Potential and Electrochemical Cells

As discussed in Section 2.3, for an electrode reaction to take place one needs two electrodes a working electrode, at which the electron transfer 

Since we are interested in controlling accurately the potential of the working electrode (in order to condition the rate of the electron transfer between this electrode and the electroactive species), we must work on the difference of potential between the two electrodes. It is clear, however, that changing the applied potential between the two electrodes causes unpredictable variations in the potential of either the working electrode, or the counter electrode, or in the iR drop. This implies that it 

The eventual current flow through the electrode causes the following reaction to proceed in one of the two directions  

This type of counter electrode is defined as a reference electrode. As we will see in Chapter 3, Section 1.2, at 25° C the saturated calomel electrode (SCE) has a potential of +0.2415 V with respect to the standard hydrogen electrode (NHE), which, although difficult to use, is the internationally accepted standard for the potential scale, having conventionally E° — 0.000 V. 

Returning to the control of the potential of the working electrode in the electrochemical cell, the use of a reference electrode as a counter electrode makes every change in the applied potential difference between the two electrodes entirely assigned to the working electrode, provided that the iR drop is negligible. In this manner we would be able to control accurately the reaction rate at the working electrode. 

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Spontaneous potentials and electrochemical cells Post overburden deposition... 

Another troublesome aspect of the reactivity ratios is the fact that they must be determined and reported as a pair. It would clearly simplify things if it were possible to specify one or two general parameters for each monomer which would correctly represent its contribution to all reactivity ratios. Combined with the analogous parameters for its comonomer, the values rj and t2 could then be evaluated. This situation parallels the standard potential of electrochemical cells which we are able to describe as the sum of potential contributions from each of the electrodes that comprise the cell. With x possible electrodes, there are x(x - l)/2 possible electrode combinations. If x = 50, there are 1225 possible cells, but these can be described by only 50 electrode potentials. A dramatic data reduction is accomplished by this device. Precisely the same proliferation of combinations exists for monomer combinations. It would simplify things if a method were available for data reduction such as that used in electrochemistry. 

Analytical methods based upon oxidation/reduction reactions include oxidation/reduction titrimetry, potentiometry, coulometry, electrogravimetry and voltammetry. Faradaic oxidation/reduction equilibria are conveniently studied by measuring the potentials of electrochemical cells in which the two half-reactions making up the equilibrium are participants. Electrochemical cells, which are galvanic or electrolytic, reversible or irreversible, consist of two conductors called electrodes, each of which is immersed in an electrolyte solution. In most of the cells, the two electrodes are different and must be separated (by a salt bridge) to avoid direct reaction between the reactants. 

Consequently, a wealth of information on the energetics of electron transfer for individual redox couples ("half-reactions") can be extracted from measurements of reversible cell potentials and electrochemical rate constant-overpotential relationships, both studied as a function of temperature. Such electrochemical measurements can, therefore, provide information on the contributions of each redox couple to the energetics of the bimolecular homogeneous reactions which is unobtainable from ordinary chemical thermodynamic and kinetic measurements. 

In this present book, we will look at the analytical use of two fundamentally different types of electrochemical technique, namely potentiometry and amper-ometry. The distinctions between the two are outlined in some detail in Chapter 2. For now, we will anticipate and say that a potentiometric technique determines the potential of electrochemical cells - usually at zero current. The potential of the electrode of interest responds (with respect to a standard reference electrode) to changes in the concentration of the species under study. The most common potentiometric methods used by the analyst employ voltmeters, potentiometers or pH meters. Such measurements are generally relatively cheap to perform, but can be slow and tedious unless automated. 

Activity data for electrolytes usually are obtained by one or more of three independent experimental methods measurement of the potentials of electrochemical cells, measurement of the solubility, and measurement of the properties of the solvent, such as vapor pressure, freezing point depression, boiling point elevation, and osmotic pressure. All these solvent properties may be subsumed under the rubric colligative properties. 

Electrical energy is considered to allow us to discuss batteries and electrochemical cells, as well as motors and resistance heating. If a charge Q is transferred to a system at an electrostatic potential (voltage) < ) with respect to the surroundings, the work done is Q ... 

Cells with Liquid Junctions and Elimination of Junction Potentials. When electrochemical cells are employed to obtain thermodynamic data, high accuracy ( 0.05 mV) requires the use of cells that are free from liquid junction (in the sense that the construction of the cell does not involve bringing into contact two or more distinctly different electrolyte solutions). Otherwise, the previously discussed uncertainties in the calculation of liquid-junction potentials will limit the accuracy of the data. 

Potentiometric methods of analysis are based on measuring the potential of electrochemical cells without drawing appreciable current. For nearly a century, potentiometric techn iques have been used to locate end points in titrations. In more recent methods, ion concentrations are measured directly from the potential of ion-selective membrane electrodes. These electrodes are relatively free from interferences and provide a rapid, convenient, and nondestructive means of quantitatively determining numerous important anions and cations. ... 

In Feature 21-5, we described the use of operational amplifiers to measure the potential of electrochemical cells. Op amps also can be used to measure currents and carry out a variety of other control and measurement tasks. Consider the measurement of current, as illustrated in Figure 23F-1. 

Electrochemical Cells Better and Better Batteries MiniLab 17.2 The Lemon with Potential ChemLab Oxidation-Reduction and Electrochemical Cells... 

Potentiometry—the measurement of electric potentials in electrochemical cells—is probably one of the oldest methods of chemical analysis still in wide use. The early, essentially qualitative, work of Luigi Galvani (1737-1798) and Count Alessandro Volta (1745-1827) had its first fruit in the work of J. Willard Gibbs (1839-1903) and Walther Nernst (1864-1941), who laid the foundations for the treatment of electrochemical equilibria and electrode potentials. The early analytical applications of potentiometry were essentially to detect the endpoints of titrations. More extensive use of direct potentiometric methods came after Haber developed the glass electrode for pH measurements in 1909. In recent years, several new classes of ion-selective sensors have been introduced, beginning with glass electrodes more or less selectively responsive to other univalent cations (Na, NH ", etc.). Now, solid-state crystalline electrodes for ions such as F , Ag", and sulfide, and liquid ion-exchange membrane electrodes responsive to many simple and complex ions—Ca , BF4", CIO "—provide the chemist with electrochemical probes responsive to a wide variety of ionic species. 

Note, however, that the half-wave potential Ey is usually similar but not exactly equivalent to the thermodynamic standard potential First, the product of reduction may be stabilized by amalgam formation in metal ion reductions second, there will always be a small liquid junction potential in electrochemical cells of this type that should be corrected for and hnally, it can be shown that the potential Ey is the sum of two terms ... 

Bale] and coworkers published a number of papers on the thermodynamics and the reversible potential of sodium amalgam in contact with Na" " [18-27]. They measured the reversible potential of electrochemical cells consisting of... 

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