Nemstian

Electrochemical methods may be classified into two broad classes, namely potentiometric metiiods and voltannnetric methods. The fonner involves the measurement of the potential of a working electrode iimnersed in a solution containing a redox species of interest with respect to a reference electrode. These are equilibrium experiments involving no current flow and provide themiodynamic infomiation only. The potential of the working electrode responds in a Nemstian maimer to the activity of the redox species, whilst that of the reference electrode remains constant. In contrast, m voltannnetric methods the system is perturbed... 

The general theoretical treatment of ion-selective membranes assumes a homogeneous membrane phase and thermodynamic equilibrium at the phase boundaries. Obvious deviations from a Nemstian behavior are explained by an additional diffusion potential inside the membrane. However, allowing stationary state conditions in which the thermodynamic equilibrium is not established some hitherto difficult to explain facts (e.g., super-Nemstian slope, dependence of the selectivity of ion-transport upon the availability of co-ions, etc.) can be understood more easily. 

In case b) a potentiometrically measured calibration curve for ion M+ would show a sigmoidal form, starting with a sub-Nernstian slope leading to a super-Nemstian... 

Thus, the peak separation can be used to determine the number of electrons transferred, and as a criterion for a Nemstian behavior. Accordingly, a fast one-electron process exhibits a AEp of about 59 mV Both the cathodic and anodic peak potentials are independent of die scan rate. It is possible to relate the half-peak potential (Ep/2. where the current is half of the peak current) to the polarographic half-wave potential, El/2 ... 

Recall that Nemstian behavior of diffusing species yields a r1 /2 dependence, hi practice, the ideal behavior is approached for relatively slow scan rates, and for an adsorbed layer that shows no intermolecular interactions and fast electron transfers. 

A Nemstian response of 59 mV per decade change in concentration is commonly observed (at 25°C). Relation to the partial pressure carbon dioxide is accomplished by the use of Henry s law. A catheter sensor configuration has been developed for in-vivo monitoring of blood carbon dioxide (61). 

In an electrode system, two different kinds of thermodynamic equilibrium play substantially important roles (1) Nemstian equilibrium with regard... 

The newly formed equilibrium, however, is broken easily and incessantly by the thermal motion of solution particles. Since the electrode system is not in Nemstian equilibrium at the potential, such a breakdown (nonequilibrium fluctuation) produces pitting dissolution. The physical quantities related to the dissolution fluctuate on one side of the electrostatic equilibrium, that is, the fluctuations take place toward the direction in which the reaction proceeds. 

However, this equilibrium is not the familiar Nemstian type, but is a state of electrostatic equilibrium in the electrical double layer. 

From an analysis of data for polypyrrole, Albery and Mount concluded that the high-frequency semicircle was indeed due to the electron-transfer resistance.203 We have confirmed this using a polystyrene sulfonate-doped polypyrrole with known ion and electron-transport resistances.145 The charge-transfer resistance was found to decrease exponentially with increasing potential, in parallel with the decreasing electronic resistance. The slope of 60 mV/decade indicates a Nemstian response at low doping levels. 

The nnmeric solution of Eq. (32.9) for the interface A" R+X with the common cation R+ is shown in Fig. 32.1. The expected Nemstian response of 59mV per decade is observed only in a limited range of the ratio of concentrations of electrolytes R+A and R X . When this ratio is too low or too high, the equilibrium potential approaches the distribution potential of the electrolyte that is present in excess. 

Orion Model 95-64). In practice, one simply determines E ntot by calibration with a standard solution without the necessity of knowing the various constants mentioned. The S02 electrode allows the determination of concentrations down to 10 8 Af with a response time of a few minutes. From the above it appears that the gas-sensing electrodes show Nemstian behaviour provided that the concentrations to be measured are not high there is little or no interference by other components in the sample solution. 

The cell voltage measurement in itself represents a point of decisive significance, where factors such as temperature of the measurement, and Nemstian behaviour and asymmetry of the electrode play a role together with the reliability and flexibility of the pH/mV meter. Such a meter consists of a null-point or a direct-reading meter. 

Irreversibility versus reversibility inpolarography. Previously in this chapter we dealt only with reversible redox systems, i.e., with truly Nemstian behaviour and merely diffusion control. This also applies to combined processess of electron transfer and chemical reaction (e.g., complexation) provided that both take place instantly. For instance, in EC such as... 

FIGURE 4.5 Typical ISE response curves to monovalent cations (solid circles) with a response slope of 59.2mV decade-1 and to divalent cations (open circles) with a Nemstian slope of 29.6mV decade-1. Intercepts of the linear ranges of sensor responses define the lower and higher detection limits. 

Depending on the fabrication techniques and deposition parameters, the pH sensitive slope of IrOx electrodes varies from near-Nemstian (about 59 mV/pH) to super-Nemstian (about 70mV/pH or higher). Since the compounds in the oxide layers are possibly mixed in stoichiometry and oxidation states, most reported iridium oxide reactions use x, y in the chemical formulas, such as lr203 xH20 and IrOx(OH)y. Such mixed oxidation states in IrOx compounds may induce more H+ ion transfer per electron, which has been attributed to causing super-Nerstian pH responses [41],... 

Considering the H+ dependent redox reaction between two oxidation states of the iridium oxide as the basis of the pH sensing mechanism, the electrode potential changes to the hydrogen ion concentration are expressed by Nemstian equation ... 

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