Indifferent electrolytes, surface

Assume that both the initial substances and the products of the electrode reaction are soluble either in the solution or in the electrode. The system will be restricted to two substances whose electrode reaction is described by Eq. (5.2.1). The solution will contain a sufficient concentration of indifferent electrolyte so that migration can be neglected. The surface of the electrode is identified with the reference plane, defined in Section 2.5.1. In this plane a definite amount of the oxidized component, corresponding to the material flux J0x and equivalent to the current density j, is formed or... 

Materials. Synthetic hematite was obtained from J. T. Baker Chemical Company, Phillipsburg, NJ. Particle size analysis using a HIAC instrument (Montclair, CA) indicated the particles to be 80 percent (number) finer than 2 microns. Using nitrogen as the adsorbate, the B.E.T. specific surface area was found to be 9 square meters per gram. The point of zero charge, as obtained from electrophoretic measurements in the presence of indifferent electrolytes, occurred at pH 8.3. 

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. 

Of the various quantities that affect the shape of the net interaction potential curve, none is as accessible to empirical adjustment as k. This quantity depends on both the concentration and valence of the indifferent electrolyte, as shown by Equation (11.41). For the present we examine only the consequences of concentration changes on the total potential energy curve. We consider the valence of electrolytes in the following section. To consider the effect of electrolyte concentration on the potential energy of interaction, it is best to use the more elaborate expressions for interacting spheres. Figure 13.8 is a plot of ne, for this situation as a function of separation of surfaces with k as the parameter that varies from one curve to another. 

The lower the concentration of indifferent electrolyte, the longer is the distance from the surface before the repulsion drops significantly. 

Potentiometric titration of an aqueous suspension of oxides in the presence of varying concentrations of indifferent electrolyte has been used successfully to determine the zero point of charge (z.p.c.) and the variation in excess surface charge with pH (I, 8). The variation in excess surface charge (rH+-r0H-) with pH and NaCl concentration is shown for goethite in Figure 4. 

If counter ions are adsorbed only by electrostatic attraction, they are called indifferent electrolytes. On the other hand, some ions exhibit surface activity in addition to electrostatic attraction because of such phenomena as covalent bond formation, hydrogen bonding, hydrophobic and solvation effects, etc. Because of their surface activity, such counter ions may be able to reverse the sign of because the charge of such ions adsorbed exceeds the surface charge. 

Example. The surfaces of dispersed Agl particles can be considered similarly to an Ag-Agl-aqueous solution reversible electrode (i.e., each phase contains a common ion that can cross the interface). Here both Ag+ and I- will be potential determining ions because either may adsorb at the interface and change the surface potential. In this case, NaN03 is an example of an indifferent electrolyte as far as the electrode potential goes. 

Figure 5.10 Illustration of critical coagulation concentrations for a range of surface (Stern layer) potentials and indifferent electrolyte counterion charge numbers of 1 through 3. The curves were calculated taking A = 10-19 J, s/s0 = 78.5 and T = 298 K. The sol is predicted to be stable above and to the left of each curve and flocculated below and to the right. From Shaw [60], Copyright 1966, Butterworths.

Overpotential Departure from equilibrium (reversible) potential due to passage of a net current. Concentration overpotential results from concentration gradients adjacent to an electrode surface. Surface overpotential results from irreversibilities of electrode kinetics. Supporting (inert or indifferent) electrolyte Compounds that increase the ionic conductivity of the electrolyte but do not participate in the electrode reaction. 

Oxide surfaces have zero net charge and zero net surface potential at pH0 when immersed in an indifferent electrolyte solution. Therefore, the second term of the right side of equation 4.14 vanishes. Any change in the concentration of the supporting electrolyte does not change the surface potential and thus pH0 is independent of the electrolyte concentration. This independence is evidenced by a crossing point of the proton adsorption curves and pH0 coincides with PZC. 

For the cases of oxides and latices. as mentioned above, there is little or no mobility of the surface charges. In such systems the mutual distance between the charges ( ) becomes a characteristic parameter but It is not the absolute value of t that counts, but its relation to the thickness of the diffuse part of the double layer, characterized by x". For kI 1 the diffuse layer is so much thicker than that the assumption of a smeared-out surface charge remains tenable, whereas for Kt 1 the charges are relatively so far apart that around each of them a hemispherical diffuse layer is formed. As x is the yardstick, which decreases with, it follows that the surface charge has a more discrete nature at higher indifferent electrolyte concentration. 

Even with the "well-behaved" silver halides and oxides, indifferent electrolyte. For Insoluble oxides In pure water, containing only H and OH Ions, these Ions constitute not only the surface charge but also the counter charge. (The situation Is a bit academic because carrying out a titration requires the Introduction of other ions anjnvay. Moreover, few oxides are completely insoluble and some Ions may be Introduced by the wall of the vessel (silicates from the glass).) We shall therefore only consider the realistic cases that c ., etc. An additional... 

The whole electrode system is electrically equivalent to the circuit shown in Fig. 3 (a), where C, and J , are the capacity and resistance equivalent to the electrode reaction, C, the ordinary double layer capacity of the electrode surface and the electrolyte resistance between the electrode and the platinum gauze. Rc was obtained by measuring the total impedance, with the usual indifferent electrolyte, but cutting down the impedance due to C, and / , to about 3 ohms by using a high (M./20)... 

The underlying assumption of reactions (37) to (39) is that the first decomposition intermediate and not the free hole h+ constitutes the mobile species involved in the consecutive steps of the dissolution reaction, and hence that the decomposition intermediate Xj is mobile within a two-dimensional surface layer. In other words, a bonding electron can jump from an unbroken surface bond to a neighboring electron-deficient surface bond. Note that here, the only step consuming holes is step (4), so that in an indifferent electrolyte, Eqs. (3) and (5) are expected to hold, with n = 1. 

Equations (2)-(4) show that the total potential energy of interaction between two colloidal spherical particles depends on the surface potential of the particles, the effective Hamaker constant, and the ionic strength of the suspending medium. It is known that the addition of an indifferent electrolyte can cause a colloid to undergo aggregation. Furthermore, for a particular salt, a fairly sharply defined concentration, called critical aggregation concentration (CAC), is needed to induce aggregation. 

The space charge density p(z) neutralizing the surface charge is related to the local electrolyte concentrations. For a symmetrical indifferent electrolyte p(z) can be written as ... 

The screening effect of the indifferent electrolyte is shown in panel (a). For high salt concentration (2M) the electrostatic interaction is small and the (1 — h) vs. pH curve resembles the (1 — h) vs. pH. curve shown in Fig. 2a. The lower the salt concentration, the larger become the deviations from the (1 — 0h) vs. pH curve. The effect of Ci on the charging behaviour is illustrated in panel (b). The curve for Ci = oo corresponds with the curve at O.IM salt in panel (a). For low Stern layer capacitances the screening of the surface charge by electrolyte is very poor and charging the surface becomes extremely difficult. Panel (c) shows a plot of vs. pH at the same salt concentrations as shown in panel (a). 

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