Electromotive force equilibrium electrode potential

L electromotive force, emf (in an electrochemical cell) equilibrium electrode potential in volts... 

The electrode reactions taking place at the electrodes of direct methanol fuel cells, the overall current-producing reactions, and the corresponding thermodynamic values of equilibrium electrode potentials EP and electromotive force (EMF) of the direct methanol fuel cell are given as follows ... 

Electrode Potential, Equilibrium Electrode Potential, Standard Electrode Potential, Electromotive Force... 

The electrode potential of an electrode reaction at equilibrium can be measured as the electromotive force of an electrochemical cell composed of both the reaction electrode and the normed hydrogen electrode. The potential of the reaction electrode thus measured is taken as the equilibrium potential of the electrode reaction relative to the normal hydrogen electrode. 

After a calibration has been established, we may proceed with the sample measurements, which can be done with a submersible probe, in a flow-through cell, or in a plastic cup filled sampled water. Temperature variations are a source of several errors that affect pH measurements, causing changes in the equilibrium of the calibration solutions and samples and the drift of reference electrode. That is why field pH meters have a built-in temperature probe and a temperature compensation capability. Although the meter measures the electromotive force or potential in millivolts, the meter software delivers the readings in pH units. For routine work, we... 

The electromotive force of this cell is equivalent to the redox potential Fe2. of the reaction referred to the standard hydrogen electrode, i.e. the equilibrium potential of the redox reaction, and it is given by Eq. 9.33 at the standard temperature and pressure ... 

Let the terminals of the cell be of the same metal, M, as one of the electrodes. The electrostatic potentials, when the cell has reached equilibrium with the terminals disconnected from each other, are indicated above the phases there are three phase boundary potentials concerned in producing the final difference of potential, Fa— Va between the terminals. These are Va--Vs, —(VP—VS), and the metal-metal contact potential F — V their algebraic sum is the difference in electrostatic potential of the terminal wires. It is the electromotive force of the whole cell, and can, of course, be measured with a potentiometer. 

The cell is held under fixed conditions of temperature and pressure, and the electrodes are connected externally to a potentiometer. If the electromotive force produced by the cell is exactly balanced by the potential difference of the potentiometer, the reaction is held in equilibrium. The reaction may be made to proceed in the forward direction by a slight decrease in the opposing potential difference, and it may be reversed by a corresponding increase in the potential difference above the emf of the cell. 

In corrosion the dynamic electrochemical processes are of importance and hence considerations of the consequences of perturbation of a system at equilibrium are considered. Let us consider the familiar Daniel cell consisting of copper metal in copper sulfate, and zinc metal in zinc sulfate solution. This, as depicted in Figure 1.18 gives an electromotive force of 1.1 V when there is no current flow. When a small current flows through the resistance R, the potential decreases below 1.1 V. On continued flow of current, the potential difference between the electrodes approaches a value near zero, and... 

The reduction potential is an electrochemical concept. Consider a substance that can exist in an oxidized form X and a reduced form X . Such a pair is called a redox couple. The reduction potential of this couple can be determined by measuring the electromotive force generated by a sample half-cell connected to a standard reference half-cell (Figure 18.6). The sample half-cell consists of an electrode immersed in a solution of 1 M oxidant (X) and 1 M reductant (X ). The standard reference half-cell consists of an electrode immersed in a 1 M H+ solution that is in equilibrium with H2 gas at 1 atmosphere pressure. The electrodes are connected to a voltmeter, and an agar bridge establishes electrical continuity between the half-cells. Electrons then flow from one half-cell to the other. If the reaction proceeds in the direction... 

The potential difference between the electrodes in a working electrochemical cell is called the cell potential. The cell potential is not a constant and changes with time as the cell reaction proceeds. Thus the cell potential is a potential difference measured under non-equilibrium conditions as electric current is drawn from the cell. Electromotive force is the zero-current cel potential and corresponds to the potential difference of the cell when the cell (not the cell reaction) is at equilibrium. Infinitesimally small changes from this equilibrium are reversible with constant concentration and, consequently, it is possible to relate emf to thermodynamic properties. 

Thus, the electromotive force in a galvanic cell at equilibrium is equal to the potential difference between the electrodes. Substitution of Eq. (13-30) into Eq. (13-25) results in... 

Under conditions of oxygen or electrolyte concentration gradients, or due to heterogeneities of the metallic substrate, the cathodic and anodic sites may be separated. For each of the two electrodes, the equilibrium potential for their actual conditions can be determined by the Nemst equation. The electromotive force (EMF) for the corrosion process to occur is the difference between the two equilibrium potentials. When the cathode and the anode are short-circuited, a mixed potential results, known as corrosion potential, f corr- The value of corr lies between the two separate electrode potentials, although shifted towards the equilibrium potential of the faster reaction. This situation can be easily visualized with the help of the... 

For biological systems this tendency is expressed by the standard reduction potential, Eq, defined as the electromotive force (emf) in volts given by a half-cell in which the reductant and oxidant species are both present at 1.0 M concentration unit activity, at 25°C and pH 7.0 in equilibrium with an electrode which can reversibly accept electrons from the reductant species, according to the equation ... 

The chemical potential of the monolayer T may be determined from the potential (Er) of the working electrode with respect to an electrode of pure T, both in equilibrium with T ions in the solution, that is, the electromotive force (emf) of the cell TIT" " T Au is considered... 

When no current is flowing, the electrochemical changes occurring at an electrode are in steady state, i.e., atoms leave the electrode and become ions and the ions move to the electrode and becomes atoms. The process continuous till equilibrium is reached. A potential difference exists between electrode-electrolyte interfaces, which is known as electrode potential. The electrode potential between the electrode and electrolyte acts as a barrier to a faster rate of reaction, which is the electromotive force (emf) of the cell. External energy must be supplied so that ions are discharged at the required rate to promote current flow. Thus, when the current flows between the electrodes, several phenomena occur at the electrode surface that produce emf, which opposes the current flow. The deparmre of electrode potential from the equilibrium value upon passage of current is termed as polarization. ... 

The different procedures based on the measurement of electromotive force that may be applied in many varied ways in the equilibrium chemistry of aqueous solutions have a much more restricted use in non-aqueous solutions. It is difficult to construct measuring cells that have small and readily reproducible diffusion potentials or that are without a liquid junction. The glass electrode, perhaps the most extensively used type in aqueous solutions, does not function at all in some non-aqueous solutions, and with very low accuracy in others. Evaluation of the data obtained with its use is hampered by the limits of the acidity scales employed in such systems. The most promising type of ionselective membrane electrodes, the liquid ionic exchange membrane electrode, virtually cannot be employed in non-aqueous solutions. 

The potential difference is closely related to the difference of the electrochemical potential based on the electrochemical affinity. If we could measure A(p directly, we could organize the table of electromotive forces based on the Galvani potential difference. However, A

reference electrode to measure the half cell potential at an electrode. When a certain electrcxle is coupled with a reference electrode, then the electromotive force can be measured. Since we usually use some reference electrodes as standards, the electromotive force is defined as the equilibrium potential of the reaction. The table was made in such a way and the hydrogen reference electrode was used to measure and calculate potentials for the half cell reactions. 

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