Electrochemical cells definition

Equations 11.19-11.21 are defined for a potentiometric electrochemical cell in which the pH electrode is the cathode. In this case an increase in pH decreases the cell potential. Many pH meters are designed with the pH electrode as the anode so that an increase in pH increases the cell potential. The operational definition of pH then becomes... 

Electrochemical cells may be used in either active or passive modes, depending on whether or not a signal, typically a current or voltage, must be actively appHed to the cell in order to evoke an analytically usehil response. Electroanalytical techniques have also been divided into two broad categories, static and dynamic, depending on whether or not current dows in the external circuit (1). In the static case, the system is assumed to be at equilibrium. The term dynamic indicates that the system has been disturbed and is not at equilibrium when the measurement is made. These definitions are often inappropriate because active measurements can be made that hardly disturb the system and passive measurements can be made on systems that are far from equilibrium. The terms static and dynamic also imply some sort of artificial time constraints on the measurement. Active and passive are terms that nonelectrochemists seem to understand more readily than static and dynamic. 

The most appropriate experimental procedure is to treat the metal in UHV, controlling the state of the surface with spectroscopic techniques (low-energy electron diffraction, LEED atomic emission spectroscopy, AES), followed by rapid and protected transfer into the electrochemical cell. This assemblage is definitely appropriate for comparing UHV and electrochemical experiments. However, the effect of the contact with the solution must always be checked, possibly with a backward transfer. These aspects are discussed in further detail for specific metals later on. 

It must be emphasized that Equations (5.24) and (5.25) stem from the definitions of Fermi level, work function and Volta potential and are generally valid for any electrochemical cell, solid state or aqueous. We can now compare these equations with the corresponding experimental equations (5.18) and (5.19) found to hold, under rather broad temperature, gaseous composition and overpotential conditions (Figs. 5.8 to 5.16), in solid state electrochemistry ... 

Electrochemical cells can be constructed using an almost limitless combination of electrodes and solutions, and each combination generates a specific potential. Keeping track of the electrical potentials of all cells under all possible situations would be extremely tedious without a set of standard reference conditions. By definition, the standard electrical potential is the potential developed by a cell In which all chemical species are present under standard thermodynamic conditions. Recall that standard conditions for thermodynamic properties include concentrations of 1 M for solutes in solution and pressures of 1 bar for gases. Chemists use the same standard conditions for electrochemical properties. As in thermodynamics, standard conditions are designated with a superscript °. A standard electrical potential is designated E °. 

The existence and use of batteries is thought to have roots in prehistoric times, whereby, through archeological discoveries, it was discovered that prehistoric people had created an electrochemical cell that would qualify, under today s definition, as a battery. A curiosity found in Baghdad in 1932 was probably representative of battery technology dating as far back as 2500 years.1 Such a primitive... 

An electrochemical cell is defined as two or more half-cells in contact with a common electrolyte . We see from this definition how a cell forms within the mouth, with aluminium as the more positive pole (the anode) and the fillings acting as the more negative pole (the cathode). Saliva completes this cell as an electrolyte. All the electrochemical processes occurring are contained within the boundaries of the cell. 

Reference Electrodes By definition, the normal hydrogen electrode (N H E) is the reference for electrode potentials (see Sect. 2.3.2.1), but practically it is scarcely usable. A reference electrode (RE) has to provide a well-defined potential between the electrolyte and its electric connector, joined with the input of the measuring instrument. Usually, a metal and a slightly soluble salt of this metal is applied (secondary electrode) [76, 77]. The electrolyte in the RE is connected to the electrolyte in the electrochemical cell via a diaphragm, which has to separate both electrolytes, as far as possible without a potential difference (see below). 

In electrochemical mass spectrometry (ECMS) [24], the electrochemical cell is directly coupled with the mass spectrometer and the volatile product or intermediate at the electrode are introduced into the mass spectrometer. Thus, from the mass signal-potential curve measured in situ and simultaneously with the current-potential curve, it is possible to definitely identify the volatile substance and the potential of its formation. 

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... 

This is not the official definition, which is a practical definition based on the measured potentials of some exactly defined electrochemical cells in which the hydrogen ion activity is calculated by an extended Debye-Huckel theory. 

Consider Equation (6) - that is, the definition of AG. On this basis, one may formulate the equilibrium potential of an electrochemical cell in the following way ... 

The WE and CE combination represents a driven electrochemical cell. The presence of the RE allows the separation of the applied potential into a controlled portion (between the RE and the WE) and a controlling portion (between the RE and the CE). The voltage between the RE and the CE is changed by the potentio-stat in order the keep the controlled portion at the desired value. Consider the application of a potential Vin to the WE that is more positive than its rest potential, VffiSt, with respect to RE. By definition, polarization of the WE anodically (i.e., in a positive direction) would lead to an anodic current through the WE-solution interface and a release of electrons to the external circuit. These electrons would be transported by the potentiostat to the CE. A reduction reaction would occur at the CE-solution interface facilitated by a more negative potential across it. The circuit would be completed by ionic conduction through the solution. 

Stockholm convention — (1953) Rules for the definite description of an -> electrochemical cell. 

However, there are penalties to pay if one uses the electrochemical cell method. First, there is the question of the value of —it should be known as a function of concentration, and such values are often not available and imply the need for a separate determination. Further, there is a nasty experimental point. One talks of the liquid-junction potential as though it were a clear and definite entity. The thermodynamic equation [Eq. (4.291)] assumes that there is a sharp boundary with linear change of concentration across a small distance (see Section 4.5.9). These conditions assumed in the deduction only last for a short time after the two solutions have been brought... 

The foregoing example illustrates how equilibrium constants for overall cell reactions can be determined electrochemically. Although the example dealt with redox equilibrium, related procedures can be used to measure the solubility product constants of sparingly soluble ionic compounds or the ionization constants of weak acids and bases. Suppose that the solubility product constant of AgCl is to be determined by means of an electrochemical cell. One half-cell contains solid AgCl and Ag metal in equilibrium with a known concentration of CP (aq) (established with 0.00100 M NaCl, for example) so that an unknown but definite concentration of Kg aq) is present. A silver electrode is used so that the half-cell reaction involved is either the reduction of Ag (aq) or the oxidation of Ag. This is, in effect, an Ag" Ag half-cell whose potential is to be determined. The second half-cell can be any whose potential is accurately known, and its choice is a matter of convenience. In the following example, the second half-cell is a standard H30" H2 half-cell. 

Fig. 3 Electrochemical cell for the definition of cell voltage measurement according to the IUPAC convention [51].

The first step is to draw a schematic diagram of the battery with a clear definition of the desired predicted variables (dependent variables) and the provided variables (independent variables). As mentioned before, a battery is a combination of several electrochemical cells connected in series or in parallel depending on the desired voltage and capacity, which increases the complexity of the model. 

Isoelectric point The pH at which an amino acid has no tendency to migrate under the influence of an electric field. lUPAC convention A set of definitions relating to electrochemical cells and their potentials also known as the Stockholm convention. 

Polarization (1) In an electrochemical cell, a phenomenon in which the magnitude of the current is limited by the low rate of the electrode reactions (kinetic polarization) or the slowness of transport of reactants to the electrode surface (concentration polarization). (2) The process of causing electromagnetic radiation to vibrate in a definite pattern. 

Unfortunately, simultaneous analytical solution of the mass transfer and kinetic equations of an electrochemical cell is usually complex. Thus, the cell is usually operated with definitive hydrodynamic characteristics. Operational techniques, relating to controlling either the potential or the current, have been developed to simplify the analysis of the electrochemical cell. Description of these operational techniques and their corresponding mathematical analyses are well discussed elsewhere. 

The arm of this Handbook is to combine the fundamental information and to provide a brief overview of recent advances in solid-state electrochemistry, with a primary emphasis on methodological aspects, novel materials, factors governing the performance of electrochemical cells, and their practical applications. The main focus is, therefore, centered on specialists working in this scientific field and in closely related areas, except for a number of chapters which present also the basic formulae and relevant definitions for those readers who are less familiar with theory and research methods in solid-state electrochemistry. Since it has been impossible to cover in total the rich diversity of electrochemical phenomena, techniques and appliances, priority has been given to recent developments and research trends. Those readers seeking more detailed information on specific aspects and applications are addressed to the list of reference material below, which includes both interdisciplinary and specialized books [8-20]. 

The over potential plays a central role in electrochemistry as it controls the electrochemical reactions. By convention it is generally measured as a positive value for reactions where electrons are transferred to the electrode. The associated current is also counted positively. In this case the electrode is called an anode. If electrons are transferred from, the electrode to the ions of the electrolyte, the over potential and the associated current are measured as negative values. The electrode is termed cathode. Using the definition of the over potential, the terminal voltage for an electrochemical cell is given by (see Fig. 3.2(b)) ... 

Figure 3.3 Potential distribution in an electrochemical cell (see text for definitions of the different physical quantities).

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