Reference-electrode potentials, temperature

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. 

In addition, the temperature dependence of the diffusion potentials and the temperature dependence of the reference electrode potential itself must be considered. Also, the temperature dependence of the solubility of metal salts is important in Eq. (2-29). For these reasons reference electrodes with constant salt concentration are sometimes preferred to those with saturated solutions. For practical reasons, reference electrodes are often situated outside the system under investigation at room temperature and connected with the medium via a salt bridge in which pressure and temperature differences can be neglected. This is the case for all data on potentials given in this handbook unless otherwise stated. 

A typical set of experimental data290a,290b is shown in Fig. 11. All measurements converge to the value measured by Grahame.286 At present, the of Hg in water can be confidently indicated5 as -0.433 0.001 V (SCE), i.e., -0.192 0.001 V (SHE). The residual uncertainty is related to the unknown liquid junction potential at the boundary with the SCE, which is customarily used as a reference electrode. The temperature coefficient of of the Hg/H20 interface has been measured and its significance discussed.7,106,1 8,291... 

Another remarkable feature of that probe is its complete independence of the reference electrode potential and nearly perfect temperature compensation. Thus, a pseudo-reference such as silver wire, functioning only as a signal return, can be used. 

Most factors in parameter G (Chapter 4, Equations 4.55 and 4.56) and the potential of the reference electrode are temperature-dependent this is possibly also the case for factor x. Equation 4.56 also represents the concentration of the hydroxide ion. This means that a potential sensor based on the prewave will also have to contain a pH sensor. The hydroxide ion concentration derived from the output signal of this additional sensor needs to be introduced in the algorithm for the calculation of the hydrogen peroxide concentration. As an additional sensor, a glass electrode is an obvious choice, with a temperature-dependent potential, which is the case also for the potential of the reference electrode associated with the glass electrode and for the pH of the buffers. In this research work, the influence of... 

Variation of Reference Electrode Potentials with Temperature pH Values of Standard Solutions Used in the Calibration of Glass Electrodes Temperature vs. pH Correlation of Standard Solutions Used for the Calibration of Electrodes Solid Membrane Electrodes Liquid Membrane Electrodes... 

VARIATION OF REFERENCE ELECTRODE POTENTIALS WITH TEMPERATURE... 

Variation of Reference Electrode Potentials with Temperature... 

Reference electrode potentials change with temperature. Both electrochemical reactions (Nernstian thermodynamics) and chemical solubilities, e.g. of the inner reference electrode solution, are affected. Accordingly, the temperature coefficient, dE/dT (mV °C4), varies from one type of reference electrode to another. To minimise errors in potential readings the coefficient should be low and at least known. Examples of temperature coefficients are given in Table 2.2. 

In practice, the value of k is never obtained as such, because the meter is adjusted so that the standard reads the correct value for its pX, the scale being Nernstian. As k contains in addition to the reference electrode potentials, a liquid-junction potential and an asymmetry potential, frequent standardization of the system is necessary. The uncertainty in the value of the junction potential, even when a salt bridge is used, is of the order of 0.5 mV. Consequently the absolute uncertainty in the measurement of pX is always at least 0.001/(0.059// ) or 0.02 if n = I, i.e. a relative precision of about 2% at best. For the most precise work a standard addition technique (p. 32) and close temperature control are desirable. All measurements should be made at constant ionic strength because of its effect on activities. Likewise,... 

The problem of the variation of reference-electrode potential can, of course, be avoided if the reference electrode is kept at constant T while the temperature of the working electrode is varied. However, then a thermal diffusion junction p.d. is introduced that depends on the temperature difference, as is well known. In deriving the result in Eq. (52) from Eq. (50), it is usually assumed that over relatively small ranges of T, itself does not vary with 7, i.e., ACp is small. This is not always the case, however, for reactions involving change of charge in hydroxylic, structured solvents (see Ref. 55). 

The term on the left-hand side in Eq. (1.7) is the potential of the electrochemical equilibrium A/B at the working electrodes with respect to the reference electrode. The first two terms on the right-hand side correspond to the equilibrium potential (versus the reference electrode potential) when the activities of the electroactive species are unity yg [j] = 1. When measured relative to a standard hydrogen electrode (SHE) this is the so-called standard electrode potential, the value of which is characteristic of the redox couple A/B for given temperature and pressure. 

General Electrochemical Setup. Catalytic studies to probe formic acid electrooxidation efficiencies are commonly not performed in a complex fuel cell, but using a three-electrode electrochemical cell at room temperature, consisting of a working (catalyst of interest), a counter (Pt mesh), and a reference electrode. Potentials are typically referenced against an RHE, saturated calomel electrode (SCE), or sUver/silver chloride (Ag/AgCl). 

Figure 23.14. Impact of ruthenium on oxygen reduction performance (a) CO stripping scans for the cathode and anode, (b) steady-state anode polarization plots before and alter contamination of the eathode, (c) H2-air steady-state polarization curves, and (d) DMFC steady-state polarization curves. Methanol concentration 0.3 M, anode potential during contamination 1.3 V vs. hydrogen counter/quasi-reference electrode, cell temperature 75 °C [65]. (Reprinted by permission of ECS— The Electrochemical Society, from Piela P, Eickes C, Brosha E, Garzon F, Zelenaya P. Ruthenium crossover in direct methanol fuel cell with Pt-Ru black anode.)...

The potential of the Ag/AgCl reference electrode is temperature dependent. At 25°C with 3.5 M KCl it has a value of +200 5mV. This amount is to be added to each measured EMF to refer it to the normal hydrogen electrode. If the potential of the Ag/AgCl reference electrode is determined at various temperatures between 0 and 95 C by measuring the EMF relative to a hydrogen electrode, whenever the electrode is warmed or cooled back to room temperature, the electrode potential returns to within 2mV of its original value. This temperature hysteresis effect is thus very small with the Ag/AgCl reference electrode. For this reason it is recommended for all applications in which the temperature cannot always be held constant or lies above 80°C. 

TABLE 8.20 Potentials of Reference Electrodes in Volts as a Function of Temperature Liquid-junction potential included. 

In Section 8, the material on solubility constants has been doubled to 550 entries. Sections on proton transfer reactions, including some at various temperatures, formation constants of metal complexes with organic and inorganic ligands, buffer solutions of all types, reference electrodes, indicators, and electrode potentials are retained with some revisions. The material on conductances has been revised and expanded, particularly in the table on limiting equivalent ionic conductances. 

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. 

Reference electrode Me/Me" system Electrolyte Potential at 25°C (V) Temperature dependence (mV/°C) Application... 

Potential control with zinc reference electrodes presented a problem because deposits of corrosion products are formed on zinc in hot water. This caused changes in the potential of the electrode which could not be tolerated. Other reference electrodes (e.g., calomel and Ag-AgCl reference electrodes) were not yet available for this application. Since then, Ag-AgCl electrodes have been developed which successfully operate at temperatures up to 100°C. The solution in the previous case was the imposition of a fixed current level after reaching stationary operating conditions [27]. 

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