Potential-determining ions electrical

Using a simple amphoteric model for the mineral surface, we have demonstrated the role specific chemical binding reactions of potential determining Ions In determining the electrical properties and thermodynamics of the oxide/solution interfaces. A by-product of our study Is that under appropriate conditions, an amphoteric surface can show marked deviations from ideal Nernstlan behaviour. The graphical method also serves to Illustrate the... 

The ion that determines the p>otential of the compact layer is called the potential-determining ion. In cases in which the potentied of compact layer is determined by the dissociation reaction of adsorbed hydroxyl groups, the potential -determining ions are hydrated protons or hydroxide ions. For cadmium sulfide electrodes, the potential-determining ions are not hydrated protons but hydrated sulfide ions the iso-electric point is at the sulfide ion concentration of 4 x 10 M [Ginley-Butler, 1978]. 

The producer must ensure that the galvanic cell is collected in such a way that the potential-determining ion-transfer processes at the ISS/GSS solution interface is exposed, while all other electric potential drops at the other interfaces are constant, see A2. 

Figure 1. Potential variation through the electrical double layer for a higher concentration of potential-determining ion (a), a lower concentration of potential-determining ion (b), and in the presence of a specifically adsorbed counter ion with a potential-determining ion below the point-of-zero charge (c). Note that the potential in all three instances could be identical. (Reproduced, with permission, from Ref. 1. Copyright 1970, International Union of Pure and Applied Chemists.)...

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. 

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). 

Ions causing electrical charge of a mineral surface are called potential-determining ions (PDI). The surface charge density of a mineral os according to the mechanism a) can be calculated from the change of pH value in an aqueous suspension of the solid by using Eq. (1) ... 

The electrostatic free energy contribution is the sum of the electrostatic energy of the solution and the chemical contribution of the surface, if the potential determining ions are in thermodynamic equilibrium with those in solution. At constant surface electrical potential Pq, the chemical contribution is given by... 

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. 

The electrical double layer at the metal oxide/electrolyte solution interface can be described by characteristic parameters such as surface charge and electrokinetic potential. Metal oxide surface charge is created by the adsorption of electrolyte ions and potential determining ions (H+ and OH-).9 This phenomenon is described by ionization and complexation reactions of surface hydroxyl groups, and each of these reactions can be characterized by suitable constants such as pKa , pKa2, pKAn and pKct. The values of the point of zero charge (pHpzc), the isoelectric point (pH ep), and all surface reaction constants for the measured oxides are collected in Table 1. 

Note that variably charged soil mineral surfaces do not exactly obey the Nernst equation. The assumption is only valid at approximately one pH unit above or belov the PZC or at approximately 25 mV of surface electrical potential. The assumption is only used for demonstration purposes. Since H+ and OH are considered to be potential determining ions (PDIs), Equation 3.15 can be rewritten as... 

Explain the relationship between variably charged soils and surface electric potential and the relationship between constant charge soils and surface electric potential. Define the role of potentially determining ions in variably and constant charged soils. Discuss the practical meaning of the above. 

If lattice ions or other potential-determining ions are adsorbed on a solid surface, they may be regarded as belonging to the solid and imparting an electrical charge to it. For the sake of overall electrical neutrality, an equivalent number of oppositely charged ions (counterions) exist in solution, drawn to the charged surface by electrical attraction. The counterions and the adsorbed lattice ions form an electrical double layer. The closest counterions cannot be nearer the surface than a finite distance (inner Helmholtz plane ) that depends on the ionic radius. 

Two parameters were introduced into the description of double electrical layer. One of them is the point of zero charge (PZC) which according to lUPAC definition [101] can be expressed as concentration of potential-determining ions PDI at which the surface charge is equal to zero ( o = 0), as well as the surface potential (V>o = 0). Another parameter is isoelectric point (lEP) defined [101] as concentration of PDI at which the electrokinetic potential is equal to zero (( = 0). 

Indifferent Electrolyte An electrolyte whose ions have no significant effect on the electric potential of a surface or interface, as opposed to potential-determining ions, which have a direct influence on surface charge. This distinction is most valid for low electrolyte concentrations. Example For the Agl surface in water, NaN03 would be an indifferent electrolyte, and both Ag+ and I" would be potential-determining ions. 

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