Measured electrode potential

Similar designs are used for other REs on the basis of poorly soluble mercury compounds (1) the mercury-mercurous sulfate RE with H2SO4 or K2SO4 solutions saturated with Hg2S04, for which = 0.6151V and (2) the mercury-mercuric oxide RE, for measuring electrode potentials in alkaline solutions, with KOH solution saturated with HgO, for which = 0.098 V and E = 0.920 V. 

When the two phases have the same chemical composition, the chemical potentials are equal, and then AV — — cf>2, which was already pointed out in Section 2.1. In our case both leads are made of the same material, platinum so the measured electrode potential,... 

Keeping in view the above serious anomalies commonly encountered with direct potentiometry, such as an element of uncertainty triggered by liquid junction potential (E.) and high degree of sensitivity required to measure electrode potential (E), it promptly gave birth to the phenomenon of potentiometric titrations,... 

Worked Example 3.11. We know the concentration of copper sulfate to be 0.01 mol dm from other experiments, and so we also know (from suitable tables) that the mean ionic activity coefficient of the copper sulfate solution is 0.404. The measured electrode potential was Ec j+ — 0.269 V and = 0.340 V. We will calculate the... 

The term A (Pt,M) appears in all measurements and thus does not influence the order of the measured electrode potentials. It is the potential difference that appears when two dissimilar conductors come into contact. Since the Fermi energies of two different metals are in general different, a flow of electrons occurs that tends to equalize the Fermi energies (i.e., their chemical potential). The Fermi level is either (1) the uppermost (the top) filled energy level in a partially occupied valence band of electrons in a solid, or (2) the boundary between the filled and the empty states in a band of electrons in a solid (Chapter 3). This electron flow charges up one conductor relative to the other and the contact potential difference results (Fig. 5.3). 

This equation is virtually identical to the Jdnetically deduced version of Eq. (7.40). However, it is not yet formally identical with that of Nernst, which was deduced long before the concept of a Galvani potential difference (MdS< >) across the metal/solution interface was introduced (Lange and Misenko, 1930). Nernst s original treatment was in terms of the electrode potential and symbolized by V. It is possible to show (see Section 3.5.15) that for a given electrode, M S< > - V + const. (i.e., the factors that connect the measured electrode potential to the potential across the actual interface) do not depend on the activity of ions in the solution. Hence, using now the relative electrode potentials, Vt in place of the absolute potentials ,... 

In aqueous solutions, the method of measuring electrode potentials has been well established. The standard hydrogen electrode (SHE) is the primary reference electrode and its potential is defined as zero at all temperatures. Practical measurements employ reference electrodes that are easy to use, the most popular ones being a silver-silver chloride electrode and a saturated calomel electrode (Table 5.4). The magnitude of the liquid junction potential (LJP) between two aqueous electrolyte solutions can be estimated by the Henderson equation. However, it is usual to keep the LJP small either by adding the same indifferent electrolyte in the two solutions or by inserting an appropriate salt bridge between the two solutions. 

In contrast, in non-aqueous solutions, the method of measuring electrode potentials has not been well established. The most serious problem is the reference electrode there is no primary reference electrode like the SHE in aqueous solutions and no reference electrode as reliable as the aqueous Ag/AgCl electrode. Thus, various reference electrodes have been employed in practical measurements, making the comparison of potential data difficult. As will be described later, various efforts are being made to improve this situation. 

When you calibrate an electrode with standard buffers, you measure a voltage with the electrode in each buffer. The pH of buffer SI is pHs, and the measured electrode potential in this buffer is Es>- The pH of buffer S2 is pHS2 and the measured electrode potential is s2-The equation of the line through the two standard points is... 

With the apparatus in Figure 16-1. we measure electrode potential. , versus volume of titrant, V, during a redox titration. The end point is the maximum of the first derivative, AE/AV, or the zero crossing of the second derivative. A(AE/AV)/AV (Figure 11-6). 

E = measured electrode potential E0 = reference potential (a constant)... 

Ion-selective electrodes belong to the group of potentiometric methods. Many electrode systems, partly well known, partly in development and under investigation, show a Nemstian relationship between the measured electrode potential and the activity of a species in solution. Important conditions to be fulfilled for the development of ion-selective electrodes are the affinity of a membrane surface for a typical ion or molecule and a minimum ion conductivity over the membrane. If possible, but not necessarily, these conditions should be fulfilled at room temperature. 

The pH meter used for pH measurement consists of these glass and reference electrodes and a potentiometer for measuring electrode potential. The pH meter must be daily calibrated against the standard buffer solutions of pH 4, 7, and 10. pH measurements are affected by temperature and the presence of very high concentrations of suspended matter. 

In order to relate redox, EF, and electrode potentials it is important to utilize the same reference state, namely vacuum14. In relation to vacuum the energy of the standard hydrogen electrode is —4.44eV (Fig. 4.7). When we measure electrode potentials, we measure the corresponding value of Ef through the relation... 

The potentials of various metals shown in Table 1.5 are with respect to the standard hydrogen electrode (SHE). The hydrogen electrode is not very convenient to use in practice as a reference electrode in the context of measuring electrode potentials. Some of the reference electrodes used in practice are detailed below (see Table 1.6). 

The measured electrode potential with respect to calomel electrode is added to the value of calomel electrode to obtain the value of the potential with respect to the standard hydrogen electrode. 

When an electrochemical reaction is perturbed from its equilibrium state, the relative stabilities of the species in the reaction are changed. The change due to the perturbation is reflected in the measured electrode potential, which differs from the equilibrium... 

One way of correcting for this H+ diffusion effect is to evaluate the reference voltage for each nitrate activity and measured electrode potential and make the reference voltage functionally dependent on the concentration of the H+ ions in the solution. Thus, through careful calibration with pure HN03 solutions and a knowledge of the H+ concentration of the sample, the reference voltage can be calculated and nitrate ion electrode can be used to determine the nitrate ion activity of the sample. The individual calibration corresponds to a series of experimental nitrate... 

The rate equations may be expressed in the more practical form as functions of the measured electrode potential, E, as follows ... 

Since aqueous solutions contain hydrogen ions, water molecules,and generally oxygen, the measured electrode potential of metals will usually be a mixed potential. Taking the... 

The valne (E°) for the standard electromotive force of a cell in which hydrogen nnder standard conditions is oxidized to hydroninm ions (solvated protons) at the left-hand electrode. This value is used as a standard to measure electrode potentials. 

Recommendations for measuring electrode potentials in nonaqueous solvents are given by Gritzner and Kuta [63]. 

By analogy to pH titration curves of acids and bases, it is customary in precipitation titrations to plot the quantity pM (defined by either — log [M " ] or — log a m ) against titration volume. For certain metals that form reversible electrodes with their ions, the measured electrode potential is a linear function of the logarithm of ion activity, so the titration curve can be realized experimentally in a potentiometric titration. In any case, the curve gives a useful indication of the sharpness of an endpoint break. 

It is necessary to distinguish between the concept of a potential and the measurement of a potential. Redox or electrode potentials (quoted in tables in Stability Constants of Metal-Ion Complexes or by Bard et al., 1985) have been derived from equilibrium data, thermal data, and the chemical behavior of a redox couple with respect to known oxidizing and reducing agents, and from direct measurements of electrochemical cells. Hence there is no a priori reason to identify the thermodynamic redox potentials with measurable electrode potentials. 

No electrode is fully selective other species often similar to that to be measured may affect the electrode response. The effect of another (disturbing) ion, S, on the measured electrode potential can typically be expressed by... 

In changing the concentration of ionic species to determine reaction orders, one should remember to maintain a high constant ionic strength of the solution by using nonelectroactive supporting electrolytes. Otherwise the measured electrode potential would include the potential drop within the diffuse double layer, which is concentration dependent (cf. Section III,A,1). 

Table 7.1. This may be why, in aerated soil solutions, measured electrode potentials tend to be around 500 mV, while theoretical O2 reduction potentials are considerably higher.

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