it is necessary to have values of ion exchange coefficients, solubility products and also surface acidity and constant of surface complex formation for individual minerals. For nonpolar and chemically passive components are needed values of partition coefficients or solubility. Due to the shortage of these data and complexity of computations at... 

Coulometric titration techniques were used to measure chemical diffusion at between 700 and lOOOC. The transient current response to a potentiostatic step was transformed from the time domain to the frequency domain. The equivalent circuit which was used to fit the resultant impedance data contained an element which described the finite-length diffusion of O into the sample. Other elements which were included were the gas-phase capacitance, and the sum of the gas-phase diffusion resistance and that which was associated with the limited surface exchange kinetics of the sample. The chemical diffusion coefficient of the perovskite, Laq gSrq 2C0O3,... 

Fig. 23 shows the Arrhenius plots of D measured on two different samples of the same composition La2Cuo5Nio.504+s, but with two different thicknesses. Oxygen chemical diffusion coefficients, obtained for both samples, are equal over a wide temperature range. This observation confirms that the relaxation process is not controlled by the surface exchange, but by the bulk diffusion (i.e., this latter process is the limiting step). 

In all of these cases, the structure of the organic sorbate, the composition of the surface, and the conditions of the vapor or solution exchanging with the solid must be considered. However, it is important to note that with some experience in thinking about the organic chemicals and environmental situation involved, we can usually anticipate which one or two sorption mechanisms will predominate. For example, in Chapter 9 we wrote an expression reflecting several simultaneously active sorption mechanisms, each with their own equilibrium descriptor, to estimate an overall solid-water distribution coefficient for cases of interest (Eq. 9-16) ... 

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