Total site concentrations

Surface site densities used in the computation of the oxide site concentrations presented in this paper were determined by either rapid tritium exchange or acquired from published values (18). Reported total site densities for hydrous metal oxides show relatively little variation generally they range by less than a factor of 3. Since [M], [SOM], [H] and x are known or can be determined from experimental data, uncertainties in estimates of the total site concentration are directly translated into uncertainties in the calculated partitioning coefficient. 

Some prefer to write the surface reaction rate in terms cf the fraction of the surface of sites covered (i.e.,/ ) rather than the number of sites C. s covered, the difference being the multiplication factor of the total site concentration, C, In any event, the final form of the rate law is the same because C, k, and so on, are all lumped... 

Figure 13.37 Adsorption of Np(V) by different minerals, for ZNp(V) = (1.1 to 1.3) x 10" M, in 7=0.01 M NaC104 solutions. Total site concentration [SOH] = 5 x 10" M. From Kohler et al. (1992). Reprinted from Kbaraka, Y. K. A. S. Maest (eds.), Water-rock interaction—Proceedings of the 7th international symposium, WRI-7, Park City, Utah, 13-18 July 1992. 1992. Volume 1. Set of two volumes, 1730 pp., Hfl. 325AJSS 190.00. Please order from A. A. Balkema, Old Post Road, Brookfield, Vermont 05036 (telephone 802-276-3162 telefax 802-276-3837).

Since both fractional coverages are dimensionless, the dimensions of the reaction rate reside in the catalytic rate constant k, which is itself a composite parameter containing a term for the total site concentration, So, squared ... 

The frequency of collisions is proportional to the partial pressure, and the probability of adsorption is incorporated in the rate constant k, which has an exponential dependence on temperature. The concentration of vacant sites is expressed as (1 — ff), where 6 is the fraction of occupied sites, and the total site concentration is included in the rate constant kj. Molecules already adsorbed are assumed to have no effect on the rate of adsorption for nearby vacant sites ... 

Let the number of binding sites per antibody be n, so that the total site concentration is x [Ab]... 

This implies that the total site concentration is constant. This fact allows us to write... 

Let a and b denote concentrations of reduced and oxidized species present in the film and at any time t the total site concentration Cx = a + b, whereas at time t = 0, b = 0 and Cx - a. Now charge transfer across the conducting/nonconducting boimdary is given by the Butler-Volmer expression... 

The total sites concentration may not always remain constant during use. This will be discussed further in Chapter 5 on catalyst deactivation. 

If the reaction product is weakly adsorbed, the equation for the total site concentration becomes... 

The concentration of free binding sites is negligible throughout the reaction and it can be assumed that they are in a steady state and that the total site concentration... 

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