Constant surface acidity

Uptake and release of protons can be described by the acidity constants (assuming a solution of constant ionic strength, we imply that the activity coefficients of the surface species are equal) ... 

Example 2.1 Evaluation of Surface Charge from Alkalimetric and Acidimetric Titration Curves and Determination of Surface Acidity Constants... 

The acidity constants calculated from every point in the titration curve (Figure 2.2a and b) are microscopic acidity constants (Eqs. 2.5, 2.6). Each loss of a proton reduces the charge on the surface and thus affects the acidity of the neighboring... 

Soil particles were found to have a capacity for ligand binding of 2 1(H mol g-1 these surface functional groups are characterized by an apparent "mono-protic" acidity constant... 

Much information on the functional surface groups on sulfides is not yet available. Recent studies by Ronngren et al. (1991) gave the following acidity constants for... 

Specific surface area 40 m2 g 1, acidity constants of FeOHg pK., (int) = 7.25, K 2 = 9.75, site density = 4.8 nrrr2, hematite cone = 10 mgle. Ionic strength 0.005. For the calculation the diffuse double layer model shall be used. 

Estimate the variation of surface charge of a hematite suspension (same charac-teristics as that used in Example 7.2) to which various concentrations of a ligand H2U (that forms bidentate surface complexes with the Fe(III) surface groups, FelT such a ligand could be oxalate, phtalate, salicylate or serve as a simplified model for a humic acid we assume acidity constants and surface complex formation constants representative for such ligands. The problem is essentially the same as that discussed in Example 5.1. We recalculate here for pH = 6.5. 

It is important to establish the origin and magnitude of the acidity (and hence, the charge) of mineral surfaces, because the reactivity of the surface is directly related to its acidity. Several microscopic-mechanistic models have been proposed to describe the acidity of hydroxyl groups on oxide surfaces most describe the surface in terms of amphoteric weak acid groups (14-17), but recently a monoprotic weak acid model for the surface was proposed (U3). The models differ primarily in their description of the EDL and the assumptions used to describe interfacial structure. "Intrinsic" acidity constants that are derived from these models can have substantially different values because of the different assumptions employed in each model for the structure of the EDL (5). Westall (Chapter 4) reviews several different amphoteric models which describe the acidity of oxide surfaces and compares the applicability of these models with the monoprotic weak acid model. The assumptions employed by each of the models to estimate values of thermodynamic constants are critically examined. 

Gouy-Chapman, Stern, and triple layer). Methods which have been used for determining thermodynamic constants from experimental data for surface hydrolysis reactions are examined critically. One method of linear extrapolation of the logarithm of the activity quotient to zero surface charge is shown to bias the values which are obtained for the intrinsic acidity constants of the diprotic surface groups. The advantages of a simple model based on monoprotic surface groups and a Stern model of the electric double layer are discussed. The model is physically plausible, and mathematically consistent with adsorption and surface potential data. 

The representation of surface groups as diprotic weak acids is appealing because it includes a modest degree of complexity (two acidity constants), allows convenient representation of the condition of zero surface excess of hydrogen ion, and is still quite manageable mathematically. However, it must be borne in mind that this model is still a grossly simplified representation of the actual surface. It remains to be shown that this simplification is significantly better than any other simplification. 

Figure 6. Use of Method I, Equation 22, to calculate surface acidity constants with Ng - 12 sites nm" log Ka = -3.5 and log Ka2 = -8.1 or, in other terms, pHZpC = 5.8 and log Kd - -2.3. Capacitances were determined from the slopes according to Equation 22 acid branch, 0.77 F m base branch, 0.89 F m. Data are from Figure 5, Ti02 in 0.1 M KNO3 (32). ...

F" Exchange Capacity, mmoles/g Surface Acidity Constants 2 2.27 2.58... 

Formic acid did not adsorb on clean Ag(llO) above 180 K. In order to obtain the adsorbed species, the surface was predosed with oxygen to produce adsorbed oxygen atoms. Surface species could then be stabilized by reacting formic acid with surface oxygen to produce water and the formate (102). Subsequent heating of the surface produced decomposition near 425 K. Only CO2 and H2 were observed as products from the HCOO intermediate. Additionally, some back reaction to reform HCOOH occurred between HCOO(a) and the H(a) liberated by the decomposition. The rate constant for the decomposition was... 

Intrinsic acidity constants (K im) represent the dissociation of surface hydroxyl groups at zero surface charge. These constants carmot be measured directly. They are obtained from the experimentally accessible conditional constants, Kcond, either by extrapolation to a situation of zero surface charge, or by fitting the experimental data to an appropriate double layer model (section 10.3) to compute the electrostatic component. 

The acidity constants for the surface protolysis reactions provide a measure of the extent to which the reaction will proceed. For the iron oxides the values of the pJC,s usually fall between 5 and 10, but there is considerable variation in the values for dif... 

By substituting expressions for the apparent acidity constants the two intrinsic surface acidity constants K J[ K for surface reactions are defined as... 

The exponential term can be thought of as a surface activity coefficient (Dzombak and Morel, 1990) that corrects the apparent acidity constants for surface charge effects. 

Therefore, from a knowledge of the intrinsic acidity constants, Ns and the value of kj, the distribution of surface species, using data from Lumsdon and Evans (1994). 

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