Test electrodes

The method applies a small potential (usually 10-30 mV) to a test electrode on either side of the corrosion potential ( corr)- The resultant current... 

Figure 9. Procedure for the preparation of the test electrode for aqueous electrolytes (9 mol L 1 KOH or ZnCl2 solution). (1) the sample is mixed by shaking in a plastic container 20 mm (diam.) x 40 mm (height) (2) the mixture is made into a thin film by grinding with a pestle in a ceramic mortar (3) the metal screen is prepared (4) the three layers (A, B, C) are pressed between the steel blocks.

As the temperature is varied, the Galvani potentials of all interfaces will change, and we cannot relate the measured value of d"S dT to the temperature coefficient of Galvani potential for an individual electrode. The temperature coefficient of electrode potential probably depends on the temperature coefficient of Galvani potential for the reference electrode and hence is not a property of the test electrode alone. 

We might try to measure the temperature coefficient of the Galvani potential for an individual electrode under nonisothermal conditions then only the temperature of the test electrode would be varied, while the reference electrode remains at a constant temperature and retains a constant value of Galvani potential (Fig. 3.2). 

One distingnishes practical and standard reference electrodes. A standard RE is an electrode system of particnlar confignration, the potential of which, nnder specified conditions, is conventionally taken as zero in tfie corresponding scale of potentials (i.e., as the point of reference nsed in finding tfie potentials of otfier electrodes). Practical REs are electrode systems having a snfficiently stable and reproducible value of potential which are nsed in the laboratory to measure the potentials of other electrodes. The potential of a practical reference electrode may difier from the conventional zero potential of the standard electrode, in which case the potential of the test electrode is converted to this scale by calculation. 

Figure 33.2 shows results obtained by studies of electrochemical noise for the corrosion behavior of carbon steel A516-70 in carbonate solutions with and without NaCl as an activator (Cheng et al., 2000). It can be seen that in ordinary carbonate solution the fluctuations of potential of a test electrode and the fluctuations of current flowing between a pair of identical electrodes are small. Added NaCl causes a drastic increase in intensity of the electrochemical noise. The PDS plots (Fig. 33.3) differ accordingly. 

Each test electrode was transferred to the EC chamber and subjected to electrochemical stabilization. The EC chamber was then evacuated rapidly by two sorption pumps and a cryopump to transfer the electrode to the XPS chamber again. 

In practice, it is very often necessary to determine the potential of a test (indicator) electrode connected in a cell with a well defined second electrode. This reference electrode is usually a suitable electrode of the second kind, as described in Section 3.2.2. The potentials of these electrodes are tabulated, so that Eq. (3.1.66) can be used to determine the potential of the test electrode from the measured EMF. The standard hydrogen electrode is a hydrogen electrode saturated with gaseous hydrogen with a partial pressure equal to the standard pressure and immersed in a solution with unit hydrogen ion activity. Its potential is set equal to zero by convention. Because of the relative difficulty involved in preparing this electrode and various other complications (see Section 3.2.1), it is not used as a reference electrode in practice. 

In electrochemical kinetics, the concept of the electrode potential is employed in a more general sense, and designates the electrical potential difference between two identical metal leads, the first of which is connected to the electrode under study (test, working or indicator electrode) and the second to the reference electrode which is in a currentless state. Electric current flows, of course, between the test electrode and the third, auxiliary, electrode. The electric potential difference between these two electrodes includes the ohmic potential difference as discussed in Section 5.5.2. 

The ohmic potential difference in an electrolytic cell consisting of a spherical test electrode, termed, for a small radius r0, ultramicroelectrode, in the centre and another very distant concentrical counter-electrode is given by the equation... 

Relaxation methods for the study of fast electrode processes are recent developments but their origin, except in the case of faradaic rectification, can be traced to older work. The other relaxation methods are subject to errors related directly or indirectly to the internal resistance of the cell and the double-layer capacity of the test electrode. These errors tend to increase as the reaction becomes more and more reversible. None of these methods is suitable for the accurate determination of rate constants larger than 1.0 cm/s. Such errors are eliminated with faradaic rectification, because this method takes advantage of complete linearity of cell resistance and the slight nonlinearity of double-layer capacity. The potentialities of the faradaic rectification method for measurement of rate constants of the order of 10 cm/s are well recognized, and it is hoped that by suitably developing the technique for measurement at frequencies above 20 MHz, it should be possible to measure rate constants even of the order of 100 cm/s. 

All the three electrodes (test electrode, counter electrode, and reference electrode) are made from the same smooth, bright, polished metal foil (A.R.). The metal foil is cut in rectangular shapes... 

The cell and the circuit diagram are shown in Fig. 1. The cell consists of a test electrode, El, reference electrode, R, and counter electrode, E2. The ac potential between the test electrode Ej and the reference electrode R is measured by connecting them to a sensitive ac millivoltmeter through the contact key (by connecting point a to point b). The rectified voltage between Ej and R is measured across a dc microvoltmeter (sensitivity, 1 tV/smallest... 

Before starting any experiment, the potential of the test electrode Ej is measured with reference to a saturated calomel electrode which is connected to the experimental cell through a bridge containing the same supporting electrolyte solution. Such measurements are taken whenever the concentration of the metal ion is changed. The cell is kept immersed in a thermostated bath maintained at a known temperature. 

To measure the electrode potential of a test electrodes, M, we usually use an electrochemical cell consisting of test electrode M and reference electrode both of which are coimected by a metal lead of A and A" of the same metallic conductor to a potentiometer outside the cell as shown in Fig. 4-23. The difference in the electrode potential, E, measured between the test electrode and the reference electrode, conventionally called the electromotive force, equals the difference in the Fermi level of electrons between the two electrodes E = - 8j(M) - EjtM )... 

In the cell used for measuring the electrode potential, in which the two electrodes are immersed in a single phase of electrolyte solution, the outer potential, tps, ofthe test electrode-solution is equal to the outer potential, ips, of the reference electrode-solution as shown in Fig. 4—24. The difference in the Fermi level of electrons, CFtu)- between the test electrode M and the reference electrode M , then, is represented by the difference in the real potential of electrons, M/aw) - .(M0/ V). tuid hence by the difference in the electrode potential (absolute electrode potential), AE = E-E°, between the two electrodes. This difference also equals the difference in the work function, 4>no/3/v - 4>ji/s/v> between the two electrodes. Thus, the potential E of the test electrode relative to the reference electrode is the difference in the electrode potential (absolute electrode potential) between the two electrodes as indicated in Eqn. 4-35 . 

Pig. 4-23. Measurement of relative electrode potential by a potentiometer M = test electrode M° = reference electrode F = potentiometer = Fermi level of electron in electrodeM = Fermi level of electrons in terminal A E = relative electrode potential. 

Fig. 4-24. Electron energy levels for electrode potential relative to a reference electrode E = electrode potential (absolute) E = relative electrode potential Ps = outer potential of electrolyte solution of test electrode = outer potential of electrolyte solution of reference...

The relative work function and the relative electrode potential of electrodes in aqueous solutions and in inactive gases can be measured by a vibrating capacitor technique called Kelvin s method [Samec-Johnson-Doblhofer, 1992]. The Kelvin method estimates the difference in the work function between a test electrode and a Kelvin probe (KF) by measuring the applied voltage V at which the difference in the outer potential ij s- l KP between the test electrode and the Kelvin probe becomes zero (V = liJs - i Kp) as shown in Pig. 4—28. 

The electrode potential is the EMF of a cell in which the electrode on the right-hand side in the scheme is the test electrode, while the electrode placed on the left-hand side is a standard hydrogen electrode (i.e. a hydrogen electrode saturated with hydrogen under standard pressure Po = 1 -013 x 10 Pa and placed in a solution with an activity of hydronium ions equal to one). 

Diffusion-Layer Model Let us consider again the general electrochemical reaction (6.6). Initially, at time before electrolysis, the concentration of the solution is homogeneous at all distances x from the electrode, equal to the bulk concentration of reactant Ox. In a more rigorous consideration, one would say that the concentration of the solution is homogeneous up to the outer Helmholtz plane (OHP), that is, up to x = xqhp-When a constant current is applied to the test electrodes and counterelectrodes such that the reaction... 

As the electrolysis proceeds, there is a progressive depletion of the Ox species at the interface of the test electrode (cathode). The depletion extends farther and farther away into the solution as the electrolysis proceeds. Thus, during this non-steady-state electrolysis, the concentration of the reactant Ox is a function of the distance x from the electrode (cathode) and the time f, [Ox] = Concurrently, concentration of the reaction product Red increases with time. For simplicity, the concentrations will be used instead of activities. Weber (19) and Sand (20) solved the differential equation expressing Pick s diffusion law (see Chapter 18) and obtained a function expressing the variation of the concentration of reactant Ox and product Red on switching on a constant current. Figure 6.10 shows this variation for the reactant. 

Figure 6.16. Three-compartment three-electrode electrochemical cell RE, reference electrode LC, Lugin capillary TE, test electrode GF, glass frit CE, counterelectrode.

Galvanostatic Transient Technique, In the galvanostatic technique the current between the test electrode and the auxiliary (counter-) electrode is held constant with a current source (galvanostat), and the potential between the test electrode and the reference electrode is determined as a function of time. The potential is the dependent variable, which is recorded with suitable recording systems, such as recorders or oscilloscopes (Fig. 6.17). 

Figure 6.17. Schematic diagram of apparatus for galvanostatic measurements P, constant current power supply e, test electrode e2, reference electrode counter (auxiliary)-electrode V, potential-time recording instrument.

Figure 6.18. Variation of potential of the test electrode, E, with time during galvanostatic electrolysis (milisecond range) Eq, equilibrium potential potential of the test electrode at beginning of electrolysis at constant current density i.

Figure 6.19. Simplified equivalent circuit for single-electrode reaction [e.g., Eq. (6.6)] Qi, double-layer capacitance of test electrode charge-transfer resistance of electrode reaction.

Potential Sweep Method, In the transient techniques described above, a set of measurements of the potential for a given current or the current for a given potential is measured in order to construct the current-potential function, i = f(E). For example, the Tafel lines shown in Figure 6.20 were constructed from a set of galvanostatic transients of the type shown in Figure 6.18. In the potential sweep technique, i = f(E), curves are recorded directly in a single experiment. This is achieved by sweeping the potential with time. In linear sweep voltammetry, the potential of the test electrode is varied linearly with time (Fig. 6.23a). If the sweep rate is... 

V (mV/s), the potential of the test electrode at time t in the cathodic polarization is given by... 

To determine an overpotential, however, it is necessary to alter the above two-electrode system by introducing an extra auxiliary electrode, which is termed the auxiliary or counter-electrode. Thus a three-electrode arrangement is set up as shown in Fig. 736. In such a setup, the counter electrode is connected to the test electrode via a polarizing circuit (e.g., a power source) through which a controllable current is made to pass and produce alterations in the potential of the test electrode. Between the nonpolarizable reference electrode and test electrode is connected an instrument that is capable of measuring the potential difference between these electrodes. 

When no current flows through the polarizing circuit and there is equilibrium at the test electrode/electrolyte interface, the potential difference, Ee, between the test and reference electrodes is given by... 

When a current I is passed through the polarizing circuit (i.e., between the test and counter-electrodes), (1) the potential of the test electrode changes from d(J)c to... 

In this equation IR is the potential drop developed when the polarizing current I overcomes the resistance R = 1/cA of the electrolyte between the test electrode and the Luggin tip or probe by means of which the reference electrode makes ionic or electrolytic contact with the test electrode. 

The IR error can be reduced by minimizing R, i.e., by choosing high-conductivity electrolytes and using small distances between the test electrode and Luggin tip (see Section 7.5.7.2). 

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