Nernst equation definition

Moreover, as the ratio uf/uf is by definition the partition coefficient of I, P , this parameter can be defined by putting into evidence the ratio of the ion activities in the Nernst equation ... 

It is very often necessary to characterize the redox properties of a given system with unknown activity coefficients in a state far from standard conditions. For this purpose, formal (solution with unit concentrations of all the species appearing in the Nernst equation its value depends on the overall composition of the solution. If the solution also contains additional species that do not appear in the Nernst equation (indifferent electrolyte, buffer components, etc.), their concentrations must be precisely specified in the formal potential data. The formal potential, denoted as E0, is best characterized by an expression in parentheses, giving both the half-cell reaction and the composition of the medium, for example E0,(Zn2+ + 2e = Zn, 10-3M H2S04). 

Let us now suppose that the waveform of figure 16.3 is applied to study the reversible oxidation of a species R to R in a given solvent. The reaction occurs at the working electrode (anode), and /i°(R/R ) is the standard potential of the R/R- couple. Because the standard potential of the reference electrode in our cell is known accurately relative to the standard potential of the SHE (E° = 0 by definition), we can write the cell reaction and the Nernst equation as... 

The ion-selective membrane is the key component of all potentiometric ion sensors. It establishes the preference with which the sensor responds to the ion of interest in the presence of various other ionic components of the sample. By definition, the ion-selective membrane forms a nonpolarized interface with the solution. If the interface is permeable to only one ion, the potential difference at that interface is expressed by the Nernst equation (6.6). If more than one ion can permeate, the interface can be anything between the liquid junction and the mixed potential. The key distinguishing feature is the absolute magnitude of the total exchange current density. 

The right hand side of Equation (3.71) is the same as that of the Nernst Equation (3.68) and so the left hand side of Equation (3.71) must be equal to the left hand side of the Nernst Equation (3.68), thus leading to Equation (3.67). An additional definition is... 

In complicated cases the number of independent components can be obtained most easily as follows add together all the molecular species which take part in the equihbrium, and subtract the number of chemical equations which come into play when the system is displaced from equihbrium. Nernst s definition of the number of independent components is equivalent to this. According to Nernst, n is the minimum number of molecular species which is sufficient to build up all the phases of the system. ... 

This equation is often called the Nernst equation for the ITIES, and the term A is in fact the standard Gibbs energy of transfer AG°t ° expressed on a potential scale, since, by definition, AG is equal to the difference between the standard Gibbs energy of solvation fif0 and the standard Gibbs energy of hydration q. 1 ... 

In the foregoing treatment of electrodes and cells we assumed implicitly that the electrode or cell was in equilibrium with respect to certain chemical and electrical transformations. By definition such an electrode or cell is reversible. To correlate measured values of cell potentials with the ones calculated by the Nernst equation, the measured values must be equilibrium or reversible values the potentiometric measurement in which no current is drawn from the cell is ideally suited for the measurement of reversible potentials. 

The for a half-reaction is the potential of that reaction versus the standard hydrogen electrode, with all species at unit activity. Most reduction potentials are not determined under such conditions, so it is expedient to define a formal reduction potential. This is a reduction potential measured under conditions where the reaction quotient in the Nernst equation is one and other nonstandard conditions are described solvent, electrolyte, pN, and so on. Formal reduction potentials are represented by °. Reduction potentials determined by cyclic voltammetry are usually formal potentials. The difference between standard and formal potentials is not expected to be great. Other definitions of the formal potential are offered. ... 

The equilibrium electrode potential Eq depends of course on the actual concentrations c or the activities a = c/ of the dissolved species according to the Nernst equation (where / is the activity coefficient). For the discussed electrode reactions with T = 298 K and the definition pH =-log Ch... 

The derivation of the Nernst equation is based on the assumption of chemical equi-libriumo It is, therefore, only to be applied under equilibrium conditions. Frequently, too little attention is paid to this restriction. If one theoretically considers the definition of chemical potential ... 

If the difference in concentration of the measured ion between the inner solution and the sample solution is too large (> 3 or 4 orders of magnitude), then deviations from the Nernst equation can be detected due to a diffusion potential inside the ion-selective membrane [68]. For more accurate measurements in such cases, the concentration of the inner fQling solution is adjusted to roughly match that of the sample solution. Most manufacturers use a 10 to 10" M inner solution. If the inner solutions is changed, one must remember to add a definite amount of chloride ion for a stable equilibrium Galvani potential at the Ag/AgCl shunt element. 

The standard potential of equation 8.176 is = 1.228 V. At standard state, the activity of gaseous oxygen is 1 by definition, and standard potential thus refers to H2O in equilibrium with an atmosphere of pure O2 at T = 25 °C and P = bar. Applying the Nernst and Faraday relations to equation 8.176 and transforming natural logarithms into base 10 logarithms, we obtain... 

Reversible Meaning comes directly from thermodynamics. Means at equilibrium" and applicable equation is that of Nernst Agrees with physical chemistry definition but extends to situation in which diffusion is rate determining interfacial processes provide negligible influence from their overpoteniial. Although i > tl, such situations called reversible and Nernst applied... 

Values of E° by definition refer to conditions under which all species are in their standard states at 298 K. For non-standard conditions the electrode potential, E, of a redox reaction is given by the familiar Nernst expression (equation 24), where... 

Arbitrary Potential Zero The Hydrogen Scale.—Since the single electrode potential [cf. equation (10)] involves the activity of an individual ionic species, it has no strict thermodynamic significance the use of such potentials is often convenient, however, and so the difficulty is overcome by defining an arbitrary zero of potential. The definition widely adopted, following on the original proposal by Nernst, is as follows ... 

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