Charge potential -determining ions

Material Potential-Determining Ion Point of Zero Charge... 

Table XI-1 (from Ref. 166) lists the potential-determining ion and its concentration giving zero charge on the mineral. There is a large family of minerals for which hydrogen (or hydroxide) ion is potential determining—oxides, silicates, phosphates, carbonates, and so on. For these, adsorption of surfactant ions is highly pH-dependent. An example is shown in Fig. XI-14. This type of behavior has important applications in flotation and is discussed further in Section XIII-4.

Aq ° is the standard ion-transfer potential (these values for some ions can be found in Ref. 74), z is the number of the charge of the potential-determining ion, and are the activities of the potential-determining ion in the oil and water phases, respectively. 

The Triple T.aver Model and the Stern Model. The ions most intimately associated with the surface are assigned to the innermost plane where they contribute to the charge Oq and experience the potential tI>q These ions are generally referred to as primary potential determining ions. For oxide surfaces, the ions H+ and 0H are usually assigned to this innermost plane. In Stern s original model, the surface of a metal electrode was considered, and the charge cjq was due to electrons. 

As mentioned before, Stern had a metal electrode in mind when he described the surface-solution interface then (7q referred to the electronic charge on the surface of the metal itself, ato the charge formed by electrostatically (or chemically) bound electrolyte ions at the IHP, and a to the charge in the diffuse layer. In the case of silver iodide, the surface charge ctq is assumed to be made up of the adsorbed "potential determining ions"... 

Oxide surfaces have usually been regarded as being similar to the Agl surface the adsorbed "potential determining ions," H+ and OH, form the charge CTq, and a and o2 are as... 

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. 

Each of the above processes has its own characteristic kinetic and rate law and, in principle, each responds differently to the process variables (illumination intensity, dopant density, presence of adsorbates, activity of surface potential determining ions, width of and potential drop in the space charge region, position of the band edges). 

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. 

The possible usefulness of this relationship — which is known as the Hiickel equation — should not be overlooked. Throughout Chapter 11 we were concerned with the potential surrounding a charged particle. Equation (11.1) provides a way of evaluating the potential at the surface, i/ o, in terms of the concentration of potential-determining ions. Owing to ion adsorption in the Stern layer, this may not be the appropriate value to use for the potential at the inner limit of the diffuse double layer. Although f is not necessarily identical to i/ o, it is nevertheless a quantity of considerable interest. 

Box 11.1 Estimating the Surface Charge <7surfex of Oxides when H+ and OH are the Potential-Determining Ions (after Stumm and Morgan, 1996)... 

Excess Surface Charge. The reaction at the goethite surface producing charged sites by adsorption of H+ and OH" as potential determining ions can be represented as follows,... 

For a reversible interface, such as Agl/aqueous solution, the electrostatic potential in the solution just outside the surface referred to zero at regions of solution infinitely remote from colloidal particles, the Volta potential, is calculated from the Nernst equation, the concentration of potential determining ions, and the zero-point-of-charge which is not usually the stoichiometric equivalence point. 

A third mechanism of surface charging dominates for oxides (e.g. Si02, TiOo, A1203) [85], proteins, and many water soluble polymers. At the surface of these substances there are groups that can dissociate. They take up or release a proton depending on pH. Examples are hydroxyl, carboxyl, sulfate, and amino groups. The potential determining ions are OH- and H+. 

The concentration of potential-determining ions at which the zeta potential is zero (C = 0) is called the isoelectric point (iep). The isoelectric point is determined by electrokinetic measurements. We have to distinguish it from the point of zero charge (pzc). At the point of zero charge the surface charge is zero. The zeta potential refers to the hydrodynamic interface while the surface charge is defined for the solid-liquid interface. 

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