Potential of an electrode

By analogy, ammonium salts should behave as acids in liquid ammonia, since they produce the cation NH4 (the solvo-cation ), and soluble inorganic amides (for example KNHj, ionic) should act as bases. This idea is borne out by experiment ammonium salts in liquid ammonia react with certain metals and hydrogen is given off. The neutralisation of an ionic amide solution by a solution of an ammonium salt in liquid ammonia can be carried out and followed by an indicator or by the change in the potential of an electrode, just like the reaction of sodium hydroxide with hydrochloric acid in water. The only notable difference is that the salt formed in liquid ammonia is usually insoluble and therefore precipitates. 

The overpotential is defined as the difference between the actual potential of an electrode at a given current density and the reversible electrode potential for the reaction. 

When the potential of an electrode of the first kind responds to the potential of another ion that is in equilibrium with M"+, it is called an electrode of the second kind. Two common electrodes of the second kind are the calomel and silver/silver chloride reference electrodes. Electrodes of the second kind also can be based on complexation reactions. Eor example, an electrode for EDTA is constructed by coupling a Hg +/Hg electrode of the first kind to EDTA by taking advantage of its formation of a stable complex with Hg +. 

Tafel Extrapolation Corrosion is an elec trochemical reac tion of a metal and its environment. When corrosion occurs, the current that flows between individual small anodes and cathodes on the metal surface causes the electrode potential for the system to change. While this current cannot be measured, it can be evaluated indirectly on a metal specimen with an inert electrode and an external electrical circuit. Pmarization is described as the extent of the change in potential of an electrode from its equilibrium potential caused by a net current flow to or from the electrode, galvanic or impressed (Fig. 28-7). 

The displacement of the potential of an electrode from its reversible value is the overpotential tj, and... 

Concentration (diffusion or transport) Overpotential change of potential of an electrode caused by concentration changes near the electrode/solution interface produced by an electrode reaction. 

Open-circuit Potential the potential of an electrode (relative to a reference electrode) from which no net current flows, so that the anodic and cathodic reactions occur at an equal rate. 

Standard Electrode Potential (E ) the equilibrium potential of an electrode reaction when the components are ail in their standard states. 

Steady-state Potential the potential of an electrode which is independent of time because its reaction occurs at a constant rate. 

Electrical methods of analysis (apart from electrogravimetry referred to above) involve the measurement of current, voltage or resistance in relation to the concentration of a certain species in solution. Techniques which can be included under this general heading are (i) voltammetry (measurement of current at a micro-electrode at a specified voltage) (ii) coulometry (measurement of current and time needed to complete an electrochemical reaction or to generate sufficient material to react completely with a specified reagent) (iii) potentiometry (measurement of the potential of an electrode in equilibrium with an ion to be determined) (iv) conductimetry (measurement of the electrical conductivity of a solution). 

SOLUTION We can determine the standard potential of an electrode by measuring the emf of a standard cell in which the other electrode has a known standard potential and applying Kq. 3. 

The standard potential of an electrode is the standard emfofa cell in which the electrode on the left in the cell diagram is a hydrogen electrode. A metal with a negative standard potential has a thermodynamic tendency to reduce Irydrogen ions in solution the ions of a metal with a positive standard potential have a tendency to be reduced by hydrogen gas. 

Determine the standard potential of an electrode from a cell emf (Example 12.5). 

When the potential of an electrode is switched from a value, where an electrode reaetion under investigation proceeds at a negligible rate, to a value, where the rate is measurable, a charge-time behavior ean be observed, which contains besides numerous other eleetroehemieal parameters the rate eonstant of the electrode reaction. Details and a complete derivation have been given elsewhere [73Rod, 75Wea, 76Wea] (Data obtained with this method are labelled CM.)... 

Chronopotentiometry has been widely used to determine diffusion coefficients in molten salts. Chronopotentiometry is an experimental procedure in which the potential of an electrode is observed as a function of time during the passage of a constant current sufficiently large to produce concentration polarization with respect to the species undergoing electrochemical reaction. 

The potential of an electrode of the second kind is determined by the activity (concentration) of anions, or more correctly, by the mean ionic activity of the corresponding electrolyte [see Eq. (3.50)]. The most conunon among electrodes of this type are the calomel REs. In them, a volume of mercury is in contact with KCl solution which has a well-defined concentration and is saturated with calomel Hg2Cl2, a poorly soluble mercury salt. The value of such an electrode is 0.2676 V (aU numerical values refer to 25°C, and potentials are reported on the SHE scale). Three types of calomel electrode are in practical use they differ in KCl concentration and, accordingly, in the values of ionic activity and potential ... 

In 1924, John Alfred Valentine Butler derived an equation for the equilibrium potential of an electrode using the equations for the finite rate of anodic and cathodic steps. 

One is by now familiar with the equation that expresses the reversible potential of an electrode as a function of the activities of participants in the electrode reaction. If the activities of resultants (or products) of the electrode reaction are standardized and taken arbitrarily to be unity, this equation or expression takes the form of one in which the electrode potential is a function of the activity of one type of ion only. It should clearly be possible to construct two similar electrodes to take up reversible potentials corresponding to different activities of this one type of ion, and emf of the cell formed by the combination of such a pair of electrodes should, therefore, take up a potential determined by both the activities concerned. Cells of this type, in which the reversible emf is a function of the different activities of one participant in the electrode reaction, are termed concentration cells. 

The potential of an electrode measured relative to a standard, usually the SHE. It is a measure of the driving force of the electrode reaction and is temperature and activity dependent (p. 230). By convention, the half-cell reaction must be written as a reduction and the potential designated positive if the reduction proceeds spontaneously with respect to the SHE, otherwise it is negative. If the sign of the potential is reversed, it must be referred to as an oxidation potential. 

A potentiometric titration is one in which the end point is detected by measuring the potential of an electrode dipped into the titrated solution. 

Figure 5 The potential of an electrode can be perturbed in order to trigger (a) reduction processes (b) oxidation processes...

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. 

In electrochemical kinetics, the plot of reaction current (reaction rate) as a fimction of electrode potential is conventionally called the polarization curve. Figure 7—4 shows schematic polarization curves of cathodic and anodic electrode reactions. The term of polarization means shifting the electrode potential from a certain specified potential, e.g. the equilibrium potential of an electrode reaction, to more negative (cathodic) or more positive (anodic) potentials. The term of polarization also occasionally applies to the magnitude of potential shift from the specified potential. 

The mixed-potential model demonstrated the importance of electrode potential in flotation systems. The mixed potential or rest potential of an electrode provides information to determine the identity of the reactions that take place at the mineral surface and the rates of these processes. One approach is to compare the measured rest potential with equilibrium potential for various processes derived from thermodynamic data. Allison et al. (1971,1972) considered that a necessary condition for the electrochemical formation of dithiolate at the mineral surface is that the measmed mixed potential arising from the reduction of oxygen and the oxidation of this collector at the surface must be anodic to the equilibrium potential for the thio ion/dithiolate couple. They correlated the rest potential of a range of sulphide minerals in different thio-collector solutions with the products extracted from the surface as shown in Table 1.2 and 1.3. It can be seen from these Tables that only those minerals exhibiting rest potential in excess of the thio ion/disulphide couple formed dithiolate as a major reaction product. Those minerals which had a rest potential below this value formed the metal collector compoimds, except covellite on which dixanthogen was formed even though the measured rest potential was below the reversible potential. Allison et al. (1972) attributed the behavior to the decomposition of cupric xanthate. 

We will need to look at the ke,-V relationship in some depth later (in Section 7.5), but until then we will merely make a mental note that the more extreme the potential of an electrode, then the faster will be the transfer of electrons across the electrode solution interface. 

We now introduce voltammetry and the subset technique of polarography. The word root voltam- tells us straightaway that we are looking at both potential ( volt- ) and current ( am- ). We will see that during any voltammetry experiment, the potential of an electrode is varied while we simultaneously monitor the current that is induced. This occurs as a result of the electrode being polarized, that is, its potential is forced away from its equilibrium value. 

The equilibrium potential of an electrode (e.g., M/M ) is defined in Section 5.2 as the voltage of the cell, Pt H2(l atm) H+(a = 1) M M, where a is the activity. Three issues have to be resolved to measure this equilibrium electrode potential (1) the selection of a reference electrode (2) the coupling of the reference electrode with the electrode whose potential is being measured, in this case M/M + and (3) the experimental method for the voltage measurement. 

Thus, the electrode potential of an electrode of the t5q)e M MA A depends on the activity of anion of the sparingly soluble compound of the electrode metal. 

The Nernst equation defines the equilibrium potential of an electrode. A simplified thermodynamic derivation of this equation is given in the Sections 5.3 to 5.5. Here we will give the kinetic derivation of this equation. 

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