Ion, potential-determining

Fig. V-3. Schematic representation of (a) the Stem layer (b) the potential-determining ions at an oxide interface (c) the potential-determining and Stem layers together.

The f potential of silver iodide can be varied over the range 75 mV, by varying the Ag or 1 concentration again demonstrating that varying the concentration of potential-determining ions can reverse the sign of the f potential. 

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.

In addition to the collector, polyvalent ions may show sufficiently strong adsorption on oxide, sulfide, and other minerals to act as potential-determining ions (see Ref. 98). Judicious addition of various salts, then, as well as pH control, can permit a considerable amount of selectivity. 

We consider an oil-water two-phase system, which contains an ionic surfactant i. If we vary the phase-boundary potential either by externally applying the voltage using two electrodes or by adjusting the solution composition of potential determining ions, the concentration of i in each phase varies accordingly, keeping the total amount of i in the system, m, constant. The condition of the latter is... 

Building on initial work [47], the main focus of SECM in the study of ET at ITIES has been to identify and understand the potential-dependence of ET rates. In these studies, the potential drop across an ITIES has been controlled by varying the concentration of potential-determining ions in the two phases. The potential drop across an ITIES follows the Nernst-Donnan equation [74,75],... 

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. 

For studies with CIO4 as the potential-determining ion, under conditions where the activity coefficients of the ion in each phase remain constant, Eq. (38) can be written as ... 

In early work on the effect of potential on ET reactions [76], Solomon and Bard showed that an ET reaction between Fe(CN)g in an aqueous phase and 7,7,8,8-tetra-cyanoquinodimethane (TCNQ) in 1,2-dichloroethane (DCE) could be promoted by judiciously adjusting the potential drop across the ITIES, using tetraphenylarsonium cation as a potential determining ion. In a similar period, Selzer and Mandler [77] reported a study of the ET reaction between aqueous IrClg and Fc in a NB phase, without any potential determining ion in either phase. A first-order rate constant of 0.013 cm s was obtained... 

As discussed in Section IV, many studies of ET kinetics with SECM have been under conditions where constant composition in phase 2 can be assumed, but this severely restricts the range of kinetics that can be studied. With the availability of a full model for diffusion, outlined in Section IV, that lifts this restriction, Barker et al. [49] studied the reaction between ZnPor in benzene or benzonitrile and aqueous reductants, using CIO4 or tetrafluoroborate as potential-determining ions. 

In these studies, CIO4 was used as the potential-determining ion, with 0.1 M THAP employed as the supporting electrolyte in the DCE phase, together with various concentrations of NaC104 in the aqueous phase. In contrast to studies of ZnPor oxidation [80],... 

The ET reaction between aqueous oxidants and decamethylferrocene (DMFc), in both DCE and NB, has been studied over a wide range of conditions and shown to be a complex process [86]. The apparent potential-dependence of the ET rate constant was contrary to Butler-Volmer theory, when the interfacial potential drop at the ITIES was adjusted via the CIO4 concentration in the aqueous phase. The highest reaction rate was observed with the smallest concentration of CIO4 in the aqueous phase, which corresponded to the lowest driving force for the oxidation process. In contrast, the ET rate increased with driving force when this was adjusted via the redox potential of the aqueous oxidant. Moreover, a Butler-Volmer trend was found when TBA was used as the potential-determining ion, with an a value of 0.38 [86]. 

Gross et al. [3] and Reid et al. [30] measured surface tension of the water-nitrobenzene interface in the presence of bromides of sodium and tetra-alkylammonium ions in water and tetra-alkylammonium tetraphenylborates in nitrobenzene, i.e., tetra-alkylammonium served as the potential-determining ion, cf. the scheme (13). The surface tension vs. the potential difference A p plot (electrocapillary curve), cf. Eq. (15), was constructed by varying the concentration of tetra-alkylammonium bromide in water, while holding... 

Concentration cells are a useful example demonstrating the difference between galvanic cells with and without transfer. These cells consist of chemically identical electrodes, each in a solution with a different activity of potential-determining ions, and are discussed on page 171. 

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. 

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... 

One other issue arises with respect to potential determining ions. In the case of Agl, the potential difference at the surface-solution interface varies with the activity of Ag+ or I" in solution according to the Nernst equation,... 

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. 

We derive the equation of state of an amphoteric surface by considering the generic dissociation reactions involving potential determining ions ... 

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