Surface equilibrium constants

Extensive tabulations on experimentally determined surface equilibrium constants (Schindler and Stumm, 1987 Dzombak and Morel, 1990) reflecting the acid-base characteristic of surface hydroxyl groups and the stability of surface metal com-... 

However, even using results from various sources in the literature for the surface charge data, we believe that the combined use of t/>o and ao curves leads to more reliable values of the surface equilibrium constants than is obtainable otherwise. 

The results found in the adsorption studies are given in Table II. From these results the surface equilibrium constants for the reactions given in Table III were determined using the SURFEQL program. In these calculations it is assumed that the Ca2+, Mg2+, Cl, SO,2, phosphate and silicate surface species are those found previously to occur on the a-FeOOH surface (33, 34). The phthalate and salicylate complexes =FeA and sFeAH (where A7 is phthalate or salicylate) were considered in these calculations. 

The charges assigned to the surface species in reaction 44 indicate relative values. Surface equilibrium constants need to be established in order to estimate the species distribution as a function of [H" ] and other potential-determining species. An aqueous suspension of CaC03 crystals in the presence of CO2 iPcoi - constant) at the point of zero charge (pHpzc) at equilibrium—in line... 

In order to be true constants, surface equilibrium constants must be written in terms of dissolved species concentrations at the oxide surface constants written in terms of bulk solution concentrations require an electrostatic correction for the effects of the electrical double layer... 

Modeling programs deal exclusively with equilibrium constants defined for the aqueous solution, so the intrinsic or surface equilibrium constants. surface obtained from the literature (e.g., Dzombak and Morel, 1990) are corrected using Equations (7.17) or (7.18). 

Sample Preparation P zpc Electrolyte Specific Surface Area Surface Equilibrium Constants... 

On the far right of the numerator we have two parameters that may be difficult to obtain independently. They are the adsorption equilibrium constant for B and the surface equilibrium constant for the reaction Asurf Bsurf- Our goal is to clear these by reexpressing them in terms of something that is unchanging. After all both of these may be strong functions of the catalyst structure and composition. The overall reaction A B is, however, one which is fixed at any temperature and pressure by the overall equilibrium constant. This is independent of the catalyst. Therefore we want to use this in the reaction rate expression. Here is how we do it ... 

Another important area of analytical chemistry, which receives some attention in this text, is the development of new methods for characterizing physical and chemical properties. Determinations of chemical structure, equilibrium constants, particle size, and surface structure are examples of a characterization analysis. 

A dye used in the coloration of materials, such as fibers, must be present as the monomer if it is to adsorb to the material s surface. This experiment describes how spectrophotometry can be used to determine the equilibrium constant between a monomer and a dimer for the dye pinacyanol iodide in water. 

The simplest mode of IGC is the infinite dilution mode , effected when the adsorbing species is present at very low concentration in a non-adsorbing carrier gas. Under such conditions, the adsorption may be assumed to be sub-monolayer, and if one assumes in addition that the surface is energetically homogeneous with respect to the adsorption (often an acceptable assumption for dispersion-force-only adsorbates), the isotherm will be linear (Henry s Law), i.e. the amount adsorbed will be linearly dependent on the partial saturation of the gas. The proportionality factor is the adsorption equilibrium constant, which is the ratio of the volume of gas adsorbed per unit area of solid to its relative saturation in the carrier. The quantity measured experimentally is the relative retention volume, Vn, for a gas sample injected into the column. It is the volume of carrier gas required to completely elute the sample, relative to the amount required to elute a non-adsorbing probe, i.e. 

The critical hydrogen content for the ductility loss increased with increasing hydrogen solubility in the alloy. The fracture surfaces were not characteristic of those found under conditions of SCC. In terms of hydrogen and deuterium solubility in a similar series of bcc alloys, the equilibrium constants were determined at infinite dilution as a function of temperature The free energy function was expressed in terms of the bound-proton model. 

The acid-base equilibrium constant for the Me residue can be determined by spectroscopic pH titration. An example for the titration is shown in Fig. 2. The electrostatic potential

difference between the apparent pK on the charged surface (pKobs) and that on an intrinsic neutral surface (pK1) by... 

In the following paper, the possibility of equilibration of the primarily adsorbed portions of polymer was analyzed [20]. The surface coupling constant (k0) was introduced to characterize the polymer-surface interaction. The constant k0 includes an electrostatic interaction term, thus being k0 > 1 for polyelectrolytes and k0 1 for neutral polymers. It was found that, theoretically, the adsorption characteristics do not depend on the equilibration processes for k0 > 1. In contrast, for neutral polymers (k0 < 1), the difference between the equilibrium and non-equilibrium modes could be considerable. As more polymer is adsorbed, excluded-volume effects will swell out the loops of the adsorbate, so that the mutual reorientation of the polymer chains occurs. 

A quite analogous treatment may be applied to the kinetics of heterogeneously catalyzed reactions. Consider a surface S that contains so active sites to which reactant A, a gaseous or solute species, may bind reversibly with an equilibrium constant KA ... 

Kr Kinetic equilibrium constant for surface reaction Exam. 10.3... 

The direction chosen for the equilibrium reaction Is determined by convenience. A scientist interested in producing ammonia from N2 and H2 would use f. On the other hand, someone studying the decomposition of ammonia on a metal surface would use eq,r Either choice works as long as the products of the net reaction appear in the numerator of the equilibrium constant expression and the reactants appear in the denominator. Example applies this reasoning to the iodine-triiodide reaction. 

Hence, the reaction order is seen to depend both on the equilibrium constant K2, which depends on temperature, and the actual partial pressure p2- We shall later see that the latter term for a catalyst is related to the extent that the surface is covered by a reactant. 

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