Electrode Potentials and Their Measurement

Potentiometric techniques employ measurement of electrode potentials and their variation with changing chemical environment. It is less usual for electrode potentials to be used for direct concentration measurements although the most noteworthy exception is the case of hydrogen ion determination in pH measurements. Titration techniques, in which the variation of potentials during the addition of titrant is recorded, are of considerable importance and versatility. 

The LUMO-HOMO gap is approximately given by the spectroscopic properties of the SO group. Since DMSO absorbs in the region of 2380 and 2560 A, we obtain96 1.24 x 104/2560 = 4.8eV. This measures the drastic enhancement in redox properties anticipated for this compound. Indeed the 4.8 V value for the ground-state potential covers the whole range of usual redox potentials. The same principle applies to the sulfones because their electrode potential and absorption properties differ little from their sulfoxide analogues. 

E is the standard electrode potential, and represents a value of E measured (or calculated) when all activities are 1, when the applied pressure p is 1 atmosphere and with all redox materials participating in their standard states. As for E, E should be cited with subscripts to describe the precise composition of the redox couple indicated. Note that is often written as thus explaining why standard electrode potentials are commonly called nought . The symbol 0 implies standard conditions i.e. 298 K, p and unit activities throughout. 

Fortunately not, but to measure the absolute potential at an interface, another reference state would have to be used, as well as the nature of the metal-electron interactions. Later, in Chapter 8, it will be shown that relevant calculations can be made of this difference of inner potentials (sometimes called the Gcdvani potential difference), but their accuracy is on the order of 0.1 V, which is not yet enough to compensate for our lack of ability to measure the quantity. In the next sections, some useful concepts will be described and in Section 6.7.2 we will return to the concept of absolute electrode potential and the possibility of creating a scale of practical absolute-electrode potentials. 

Operational amplifiers, which are the main components of an analog computer, were first used in electrochemical instrumentation at the beginning of the 1960s [26]. Because they are extremely useful in measuring and controlling the electrode potentials and the currents that flow at the electrodes, electrochemical instruments were completely modernized by their introduction. Today, most electrochemical instruments are constructed using operational amplifiers. Knowledge of operational amplifiers will help the reader to understand electrochemical instruments and to construct a simple apparatus for personal use. 

PI] The electrodes were connected to a signal generator and DC power supply for continuous-voltage operation and an amplifier for alternating-voltage operation [91], The set-up allowed one to vary the frequency and the potential and to measure their precise values. 

Thus EF is associated with the electrode potential and Eredox with the redox potential of the species since in general X = 0, we cannot assume their equivalence. A measurement of potential gives values of electrode potentials and never redox potentials. 

It is not possible to measure the potential difference between the solution and the electrode, because in order to do this the solution must be connected to a conductor, i.e. a piece of another metal must be dipped into it. On the phase boundary another electrical double layer will be formed and in fact another, unknown electrode potential is developed. It is impossible therefore to measure absolute electrode potentials, only their differences. As seen before, the e.m.f. of a cell can be measured relatively easily, and this e.m.f. is the algebraic difference of the two electrode potentials. Building up cells from two electrodes,... 

In the present chapter, the relationship between the electrode potential and the activity of the solution components in the cell is examined in detail. The connection between the Galvani potential difference at the electrode solution interface and the electrode potential on the standard redox scale is discussed. This leads to an examination of the extrathermodynamic assumption which allows one to define an absolute electrode potential. Ion transfer processes at the membrane solution interface are then examined. Diffusion potentials within the membrane and the Donnan potentials at the interface are illustrated for both liquid and solid state membranes. Specific ion electrodes are described, and their various modes of sensing ion activities in an analyte solution discussed. The structure and type of membrane used are considered with respect to its selectivity to a particular ion over other ions. At the end of the chapter, emphasis is placed on the definition of pH and its measurement using the glass electrode. 

It is impossible to measure directly the electrode potentials. Only the electromotive force (emf) of a voltaic cell arising from a combination of two electrodes can be directly measured, which is given as the arithmetical sum or difference of the two electrode potential depending upon their signs. If one of the electrode potential be accurately measured, that of the other may be calculated. The reference electrode arbitrarily chosen for this purpose is the standard hydrogen electrode. Hydrogen gas at 1 atm. pressure and at a temperature of 25°C is slowly bubbled over a platinised platinum electrode which is immersed in a solution of hydrogen ions of unit activity. By convention potential of the half cell reaction... 

The tendency of a substance to donate or accept elections, and the measure of the electron s availability, is given by its electrode potential. I11 principle, electrode potentials can be measured directly by an electrode and a voltmeter. All chemical elements can transfer electrons and thus change their oxidation states. Table 4.2 shows the standard electrode potentials of the half-reactions of several elements. The general reaction is... 

Because the potential-pH diagrams characterize equhibrium thermodynamic properties only, they cannot be used to predict the rates of reactions. They can evaluate the conditions for formation of barrier films on the metals, but they cannot estimate their effectiveness in protecting the metal in different environments. It should be noted that the Nemst equation is used to estimate the electrode potentials and is based on thermodynamic equations, which are not accurate when the concentration of the electroactive species is close to zero. All metals have a limiting critical value of their activities, (a concentration of <10 g-ions per liter), below which the Nernst equation does not agree with the experimentally measured Gibbs free-energy. 

Einthoven s electrode placement has been adopted and standardized into the ECG Lead system known as the Einthoven triangle and is shown in Fig. 17.38. Electric potential differences are measured between the three limb electrodes along the line between the electrode placements, and their potentials are called lead I, n, and III such that... 

FAST REACTIONS ACCOMPANYING THE ELECTRODE PROCESS AND RATES OF ELECTRODE PROCESS PROPER From the measurements of polarographic limiting kinetic currents (and sometimes of their half-wave potentials), and their dependence on certain parameters (mainly pH, buffer composition, drop-time etc.), it is possible to compute rate constants for the fast chemical reactions, antecedent, parallel or consecutive to the electrode process proper. Rate constants of the second order reactions of the order 10 to 10 1. mol. sec have been determined in this way. The mathematical basis and the method of computation of the rate constants is beyond the scope of this text, and the reader is referred to other texts. 

The alkali metals are a group of very reactive metals. The first three members of the group are lithium, sodium and potassium. Their atomic and physical properties are summarized in Table 3.20. The electrode potentials are a measure of reducing strength (Chapter 19). The more negative the value, the greater the tendency for the atom to lose an electron (in aqueous solution). 

Mowery and Juvet (70) have employed the spray electrification effect in a novel form as a LC detector, primarily for use with reversed-phase liquid chromatography. A stream of the eluent from the column is allowed to strike a conducting target to form a spray and the potential of the electrode is monitored. A diagram of their detector is shown in Figure 35. The target electrode is an electrically isolated conducting rod which is connected to a suitable electrometer that permits both the measurement of electrode potential and electrode current. 

In our laboratory we have examined silver, nickel, and copper powder electrodes of the first kind, and the latter two especially have been examined in detail. As a rule, powder electrodes of the first kind correspond, as far as their properties are concerned, to ordinary electrodes of the first kind apart from some characteristics resulting from the dispersion of the metal. We have found experimentally the following properties. There is a marked, but not too big, influence of the metallic powder s preparation on the value of powder-electrode potential and on the value of its normal potential. These potentials are almost equal to the values for corresponding ordinary electrodes of the first kind but they are produced far better. The discrepancy of the measured values does not exceed 0.5 to 1 mv the presence of oxygen has no influence and the size of the powder s grain is of no importance either. 

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