Cell potential Electrochemical cells. Nernst equation

In potentiometry the potential of an electrochemical cell is measured under static conditions. Because no current, or only a negligible current, flows while measuring a solution s potential, its composition remains unchanged. For this reason, potentiometry is a useful quantitative method. The first quantitative potentiometric applications appeared soon after the formulation, in 1889, of the Nernst equation relating an electrochemical cell s potential to the concentration of electroactive species in the cell. ... 

Potential and Concentration—The Nernst Equation The potential of a potentio-metric electrochemical cell is given as... 

Despite the apparent ease of determining an analyte s concentration using the Nernst equation, several problems make this approach impractical. One problem is that standard-state potentials are temperature-dependent, and most values listed in reference tables are for a temperature of 25 °C. This difficulty can be overcome by maintaining the electrochemical cell at a temperature of 25 °C or by measuring the standard-state potential at the desired temperature. 

Electrochemical methods covered in this chapter include poten-tiometry, coulometry, and voltammetry. Potentiometric methods are based on the measurement of an electrochemical cell s potential when only a negligible current is allowed to flow, fn principle the Nernst equation can be used to calculate the concentration of species in the electrochemical cell by measuring its potential and solving the Nernst equation the presence of liquid junction potentials, however, necessitates the use of an external standardization or the use of standard additions. 

As a consequence, the equilibrium potential of the single half-cell also depends on the concentrations of the compounds involved. The Nernst equation [Eq. (24)], which is one of the most important electrochemical relations, explains this context... 

R is the ideal gas constant, T is the Kelvin temperature, n is the number of electrons transferred, F is Faraday s constant, and Q is the activity quotient. The second form, involving the log Q, is the more useful form. If you know the cell reaction, the concentrations of ions, and the E°ell, then you can calculate the actual cell potential. Another useful application of the Nernst equation is in the calculation of the concentration of one of the reactants from cell potential measurements. Knowing the actual cell potential and the E°ell, allows you to calculate Q, the activity quotient. Knowing Q and all but one of the concentrations, allows you to calculate the unknown concentration. Another application of the Nernst equation is concentration cells. A concentration cell is an electrochemical cell in which the same chemical species are used in both cell compartments, but differing in concentration. Because the half reactions are the same, the E°ell = 0.00 V. Then simply substituting the appropriate concentrations into the activity quotient allows calculation of the actual cell potential. 

Tower, Stephen. All About Electrochemistry. Available online. URL http //www.cheml.com/acad/webtext/elchem/. Accessed May 28, 2009. Part of a virtual chemistry textbook, this excellent resource explains the basics of electrochemistry, which is important in understanding how fuel cells work. Discussions include galvanic cells and electrodes, cell potentials and thermodynamics, the Nernst equation and its applications, batteries and fuel cells, electrochemical corrosion, and electrolytic cells and electrolysis. 

The potential of this electrode is defined (Section 5.2) as the voltage of the cell Pt H2(l atm) H+(<2 = 1) MZ+ M, where the left-hand electrode, Et = 0, is the normal hydrogen reference electrode (described in Section 5.6). We will derive the Nernst equation on the basis of the electrochemical kinetics in Chapter 6. Here we will use a simplified approach and consider that Eq. (5.9) can be used to determine the potential E of the M/Mz+ electrode as a function of the activity of the products and reactants in the equilibrium equation (5.10). Since in reaction (5.10) there are two reactants, Mz+ and e, and only one product of reaction, M, Eq. (5.9) yields... 

This interface is also known as the perm-selective interface (Fig. 6.1a). It is found in ion-selective sensors, such as ion-selective electrodes and ion-selective field-effect transistors. It is the site of the Nernst potential, which we now derive from the thermodynamic point of view. Because the zero-current axis in Fig. 5.1 represents the electrochemical cell at equilibrium, the partitioning of charged species between the two phases is described by the Gibbs equation (A.20), from which it follows that the electrochemical potential of the species i in the sample phase (S) and in the electrode phase (m) must be equal. 

Potentiometric measurements are based on the Nernst equation, which was developed from thermodynamic relationships and is therefore valid only under equilibrium (read thermodynamic) conditions. As mentioned above, the Nernst equation relates potential to the concentration of electroactive species. For electroanalytical purposes, it is most appropriate to consider the redox process that occurs at a single electrode, although two electrodes are always essential for an electrochemical cell. However, by considering each electrode individually, the two-electrode processes are easily combined to obtain the entire cell process. Half reactions of electrode processes should be written in a consistent manner. Here, they are always written as reduction processes, with the oxidised species, O, reduced by n electrons to give a reduced species, R ... 

The cell reaction is the transfer of oxygen from one side to the other. (See also Lambda probe). In the case of an - electrochemical equilibrium (subentry of -> equilibrium) the measured -> open circuit potential (subentry of - potential) or -> equilibrium potential (subentry of -> potential) Ueq or E (emf) can be calculated by the -> Nernst equation ... 

Half-cell reaction — The redox reaction (- electrode reaction) proceeding in a half-cell. The half-cell reaction changes the ratio of the activities of the reduced and oxidized forms. When the half-cell reaction is electrochemically reversible (see reversibility), the -> Nernst equation will describe the dependence of the -> electrode potential on the ratio of the activities of the reduced and oxidized forms. 

Nernst equation — A fundamental equation in -> electrochemistry derived by - Nernst at the end of the nineteenth century assuming an osmotic equilibrium between the metal and solution phases (- Nernst equilibrium). This equation describes the dependence of the equilibrium electrode - potential on the composition of the contacting phases. The Nernst equation can be derived from the - potential of the cell reaction (Ecen = AG/nF) where AG is the - Gibbs energy change of the - cell reaction, n is the charge number of the electrochemical cell reaction, and F is the - Faraday constant. 

Reversibility — This concept is used in several ways. We may speak of chemical reversibility when the same reaction (e.g., -> cell reaction) can take place in both directions. Thermodynamic reversibility means that an infinitesimal reversal of a driving force causes the process to reverse its direction. The reaction proceeds through a series of equilibrium states, however, such a path would require an infinite length of time. The electrochemical reversibility is a practical concept. In short, it means that the -> Nernst equation can be applied also when the actual electrode potential (E) is higher (anodic reaction) or lower (cathodic reaction) than the - equilibrium potential (Ee), E > Ee. Therefore, such a process is called a reversible or nernstian reaction (reversible or nerns-tian system, behavior). It is the case when the - activation energy is small, consequently the -> standard rate constants (ks) and the -> exchange current density (jo) are high. 

Because the potential of an electrochemical cell depends on the concentrations of the participating ions, the observed potential can be used as a sensitive method for measuring ion concentrations in solution. We have already mentioned the ion-selective electrodes that work by this principle. Another application of the relationship between cell potential and concentration is the determination of equilibrium constants for reactions that are not redox reactions. For example, consider a modified version of the silver concentration cell shown in Fig. 11.11. If the 0.10 M AgN03 solution in the left-hand compartment is replaced by 1.0 M NaCl and an excess of solid AgCl is added to the cell, the observed cell potential can be used to determine the concentration of Ag+ in equilibrium with the AgCl(s). In other words, at 25°C we can write the Nernst equation as... 

Nernst equation an equation relating the potential of an electrochemical cell to the concentrations of the cell components... 

Another approach to electrochemical transduction by chemical recognition is the incorporation of the imprinted polymer as the active ingredient in a membrane of an ISE. ISEs are devices which, when incorporated in an electrochemical cell with an appropriate reference electrode, produce a potential that varies predictably with the concentration of a certain ion in solution. If the response of the electrode follows theory, the response is Nernstian and is given by the Nernst equation ... 

Electrode Potential Nernst Equation and the Electrochemical Cell 441 Tableau 8.5. Redox Equilibria of Cl Species... 

THE ELECTRODE POTENTIAL THE NERNST EQUATION AND THE ELECTROCHEMICAL CELL... 

Electrode Potential Nernst Equation and the Electrochemical Cell 447... 

Potentiometric transducers measure the potential under conditions of constant current. This device can be used to determine the analytical quantity of interest, generally the concentration of a certain analyte. The potential that develops in the electrochemical cell is the result of the free-energy change that would occur if the chemical phenomena were to proceed until the equilibrium condition is satisfied. For electrochemical cells containing an anode and a cathode, the potential difference between the cathode electrode potential and the anode electrode potential is the potential of the electrochemical cell. If the reaction is conducted under standard-state conditions, then this equation allows the calculation of the standard cell potential. When the reaction conditions are not standard state, however, one must use the Nernst equation to determine the cell potential. Physical phenomena that do not involve explicit redox reactions, but whose initial conditions have a non-zero free energy, also will generate a potential. An example of this would be ion-concentration gradients across a semi-permeable membrane this can also be a potentiometric phenomenon and is the basis of measurements that use ion-selective electrodes (ISEs). 

In general, the open-circuit potential measured between two reversible electrodes, which is also called electromotive force, /f 1, is defined by the Nernst equation. A simplified form of this equation for the electrochemical reaction (3) was given by Eq. (15). In general, the Nernst equation relates the activities (and/or fugacities) of the substances or species, a,-, in the cell s electrochemical reactions and the standard open-circuit potential, E°, of the cell as ... 

The thermodynamic analyses used in this chapter make use of the electrochemical potential. In this way the electrical aspects of the interfacial equilibria are clearly defined. Earlier work on this problem, especially that by Volta and Nernst, had led to different conclusions regarding the source of the EMF in an electrochemical cell [12]. This problem was resolved by Frumkin, essentially, by writing out the interfacial equilibria using electrochemical potentials. In this regard, all interfaces in the cell must be considered including those between different metals at the terminals of the cell. This was shown in the discussion of the thermodynamic basis of the Nernst equation. 

However, when problems are concerned with topics such as analytical methods and corrosion that involve electrochemical cells, it is often more convenient to work with potentials and the Nernst equation because this can be related directly to voltage measurements. Whatever the system or situation it is beneficial to know how to "do it both ways." We will instruct the reader how to readily use both methods interchangeably. 

It would appear that changes in the intracellular or extracellular concentration of potassium ions markedly alter the resting membrane potential. For this reason, neurophysiologists treated an excitable cell as if it were an electrochemical, or Nernst, cell. The resting potential for one permeant species could therefore be explained by the familiar Nernst equation ... 

You ve heard electrochemistry of corrosion as a lecture I shouldn t spend much time on it but I d like to describe some electrochemical effects for film formers. First the general principles. If you put a good electronic conductor (a metal) in an aqueous solution, you will typically find that an electrical potential is developed between the piece of conductor and the solution. When ions of the metal enter the solution and leave extra electrons behind a negative potential is developed. All oxidation reactions occurring on the surface are expected to produce this result. Similarly, reduction reactions that use electrons from the metal are expected to produce a more positive potential in the metal. The solution potential of the metal influences the rate of an electrochemical half-cell reaction in accordance with Le Chatelier s Principle, so it is possible to predict through the use of the Nernst Equation the potential that will exist when the only significantly rapid reactions are the oxidation and reduction parts of a reversible reaction. When more than one potentially reversible process occurs, the rate of oxidation will be expected to exceed the rate of reduction for at least one and the converse for at least one. At... 

The Nernst equation relates the reduction potential of an electrochemical cell to a standard-state reduction potential, along with the temperature, reaction quotient, and number of electrons transferred during the reaction in question. It was first developed by Walther Nernst, who won the Nobel Prize in Chemistry in 1920 for his influential work in physical chemistry. 

---