Thermodynamics reference electrode

Silver-silver halide 4 3 3 Secondary reference electrode, thermodynamic studies... 

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. 

The thermodynamic driving force behind the corrosion process can be related to the corrosion potential adopted by the metal while it is corroding. The corrosion potential is measured against a standard reference electrode. For seawater, the corrosion potentials of a number of constructional materials are shown in Table 53.1. The listing ranks metals in their thermodynamic ability to corrode. Corrosion rates are governed by additional factors as described above. 

Several significant electrode potentials of interest in aqueous batteries are listed in Table 2 these include the oxidation of carbon, and oxygen evolution/reduction reactions in acid and alkaline electrolytes. For example, for the oxidation of carbon in alkaline electrolyte, E° at 25 °C is -0.780 V vs. SHE or -0.682 V (vs. Hg/HgO reference electrode) in 0.1 molL IC0 2 at pH [14]. Based on the standard potentials for carbon in aqueous electrolytes, it is thermodynamically stable in water and other aqueous solutions at a pH less than about 13, provided no oxidizing agents are present. 

In the case of a solution with a previously known aH+ (see below), we could determine 2°H+-.H2(iatm)> provided that a reference electrode of zero potential is available however, experiments, especially with the capillary electrometer of Lippmann, did not yield the required confirmation about the realization of such a zero reference electrode16. Later attempts to determine a single electrode potential on the basis of a thermodynamic treatment also were not successful17. For this reason, the original and most practical proposal by Nernst of assigning to the standard 1 atm hydrogen potential a value of zero at any temperature has been adopted. Thus, for F2H+ H2(iatm) we can write... 

The concentration overpotential i/c is the component of the overpotential due to concentration gradients in the electrolyte solution near the electrode, not including the electric double layer. The concentration overpotential is usually identified with the Nernst potential of the working electrode with respect to the reference electrode that is, the thermodynamic electromotive force (emf) of a concentration cell formed between the working electrode (immersed in electrolyte depleted of reacting species) and the reference electrode (of the same kind but immersed in bulk electrolyte solution) ... 

Nevertheless, since in thermodynamics the values of the standard potentials are normally referred to the NHE electrode, when one wishes to obtain thermodynamic data from electrochemical experiments it is usual to transform the redox potentials obtained employing other reference electrodes into values with respect to the NHE, according to the scales given in Chapter 3, Section 1.2. 

We will now look at the effects of Ej on thermodynamic calculations, and then decide on the various methods that can be used to minimize them. One of the most common reasons for performing a calculation with an electrochemical cell is to determine the concentration or activity of an ion. In order to carry out such a calculation, we would first construct a cell, and then, knowing the potential of the reference electrode, we would determine the half-cell potential, i.e. the electrode potential E of interest, and then apply the Nemst equation. 

Having revised a few basic electrochemical ideas, such as the nature of reference electrodes, the standard hydrogen electrode and the scale based on it, we next looked briefly at thermodynamic parameters such as the electrode potential E, the standard electrode potential f and emf, and then discussed how AG, AH and AS (where the prime indicates a frustrated cell equilibrium ) may be determined. 

This is, in fact, the way electrode potentials are measured in practice. A cell is made up of the electrode of interest (the working electrode, e.g., Cu in Fig. 7.14) and a reference electrode made of Pt over which is bubbled Hj- No current passes through the reference electrode, which is therefore at its thermodynamically reversible potential. A counter-electrode (not shown in Fig. 7.14) is coupled through a power source... 

On the one hand, it is important that the current that bears the signal making the voltmeter register be very small. The reason is that it must not disturb the reference electrode from its position of thermodynamic reversibility by providing it with significant overpotential. Since, according to Eq. (7.25),... 

Note that some electrochemical cells use, instead of conventional reference electrodes, indicator electrodes. These are electrodes that are not thermodynamically reversible but which may hold then-potential constant 1 mV for some minutes—enough to make some nonsteady-state measurements (see Chapter 8). Such electrodes can simply be wires of inert materials, e.g.. smooth platinum without the conditions necessary to make it a standard electrode exhibiting a thermodynamically reversible potential. However, many different electrode materials may serve m this relatively undemanding role. 

The second factor is associated with the fact that all electrolyte solutions exhibit finite resistance to the flow of current. Thus, the potential that is measured (Em eas) between the working and reference electrodes consists of two contributors, the real thermodynamic potential Cereal) and that arising from uncompensated solution resistance (IRU)... 

It is a relatively simple process to set up a scale of redox potentials in a non-aqueous medium using the standard hydrogen electrode in that medium as the fundamental reference electrode. Thus in liquid ammonia, which is a well studied non-aqueous solvent and for which there exists a considerable amount of thermodynamic information,31 the scale of standard electrode potentials is referred to the standard hydrogen electrode in liquid ammonia (equation 25), which is assigned the value of zero volts, and in which the H+ exists as a solvated species, i.e. NH4+. 

A solid state galvanic cell consists of electrodes and the electrolyte. Solid electrolytes are available for many different mobile ions (see Section 15.3). Their ionic conductivities compare with those of liquid electrolytes (see Fig. 15-8). Under load, galvanic cells transport a known amount of component from one electrode to the other. Therefore, we can predetermine the kinetic boundary condition for transport into a solid (i.e., the electrode). By using a reference electrode we can simultaneously determine the component activity. The combination of component transfer and potential determination is called coulometric titration. It is a most useful method for the thermodynamic and kinetic investigation of compounds with narrow homogeneity ranges. For example, it has been possible to measure in a... 

From the thermodynamic point of view, this is a multiphase system for which, at equilibrium, the Gibbs equation (A.20) must apply at each interface. Because there is no charge transfer in and out of layer (4) (an ideal insulator) the sandwich of the layers (3)/(4)/(5) also represents an ideal capacitor. It follows from the Gibbs equation that this system will reach electrostatic equilibrium when the switch Sw is closed. On the other hand, if the switch Sw remains open, another capacitor (l)/( )/(6) is formed, thus violating the one-capacitor rule. The signifies the undefined nature of such a capacitor. The open switch situation is equivalent to operation without a reference electrode (or a signal return). Acceptable equilibrium electrostatic conditions would be reached only if the second capacitor had a defined and invariable geometry. 

Abstract The primary method for pH is based on the measurement of the potential difference of an electrochemical cell containing a platinum hydrogen electrode and a silver/silver chloride reference electrode, often called a Harned cell. Assumptions must be made to relate the operation of this cell to the thermodynamic definition of pH. National metrology institutes use the primary method to assign pH values to a limited number of primary standards (PS). The required comparability of pH can be ensured only if the buffers used for the calibration of pH meter-electrode assemblies are traceable to... 

Typically a metal electrode at which thermodynamic equilibrium is established between the metal atoms and the corresponding cations in solution. One example is the copper/copper sulphate reference electrode (Cu/CuS04). 

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. 

Hg/HgO reference electrodes have been widely applied in liquid alkaline solutions [28], However, the natural and most appropriate use of the electrode would probably be in a pH-measuring role, namely as an OH sensor in strongly alkaline solutions [29], It is a thermodynamically reversible and... 

Potentiometry has found extensive application over the past half-century as a means to evaluate various thermodynamic parameters. Although this is not the major application of the technique today, it still provides one of the most convenient and reliable approaches to the evaluation of thermodynamic quantities. In particular, the activity coefficients of electroactive species can be evaluated directly through the use of the Nemst equation (for species that give a reversible electrochemical response). Thus, if an electrochemical system is used without a junction potential and with a reference electrode that has a well-established potential, then potentiometric measurement of the constituent species at a known concentration provides a direct measure of its activity. This provides a direct means for evaluation of the activity coefficient (assuming that the standard potential is known accurately for the constituent half-reaction). If the standard half-reaction potential is not available, it must be evaluated under conditions where the activity coefficient can be determined by the Debye-Hiickel equation. 

Some Practical Considerations in the Use of Salt Bridges. Salt bridges are most commonly used to diminish or stabilize the junction potential between solutions of different composition and to minimize cross-contamination between solutions. For example, in working with nonaqueous solvents an aqueous reference electrode often is used that is isolated from the test solution by a salt bridge that contains the organic solvent. However, this practice cannot be recommended, except on the grounds of convenience, because there is no way at present to relate thermodynamically potentials in different solvents to the same aqueous reference-electrode potential furthermore, there is a risk of contamination of the nonaqueous solvent by water. 

Reference Electrodes For Use in Fused-Salt Systems. The kinds of investigations that require reliable reference electrodes in molten salts duplicate those in aqueous electrolytes. Specifically, these include (1) thermodynamic... 

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