Proton surface charge

Then the material balance equations can be simplified to allow the reaction quotients Qa and Qa2 to be approximated from the experimentally observed protonic surface charge <7q ... 

Figure 5. Titration of a suspension of Ti02 in KNOj at three different concentrations. CTq is the protonic surface charge, calculated from the acid-base mole balance equations, specific surface area of 20 nr g" and the solids...

The value of corresponding to values of Ka2 and Ka2 determined from Method I would be relatively small, reflecting the predominance of the neutral XOH group. (ii) The hydrogen ion activity in solution that yields zero protonic surface charge is defined from a combination of Equations 1 and 2 with nx+ set equal to nx- ... 

Then Equations 23-25 can be combined with Equations 1-4 to yield the protonic surface charge as a function of a ... 

Proton surface charge density, defined as the difference between the number of moles of complexed proton charges and of complexed hydroxyl charges per unit mass of colloids. 

The net proton surface charge density, oh, can be determined from alkali-metric/acidimetric titration, as described in Example 9.2. 

PROTON SURFACE CHARGE DENSITY. The surface density of net proton charge, cth, is defined by an expression analogous to Eq. 1.21 ... 

FIGURE 5.18 The determination of proton surface charge from potentiometric data by comparing a titration of the colloidal suspension (squares) with one of the support electrolyte alone (circles). 

We introduce here chemical modeling that considers the proton adsorption reaction leading to the protonic surface charge on an oxide surface. Typical surface reactions can be written as... 

The surface charge characterization of clay minerals, when permanent charges from isomorphic substitutions of ions in a clay crystal lattice are present besides the variable edge charges, is more complicated than that of metal oxides. In this case, the intrinsic surface charge density, Cin, can be defined as the sum of the net permanent structural charge density, oq, and the net proton surface charge density, ffo.H, i-C-, [2,... 

Of the various inorganic soil constituents, smectites (montmorillonite clays) have the greatest potential for sorption of pesticides on account of their large surface area and abundance in soils. Weak base pesticides, both protonated and neutral species, have been shown to be sorbed as interlayer complexes. Sorption of atrazine on smectites ranges from 0 to 100% of added atrazine, depending on the surface charge density of the smectite (36). 

The effects of organic molecules and phosphate on the adsorption of acid phosphatase on various minerals, and kaolinite in particular, have been investigated by Huang et al. [97]. The Langmuir affinity constant for AcP adsorption by kaolinite follows the series tartrate (K — 97.8) > phosphate (K= 48.6) > oxalate (K — 35.6) > acetate (K= 13.4). At low concentration, acetate even promoted the adsorption of acid phosphatase. It was considered that competitive interactions between anionic adsorbates can occur directly through competition for surface sites and indirectly through effects of anion adsorption on the surface charge and protonation. 

The predicted state of the sorbing surface in the two calculations differs considerably. At pH 4, the surface carries a positive surface charge and potential. The electrical charge arises largely lfom the predominance of the protonated surface species > (w)FeOH, which occupies about two thirds of the weakly binding sites. At pH 8, however, the surface charge and potential nearly vanish because of the predominance of the uncomplexed species >(w)FeOH, which is electrically neutral. 

The pH shift model of Park and Regalbuto combined (1) a proton balance between the surface and bulk liquid with (2) the protonation-deprotonation chemistry of the oxide surface (single amphoteric site), and (3) a surface charge-surface potential relationship assumed for an... 

Thus, the net surface charge of a hydrous oxide is determined by the proton transfer and reactions with other cations or anions. In general, the net surface charge density of a hydrous oxide is given by... 

Relationship between pH, surface potential, xp or Coulombic term, log P, or Coulombic free energy, AGcoui), and surface charge density, a (or surface protonation) for various ionic strengths of a 1 1 electrolyte for a hydrous ferric oxide surface (P = exp(-Fi //RT). 

As we have seen, the net surface charge of a hydrous oxide surface is established by proton transfer reactions and the surface complexation (specific sorption) of metal ions and ligands. As Fig. 3.5 illustrates, the titration curve for a hydrous oxide dispersion in the presence of a coordinatable cation is shifted towards lower pH values (because protons are released as consequence of metal ion binding, S-OH + Me2+ SOMe+ + H+) in such a way as to lower the pH of zero proton condition at the surface. 

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