Ideally polarizable interface, supporting electrolyte

Ideally Polarizable Interface with Supporting Electrolytes. 

Potentiometric Results. As shown earlier, a single salt concentration variation has no effect on the interfacial potential. Thus, to study the effect of the dye cation on the interfacial potential, other ions must be present. Supporting electrolytes, selected in such a way that an ideally polarizable interface is formed when the dye is absent, are conveniently used. 

The thermodynamics of 2D Meads overlayers on ideally polarizable foreign substrates can be relatively simply described following the interphase concept proposed by Guggenheim [3.212, 3.213] and later applied on Me UPD systems by Schmidt [3.54] as shown in Section 8.2. A phase scheme of the electrode-electrolyte interface is given in Fig. 8.1. Thermodynamically, the chemical potential of Meads is given by eq. (8.14) as a result of a formal equilibrium between Meads and its ionized form Me in the interphase (IP). The interphase equilibrium is quantitatively described by the Gibbs adsorption isotherm, eq. (8.18). In the presence of an excess of supporting electrolyte KX, i.e., c , the chemical potential is constant and... 

Adsorption on the LL interface can be observed in voltammetric curves, but a more sensitive indication of adsorption can be obtained from impedance measurements. Voltammetric studies (26, 27) showed that the addition of proteins to a so-called blocked (i.e., ideally polarizable) aqueous-nitrobenzene interface resulted in narrowing of the potential window of the supporting electrolyte system. This observation implies that the difference in hydrophilic-hydrophobic properties of the two solvents decreased. This decrease can be explained by postulating the formation of a third phase between the original two. The third phase would allow mediated, easier transport of the supporting electrolyte ions. 

In the case where the ionic species in the aqueous electrolyte are fairly hydrophilic and the organic phase features hydrophobic ions, the liquid]liquid junction behaves similarly to an ideally polarizable metal electrode. Under this condition, the Galvani potential difference can be effectively controlled by a four-electrode potentiostat [4,5]. A schematic representation of a typical electrochemical cell is shown in Fig. 1 [6]. Cyclic voltammo-grams illustrating the potential window for the water] 1,2-dichloroethane (DCE) interface for various electrolytes are also shown in Fig. 1. In the presence of bis(triphenylpho-sphoranylidene)ammonium hexafluorophosphate (BTPPA PFe) the supporting electrolyte in DCE, the potential window is limited to less than 200 mV due to the hydrophilicity of the anion. Wider polarizable potential ranges are obtained on replacing... 

Two limiting cases for the description of an electrode are the ideally polarizable electrode and the ideally nonpolarizable electrode [8, 9, 14], The ideally polarizable electrode corresponds to an electrode for which the Zfaiadaic element has infinite resistance (i.e., this element is absent). Such an electrode is modeled as a pure capacitor, with Cdi = Aq 6V (equation 26), in series with the solution resistance. In an ideally polarizable electrode, no electron transfer occurs across the electrode/electrolyte interface at any potential when current is passed rather all current is through capacitive action. No sustained current flow is required to support a large voltage change across the electrode interface. An ideally polarizable electrode is not used as a reference electrode, since the electrode potential is easily perturbed... 

Consider a metal electrode consisting of a silver wire placed inside the body, with a solution of silver ions between the wire and ECF, supporting the reaction Ag" + e <— Ag. This is an example of an electrode of the first kind, which is defined as a metal electrode directly immersed into an electrolyte of ions of the metal s salt. As the concentration of silver ions [Ag" ] decreases, the resistance of the interface increases. At very low silver ion concentrations, the Faradaic impedance Zfaradaic becomes very large, and the interface model shown in Fig. 3(a) reduces to a solution resistance in series with the capacitance C. Such an electrode is an ideally polarizable electrode. At very high silver concentrations, the Faradaic impedance approaches zero and the interface model of Fig. 3(a) reduces to a solution resistance in series with the Faradaic impedance Zfaradaic. which is approximated by the solution resistance only. Such an electrode is an ideally nonpolarizable electrode. 

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