Solution concentration potential-determining ions

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... 

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 potential of the compact layer at the flat band potential depends on the concentration of potential-determining ions in the electrolyte solution. For most semiconductor electrodes in aqueous solutions, the potential across the compact layer is determined by the dissociation of surface hydroxjd groups hence, the flat band potential is given as a linear function of pH. From Eqn. 5-87 we obtain Eqn. 5-90 ... 

Note that the potential across the compact layer can not be changed by changing the electrode potential unless the electrode interface is in the state of Fermi level pinning. In the state of band edge level pinning hi >H remains independent of the electrode potential Ma depends on the concentration of potential determining ions in aqueous solution though. 

Although we have approached the potential ip0 from the perspective of electrodes, the discussion makes it clear that the potential given by Equation (1) applies to any Agl-aqueous solution interface, and not just to electrode surfaces. We may not always know the concentrations required to use Equation (1) numerically, but so long as the bulk concentration of potential-determining ions differs from czp, a potential difference exists at the surface. The ions of water itself are potential determining for many surfaces. These as well as added solutes or ions in equilibrium with the solid mean that surface potentials at (especially, but not exclusively) aqueous interfaces are the norm rather than something exceptional. 

In subsequent chapters it will be the potential in the diffuse double layer that concerns us. It can be described relative to its value at the inner limit of the diffuse double layer, which may be either the actual surface or the Stern surface. We continue to use the symbol p0 for the potential at this inner limit. It should be remembered, however, that specific adsorption may make this quantity lower than the concentration of potential-determining ions in the solution would indicate. We see in Chapter 12 how the potential at some (unknown) location close to this inner limit can be measured. It is called the zeta potential. 

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. 

How is the surface potential quantitatively related to the concentration of the potential determining ions In equilibrium, the electrochemical potential of Ag+ ions at the crystal surface is equal to that in solution ... 

Silver iodide particles in aqueous suspension are in equilibrium with a saturated solution of which the solubility product, aAg+ai, is about 10 16 at room temperature. With excess 1 ions, the silver iodide particles are negatively charged and with sufficient excess Ag+ ions, they are positively charged. The zero point of charge is not at pAg 8 but is displaced to pAg 5.5 (pi 10.5), because the smaller and more mobile Ag+ ions are held less strongly than-the 1 ions in the silver iodide crystal lattice. The silver and iodide ions are referred to as potential-determining ions, since their concentrations determine the electric potential at the particle surface. Silver iodide sols have been used extensively for testing electric double layer and colloid stability theories. 

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. 

Potential-determining ions are those whose equilibrium between two phases, frequently between an aqueous solution and an interface, determines the difference in electrical potential between the phases. Consider a Agl dispersion in water. There will exist some concentrations of Ag+ and I" such that the surface charge of the Agl particles is zero. This is called the point of zero charge (pzc). It is usually determined by a titration method (called a colloid titration). 

Ion dissolution This is less common for pharmaceutical cases. Ionic charges are acquired by the unequal dissolution of the oppositely charged ions due to the excessive presence of ions in a solution. The concentrations of the excessive ions determine the electrical potential at the surface (i.e., potential determining ions). 

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. 

C = concentration of potential determining ion in solution C0 - concentration of potential determining ion at /0 = 0 F = Faraday s constant T = temperature... 

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. 

The point of zero charge of salt-type minerals depends on pH and on the concentration (activities) of all potential-determining ions. Thus, in the case of calcite, possible potential-determining species, in addition to H" and OH", are HCO3", CO2 and Ca various mechanisms of charge development are possible. When referring to a point of zero charge of such nonoxides, the solution composition should be specified. In the absence of complications, such... 

The second approach is to modify the conduction-band edge position, which can be tuned by pH or concentration of potential-determining ions (Finklea, 1988 Redmond and Fitzmaurice, 1993 Lyon and Hupp, 1999). It has been observed that the conduction band-edge position of metal oxides exhibits a nernstian type of dependence on the pH of the solution (Finklea, 1988 Enright et al, 1994 Lyon and Hupp, 1999). 

The reference electrode system is kept constant so that the composition of the cell up to the liquid junction does not change in the experiment. When the composition of the test electrolyte solution is changed, the cell EMF changes. These changes can be related to the concentration of a potential determining ion in the test solution via a calibration procedure. The system is also designed to keep the liquid junction potential constant and as small as possible. Thus, the observed potential drop across the cell can be written as... 

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... 

The electrode function gives the dependence between Ecell of an ion-selective electrode cell assembly and the logarithm of the potential determining ion (primary ion) activity in the sample solution. Typically, ISEs exhibit Nerns-tian electrode function in the concentration range of 10 and 10 5 M. De-... 

Where represents phthalate ion, which are considered here as potential determining ions. Khl and Kl, are the equilibrium constants, obtained by fit to adsorption data. The adsorption density as a function of phthalate aqueous concentration is illustrated in Figure 2, where the solid line shows SCF/DLM model calculation with log A // , = 16.45, log/i , = 11.28, in addition to the constants for hematite surface hydroxyl species. The model yields a good estimation at higher adsorption densities, but it overestimates the adsorbed organics at lower solution concentration. 

---