Nernst equations slope

Figure 3.16B shows a plot of EappIied versus log([0]/[R]) for the data in Figure 3.16A. The plot is linear as predicted by the Nernst equation. The slope of the plot is 30.8 mV, which corresponds to an n value of 1.92, and the intercept is 0.612 V vs. SCE, which corresponds to the E° of o-tolidine vs. SCE.

It is important to note that the electrode potential is related to activity and not to concentration. This is because the partitioning equilibria are governed by the chemical (or electrochemical) potentials, which must be expressed in activities. The multiplier in front of the logarithmic term is known as the Nernst slope . At 25°C it has a value of 59.16mV/z/. Why did we switch from n to z when deriving the Nernst equation in thermodynamic terms Symbol n is typically used for the number of electrons, that is, for redox reactions, whereas symbol z describes the number of charges per ion. Symbol z is more appropriate when we talk about transfer of any charged species, especially ions across the interface, such as in ion-selective potentiometric sensors. For example, consider the redox reaction Fe3+ + e = Fe2+ at the Pt electrode. Here, the n = 1. However, if the ferric ion is transferred to the ion-selective membrane, z = 3 for the ferrous ion, z = 2. 

The Tafel expressions for both the anodic and the cathodic reaction can be directly incorporated into a mixed potential model. In modeling terms, a Tafel relationship can be defined in terms of the Tafel slope (b), the equilibrium potential for the specific half-reaction ( e), and the exchange current density (70), where the latter can be easily expressed as a rate constant, k. An attempt to illustrate this is shown in Fig. 10 using the corrosion of Cu in neutral aerated chloride solutions as an example. The equilibrium potential is calculated from the Nernst equation e.g., for the 02 reduction reaction,... 

Ill) slopes at the te.p. The slopes (du/3pH) p decrease with Increasing c. The mobility u may be converted into f, using the Helmholtz-Smoluchowski equation if xa is large enough (no double layer polarization and no influence of surface conduction close to the zero point). Then, at low electrol3rte concentration / 9pH may approach 59 mV per pH unit at 25 , as would be the case for ilf° If the Nernst equation applies. However, such a steep slope persists only close to the zero point mostly it is much lower. Let us assume absence of specific adsorption (zeroth-order Stem theory, see flg. 3.17a) then we may write... 

It is critical that pH electrodes are calibrated often as the slope of their response curve will change with temperature, as defined by the Nernst equation [2], Calibration should be performed with at least two certified pH calibration buffers that bracket the expected pH of the sample. Calibration is performed as described in the manufacturer s instructions provided with the pH meter. Calibration should be checked by measuring the pH of the first calibration buffer. The result should be no more than 0.02 pH units different from the certified pH of the buffer after adjusted for temperature. pH measurements are best performed when calibration buffers and samples are held at a constant temperature. 

It can be seen that the corrosion current and potential depend on both the equilibrium potentials for the hydrogen evolution reaction and metal dissolution calculated from the Nernst equation, and the kinetic parameters, the exchange currents and the Tafel slopes. Table 9.1 shows the corrosion currents calculated for some typical values of these parameters it is also important to note that even a... 

This is the Nernst equation, after the physical chemist W. Nernst, who derived it at the end of the nineteenth century. As above, n is the number of electrons transferred in the cell reaction (2 in reaction 12.7), 5 the Faraday of charge, R the gas constant, and T the temperature (in kelvins). The constant 2.302 59 is used to convert from namral to base 10 logs. At 25 the quantity 2.30259 RT/3 has the value 0.05916, which is called the Nernst slope. The importance of (12.14) is that it allows calculation of the potentials of cells having nonstandard state concentrations (i.e., real cells) from tabulated values of standard half-cell values or tabulated standard Gibbs energies. 

The Nernst factor, 0.059 V for room temperature, does not always reach its theoretical value. In experimental work, it is better to use the expression Nernstian slope S of the function E = f(a). The slope must be determined empirically. The constant const in Eq. (7.8) is a combination of all terms not dependent on concentration. In practical work, the name asymmetry potential ( as) is preferred. This expression is derived from the expectation that the constant should be zero for a completely symmetric cell, i.e. if inner and outer solutions are of equal pH and if inner and outer reference electrodes are of identical types. In practice, Eas is not always zero but must be calibrated empirically by means of buffer solutions with known pH. By setting - log alHsO" ) = pH, the common form of the Nernst equation for the glass electrode results ... 

The Nernst equation is written in the form of a straight line. What are the dependent variable, the independent variable, the slope m, and the y-intercept bl... 

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