Reference electrodes standard hydrogen

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).

Measurements of the open circuit potential (OCP) were performed by linear sweep voltammetry with the anode of the electrolyser set as the working electrode, and the cathode set as both counter and reference electrodes. The hydrogen reference electrode condition was created by saturating the catholyte with H2 gas at the room temperature. The measured OCP values were refered to the standard hydrogen electrode (SHE). Since the potential of the hydrogen reference electrode varied from SHE depending on the HC1 concentration used in the experiement, this correction was taken into account for all measured OCP. 

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

In a redox electrode both components of the redox system are in solution, and their relative concentrations determine the potential of an inert electrode immersed in the system. Such electrodes have been used as reference electrodes only in special cases, because the potential is dependent on two concentrations, the ratio of which must be kept constant. The primary standard reference electrode, the hydrogen electrode, has been used as practical reference electrode in certain case, as in HF [217,218]. 

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 ... 

It is not possible to measure the voltage generated by half a cell, and it has been agreed that the voltage should be measured with respect to a reference electrode using hydrogen gas, called the standard hydrogen electrode. The reference electrode is a... 

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... 

The mercurous sulfate [7783-36-OJ, Hg2S04, mercury reference electrode, (Pt)H2 H2S04(y ) Hg2S04(Hg), is used to accurately measure the half-ceU potentials of the lead—acid battery. The standard potential of the mercury reference electrode is 0.6125 V (14). The potentials of the lead dioxide, lead sulfate, and mercurous sulfate, mercury electrodes versus a hydrogen electrode have been measured (24,25). These data may be used to calculate accurate half-ceU potentials for the lead dioxide, lead sulfate positive electrode from temperatures of 0 to 55°C and acid concentrations of from 0.1 to Sm. 

The thermodynamics of electrochemical reactions can be understood by considering the standard electrode potential, the potential of a reaction under standard conditions of temperature and pressure where all reactants and products are at unit activity. Table 1 Hsts a variety of standard electrode potentials. The standard potential is expressed relative to the standard hydrogen reference electrode potential in units of volts. A given reaction tends to proceed in the anodic direction, ie, toward the oxidation reaction, if the potential of the reaction is positive with respect to the standard potential. Conversely, a movement of the potential in the negative direction away from the standard potential encourages a cathodic or reduction reaction. 

Ion-selective electrodes are a relatively cheap approach to analysis of many ions in solution. The emf of the selective electrode is measured relative to a reference electrode. The electrode potential varies with the logarithm of the activity of the ion. The electrodes are calibrated using standards of the ion under investigation. Application is limited to those ions not subject to the same interference as ion chromatography (the preferred technique), e.g. fluoride, hydrogen chloride (see Table 10.3). 

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. 

It is apparent that since the electrode potential of a metal/solution interface can only be evaluated from the e.m.f. of a cell, the reference electrode used for that purpose must be specified precisely, e.g. the criterion for the cathodic protection of steel is —0-85 V (vs. Cu/CuSOg, sat.), but this can be expressed as a potential with respect to the standard hydrogen electrode (S.H.E.), i.e. -0-55 V (vs. S.H.E.) or with respect to any other reference electrode. Potentials of reference electrodes are given in Table 21.7. 

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... 

The most widely used reference electrode, due to its ease of preparation and constancy of potential, is the calomel electrode. A calomel half-cell is one in which mercury and calomel [mercury(I) chloride] are covered with potassium chloride solution of definite concentration this may be 0.1 M, 1M, or saturated. These electrodes are referred to as the decimolar, the molar and the saturated calomel electrode (S.C.E.) and have the potentials, relative to the standard hydrogen electrode at 25 °C, of 0.3358,0.2824 and 0.2444 volt. Of these electrodes the S.C.E. is most commonly used, largely because of the suppressive effect of saturated potassium chloride solution on liquid junction potentials. However, this electrode suffers from the drawback that its potential varies rapidly with alteration in temperature owing to changes in the solubility of potassium chloride, and restoration of a stable potential may be slow owing to the disturbance of the calomel-potassium chloride equilibrium. The potentials of the decimolar and molar electrodes are less affected by change in temperature and are to be preferred in cases where accurate values of electrode potentials are required. The electrode reaction is... 

The modern definition of pH is an operational one and is based on the work of standardisation and the recommendations of the US National Bureau of Standards (NBS). In the 1987 IUPAC definition39 the difference in pH between two solutions S (a standard) and X (an unknown) at the same temperature with the same reference electrode and with hydrogen electrodes at the same hydrogen pressure is given by ... 

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. 

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