Adsorption of potential-determining ions

The addition of acid (Ca) or base (Cb) to a CaCO system (while pco2 = constant) will change the alkalinity in solution and produce (i) a shift in the HCO3, CO3", Ca2+ equilibrium (and in pH), (ii) an adsorption of potential determining ions on the CaC03 surface, and (iii) a dissolution or precipitation of CaC03. 

In a typical inorganic oxide, the oxide surface acquires a charge by the dissociation or adsorption of potential determining ions at specific amphoteric surface groups or sites. As a consequence the equation of state of such surfaces will involve parameters that characterize surface reactions. In addition, one may also allow for the adsorption of anions and cations of the supporting electrolyte. However, in this paper we shall ignore this possibility to keep the discussion clear. Such embellishments of the model of the surface do not alter the key ideas presented here. 

The point of zero charge is the pH at which net adsorption of potential determining ions on the oxide is zero. It is also termed the point of zero net proton charge (pznpc). It is obtained by potentiometic titration of the oxide in an indifferent electrolyte and is taken as the pH at which the titration curves obtained at several different electrolyte concentrations intersect (Fig. 10.5). It is, therefore, sometimes also termed the common point of intersection (cpi). The pzc of hematite has been determined directly by measuring the repulsive force between the (001) crystal surface and the (hematite) tip of a scanning atom force microscope, as a function of pH the pzc of 8.5-8.S was close to that found by potentiometic titration (Jordan and Eggleston, 1998). This technique has the potential to permit measurement of the pzc of individual crystal faces, but the authors stress that the precision must be improved. 

Adsorption of potential-determining ions at the ferric oxide-aqueous electrolyte interface. J. Phys. Chem. 71 550-558... 

Point-o f-zero-charge Adsorption of potential-determining ions, many indirect methods based on a characteristic response to mineral processing operation... 

In many colloidal systems, the double layer is created by the adsorption of potential-determining ions for example, the potential 0o the surface of a /Silver iodide particle depends on the concentration of silver (and iodide) ions in solution. Addition of inert electrolyte increases k and results in a corresponding increase of surface charge density caused by the adsorption of sufficient potential-determining silver (or iodide) ions to keep 0O approximately constant. In contrast, however, the charge density at an ionogenic surface remains constant on addition of inert electrolyte (provided that the extent of ionisation is unaffected) and 0O decreases. 

The potential in the diffuse layer decreases exponentially with the distance to zero (from the Stem plane). The potential changes are affected by the characteristics of the diffuse layer and particularly by the type and number of ions in the bulk solution. In many systems, the electrical double layer originates from the adsorption of potential-determining ions such as surface-active ions. The addition of an inert electrolyte decreases the thickness of the electrical double layer (i.e., compressing the double layer) and thus the potential decays to zero in a short distance. As the surface potential remains constant upon addition of an inert electrolyte, the zeta potential decreases. When two similarly charged particles approach each other, the two particles are repelled due to their electrostatic interactions. The increase in the electrolyte concentration in a bulk solution helps to lower this repulsive interaction. This principle is widely used to destabilize many colloidal systems. 

For most oxides, as the pH is increased, the adsorption of potential determining ions, H" and OH, changes in correspondence with the concentration of these species in solution. For each surface, therefore, a point is reached at which the concentration of positive ions and negative ions just balance, the point of zero charge. The pH where the zeta potential, is 0, is called the isoelectric point. The isoelectric point for various ceramic materials is given in Table 9.11. 

Lange and Berger studied the adsorption of potential-determining ions, or ions that carry a charge to the solid phase. For many substances, including silver iodide, they found that adsorption follows the equation... 

Adsorption of potential-determining ions Ideally, as seen in Section 8-5, lattice ions held at the surface of a precipitate containing the same ions are adsorbed in accordance with an equation of the form... 

Aringhieri, R., and G. Pardini. 1989. Kinetics of the adsorption of potential-determining ions by positively charged soil particle surfaces. Soil Sci. 147 85-90. 

Tadros, T.F. and Lyklema, J., Adsorption of potential-determining ions at the silica-aqueous electrolyte interface and the role of some cations, J. Electroanal. Chem., 17, 267, 1968. 

Pechenyuk, S.l. and Kalinkina, E.V., Adsorption of potential-determining ions on the surface of amorphous iron(III) oxyhydrates, Kolloid. Zh., 52, 716, 1990. 

MeUs, P. et al.. Describing the adsorption of potential determining ions on variable charge mineral srrrfaces, Studi Sassar., 30, 137, 1983. 

Pechenyuk, S.L, Adsorption of potential determining ions on the surfaces of yttrium, samarium, and ytterbium oxides, Zh. Fiz. Khim., 61, 165, 1987. 

The surface charge on a solid surface can be obtained by determining the adsorption of potential-determining ions at various potentials of the interface [1]. For example, in the case of a silver iodide sol the adsorption of Ag+ and I ions is determined at various concentrations of Ag" " and I" ions in bulk solution. Similarly, for an oxide the adsorption of H" " and OH" ions Fand respectively) is determined as... 

This diffuse double-layer approach can be applied to describe the EDL of particles, if charges on particle surface are only permanent structural surface charges originating from isomorphic substitutions of ions in a clay crystal lattice (e.g., montmorillonite, which is a typical example of infinite flat plates with a constant charge density [19]) or they form by the adsorption of potential determining ions (e.g., Ag+ ions on a Agl surface is an example of the case of charged particles with constant potential [1,33,38]) and the diffuse swarm of indifferent electrolyte ions compensates surface charges. 

Lange and Berger established empirically that in many cases, including that of silver iodide, the adsorption of potential-determining ions obeys the equation... 

Fig. 31. Adsorption of potential-determining ions to Agl for differetit concentrations of decirolyres. Div iwn curves KNOg-NaNOa 7 1 (Van I.aar) 0 NaClO (Mackob) -NaNO (Mackop).

Lottermoser and Petersen determined the adsorption of potential-determining ions for AgCl, AgBr AgCNS. Their measurements have been recalculated by Keller-... 

Apart from the adsorption of potential-determining ions and the exchange adsorption of counter ions (see 9) examples of equivalent adsorption of non-potential-determining electrolyte have been reported ... 

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