Nonpolarizing electrodes

Used m the measurement of the potentials of nonpolarized electrodes under conditions of zero cuiTcnt. Seldom used in organic reactions. 

To begin our discussion, it is useful to consider current-voltage curves for an ideal polarized and an ideal nonpolarized elearode. Polarization at a single electrode can be studied by coupling it with an electrode that is not easily polarized. Such electrodes have large surface areas and have half-cell reactions that are rapid and reversible. Design details of nonpolarized electrodes are described in subsequent chapters. 

The ideal polarized electrode is one in which current remains constant and independent of potential over a wide range, Figure 22-6a is a current-voltage curve for an electrode that is ideally polarized in the region between A and B. Figure 22-6b shows the current-volutgc relationship for a nonpolarized electrode that behaves ideally in the region between A and H. For this electrode, the potential is independent of the current. 

Perfectly polarizable electrodes pass a current between the electrode and the electrolytic solution by changing the charge distribution within the solution near the electrode. Thus, no actual current crosses the electrode-electrolyte interface. Nonpolarized electrodes, however, allow the current to pass freely across the electrode-electrolyte interface without changing the charge distribution in the electrolytic solution adjacent to the electrode. Although these types of electrodes can be described theoretically, neither can be realized in practice. It is possible, however, to come up with electrode structures that approximate their characteristics. 

Theory of Electrochemical MEMS Biosensors Potentiometric MEMS Biosensors Potentiometric MEMS biosensors are a direct analytical application of the Nemst equation through measurement of the potential of nonpolarized electrodes when current is zero. A reference electrode is necessary to investigate potential changes due to biochemical reactions. Without a stable reference electrode, reliability of the biosensor cannot be achieved [3]. 

FIGURE 22-6 Current-voltage curves in blue for an ideal (a) polarized and (b) nonpolarized electrode. Gray lines show departure from ideal behavior by real electrodes. 

Every interface is more or less electrically charged, unless special care is exercised experimentally [26]. The energy of the system containing the interface hence depends on its electrical state. The thermodynamics of interfaces that explicitly takes account of the contribution of the phase-boundary potential is called the thermodynamics of electrocapillarity [27]. Thermodynamic treatments of the electrocapillary phenomena at the electrode solution interface have been generalized to the polarized as well as nonpolarized liquid liquid interface by Kakiuchi [28] and further by Markin and Volkov [29]. We summarize the essential idea of the electrocapillary equation, so far as it will be required in the following. The electrocapillary equation for a polarized liquid-liquid interface has the form... 

One important advantage of the polarized interface is that one can determine the relative surface excess of an ionic species whose counterions are reversible to a reference electrode. The adsorption properties of an ionic component, e.g., ionic surfactant, can thus be studied independently, i.e., without being disturbed by the presence of counterionic species, unlike the case of ionic surfactant adsorption at nonpolar oil-water and air-water interfaces [25]. The merits of the polarized interface are not available at nonpolarized liquid-liquid interfaces, because of the dependency of the phase-boundary potential on the solution composition. 

UMEs decrease the effects of non-Earadaic currents and of the iR drop. At usual timescales, diffusional transport becomes stationary after short settling times, and the enhanced mass transport leads to a decrease of reaction effects. On the other hand, in voltammetry very high scan rates (i up to 10 Vs ) become accessible, which is important for the study of very fast chemical steps. For organic reactions, minimization of the iR drop is of practical value and highly nonpolar solvents (e.g. benzene or hexane [8]) have been used with low or vanishing concentrations of supporting electrolyte. In scanning electrochemical microscopy (SECM [70]), the small size of UMEs is exploited to locahze electrode processes in the gm scale. 

Ion-Selective Electrodes based on Bis-Thiourea Receptors. Bis-thiourea derivatives 14, 15, and 17, which have a good membrane solubility, sufficient lipophilicity to prevent leaching into the aqueous sample solution, and a low tendency for self-aggregation in nonpolar solvents, were incorporated into PVC matrix liquid membranes for ISEs. While membrane electrodes based on the dibutyl derivative 14 gave a phosphate response almost identical to that of a conventional anion-exchanger electrode, a membrane electrode based on the phenyl-substituted bis-thiourea 15 exhibited a slightly improved phosphate response, which seems to be the result of improved complexation of phosphate in the sensor membrane. 

We begin by pointing out that this concept of covering an electrode surface with a chemically selective layer predates chemically modified electrodes. For example, an electrode of this type, the Clark electrode for determination of 02, has been available commercially for about 30 years. The chemically selective layer in this sensor is simply a Teflon-type membrane. Such membranes will only transport small, nonpolar molecules. Since 02 is such a molecule, it is transported to an internal electrolyte solution where it is electrochemically reduced. The resulting current is proportional to the concentration of 02 in the contacting solution phase. Other small nonpolar molecules present in the solution phase (e.g., N2) are not electroactive. Hence, this device is quite selective. 

---