Reference electrode standard hydrogen electrode

Pavlishchuk and Addison [207] smdied the mutual potentials between various Ag /Ag electrodes in AN and the Fc /Fc couple in AN and also between these electrodes and various types of aqueous reference electrodes (standard hydrogen electrode, SCE, etc.). In the report, they showed that the literamre data on the half-wave potential of Fc /Fc couple in AN versus aqueous reference electrodes were scattered from one report to another. For example, the data versus aqueous SCE were scattered between 315 and 480 mV. Because most of the literature did not give detailed compositions of the junction, the reasmis for the scattering are not known. But the variation in the compositimi of the liquid junction between H2O and AN solutions and thus the variation in the LJP there seem to be one reason for that. 

As mentioned previously, absolute potentials cannot be measured. Rather, potentials are measured against a reference electrode. The hydrogen electrode is perhaps the most common, and at standard conditions, it is defined to be 0 V. The hydrogen electrode reaction is... 

Figure 33 Schematic representation of the primary reference, the standard hydrogen electrode (SHE).

Ey midpoint redox potential of species) referred to standard hydrogen electrode (mV)... 

The latter condition is internationally accepted as a reference point (standard hydrogen electrode). If another electrode or another half-cell is now combined with this reference system then the corresponding cell voltage is given by (compare with Eqs. 3.34 and 3.35) ... 

Fig. 3.7 Thermodynamic potentials of methanol/Oj and methanol/hydrogen peroxide in acid systems [24]. Potentials vs. are referred against standard hydrogen electrode (SHE) at 25°C...

Since Eh and pE compare potential and activity with respect to the same reference (the standard hydrogen electrode), we may compare the oxygen electrode with the sulfur electrode by subtracting the expression for sulfur from the expression for oxygen ... 

The standard electrode potential at the above mentioned standard state conditions is denoted by °. For the MCAT, the values of the standard electrode (reduction) potentials will be given to you if you are required to solve such a question. Do not try to memorize those values. The standard electrode potentials are based on an arbitration with reference to standard hydrogen electrode. The standard hydrogen electrode potential is considered to be 0 volt. 

The electrode potential of a reaction X referred to standard hydrogen electrode = the voltage of the cell in which hydrogen reaction proceeds as oxidation andX as reduction. 

Fig. 23.2 Galvanic cell made up of a reference halfcell (standard hydrogen electrode) and a measuring half-cell containing the redox pair Rd/Ox dissolved in the corresponding electrolyte.

Upb/pbso4 = — 0.3 V (referred to standard hydrogen electrode). These values depend on acid concentration (cf. Fig. 1.2 detailed table in Ref. 5 and the actual value at certain activities in Eq. (10)). 

Table 2.1 gives the standard electrode potentials of metals with reference to standard hydrogen electrode (SHE) which is arbitrarily defined as zero. Potentials between metals are determined by taking the absolute difference between their standard potentials. The determination of standard electrode potential is shown Fig. 2.15. 

Table 2 Platinum reactions, potential listed in reference to standard hydrogen electrode [7]...

Standard Hydrogen Electrode The standard hydrogen electrode (SHE) is rarely used for routine analytical work, but is important because it is the reference electrode used to establish standard-state potentials for other half-reactions. The SHE consists of a Pt electrode immersed in a solution in which the hydrogen ion activity is 1.00 and in which H2 gas is bubbled at a pressure of 1 atm (Figure 11.7). A conventional salt bridge connects the SHE to the indicator half-cell. The shorthand notation for the standard hydrogen electrode is... 

Since the single potential of a metal cannot be measured it is necessary to use a suitable reference elecrode such as the Hg/Hg2Cl2/KCl electrode or the Ag/AgCl/KCl electrode, and although potentials are frequently expressed with reference to the standard hydrogen electrode (S.H.E.) the use of this electrode in practice is confined to fundamental studies rather than testing. 

Electrode Potential (E) the difference in electrical potential between an electrode and the electrolyte with which it is in contact. It is best given with reference to the standard hydrogen electrode (S.H.E.), when it is equal in magnitude to the e.m.f. of a cell consisting of the electrode and the S.H.E. (with any liquid-junction potential eliminated). When in such a cell the electrode is the cathode, its electrode potential is positive when the electrode is the anode, its electrode potential is negative. When the species undergoing the reaction are in their standard states, E =, the stan-... 

When the activity of the ion M"+ is equal to unity (approximately true for a 1M solution), the electrode potential E is equal to the standard potential Ee. Some important standard electrode potentials referred to the standard hydrogen electrode at 25 °C (in aqueous solution) are collected in Table 2.5.5... 

Figure 2 illustrates the resulting situation. Due to the strong acidic solution in the battery, it corresponds lo Fig. 1 for small pH values, but here the electrode potential is drawn on the vertical axis. The values are referred to the above-mentioned standard hydrogen electrode. To enlarge the scale, the range between 0 and 1.2 V is omitted. 

Details are given only if relevant for the stated value of iisHE- l All potentials refer to a standard hydrogen electrode in the used solvent. Exemptions are stated explicitly. 

All values taken from literature sources are given on the standard hydrogen electrode scale. If necessary, conversions were based on standard values for the involved reference electrodes. If a single report contains several values measured with different concentrations of the electrolyte under investigation, all values are included in the table in order to allow insight into the influence of the concentration. 

Defining a reference value for the SHE makes it possible to determine E ° values of all other redox half-reactions. As an example. Figure 19-14 shows a cell in which a standard hydrogen electrode is connected to a copper electrode in contact with a 1.00 M solution of C U . Measurements on this cell show that the SHE is at higher electrical potential than the copper electrode, indicating that electrons flow from the SHE to the Cu... 

Constant A in Eqs. (29.5) and (29.6) is about 4.4 eV when the standard hydrogen electrode is used as the reference electrode. This value has been determined from experimental values for the electron work function of mercury in vacuum, which is 4.48 eV, and for the Volta potential, between the solution and a mercury electrode polarized to = 0 V (SHE), which is -0.07 V (the work of electron transfer is 0.07 eV). The sum of these two values, according to Eq. (9.8), corresponds to the solution s electron work function at this potential (i.e., to the value of constant A with an inverted sign). 

Similar to electrode potentials, standard electrode potentials have so far been referred to the standard hydrogen electrode (SHE). These data are thus designated by vs. SHE after the symbol V, that is E(AgCI/Ag Cl- =... 

The standard electrode potential [1] of an electrochemical reaction is commonly measured with respect to the standard hydrogen electrode (SHE) [2], and the corresponding values have been compiled in tables. The choice of this reference is completely arbitrary, and it is natural to look for an absolute standard such as the vacuum level, which is commonly used in other branches of physics and chemistry. To see how this can be done, let us first consider two metals, I and II, of different chemical composition and different work functions 4>i and 4>ii-When the two metals are brought into contact, their Fermi levels must become equal. Hence electrons flow from the metal with the lower work function to that with the higher one, so that a small dipole layer is established at the contact, which gives rise to a difference in the outer potentials of the two phases (see Fig. 2.2). No work is required to transfer an electron from metal I to metal II, since the two systems are in equilibrium. This enables us calculate the outer potential difference between the two metals in the following way. We first take an electron from the Fermi level Ep of metal I to a point in the vacuum just outside metal I. The work required for this is the work function i of metal I. 

The internationally accepted primary reference is the standard hydrogen electrode (SHE). The potential of the SHE half-cell is dehned as 0.000 V at all temperatures. We say the schematic for the half-cell is... 

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