Reference electrode half cell potential measurements

8 Reference electrode (half cell) potential measurements 

The electrochemistry of corrosion, cells and half cells were discussed in Section 2.4. The standard reference electrode or half cell is a simple device. It is a piece of metal in a fixed concentration solution of its own ions (such as copper in saturated copper sulphate, silver in silver chloride, etc.). If we connect it to another metal in a solution of its own ions (such as iron in 

Very negative potentials can be found in saturated conditions where there is no oxygen to form a passive layer but with no oxygen there can be no corrosion (see Section 2.2). This shows the weakness of potential measurements. It is a measure of the thermodynamics of the corrosion, not of the rate of corrosion. Corrosion potentials can be misleading, their interpretation is based on empirical observation, not rigorously accurate scientific theory. The problem is that the potential is not purely a function of the corrosion condition but also other factors, and that the corrosion condition is not the corrosion rate. 

The reference electrode potential measurement gives an indication of the corrosion risk of the steel. The measurement is linked by empirical comparisons to the probability of corrosion. 

The major factors that affect the activity of the iron in solution are the extent to which the steel is depassivated, that is, the extent of carbonation around the steel or the presence of sufficient chloride to break down the passive layer, and the presence of oxygen to sustain the passive layer. In the absence of oxygen, iron will dissolve but will remain stable in solution as 

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Figure 2.6 Schematic of a reference electrode (half cell) potential measurement of steel in concrete.

To completely eliminate changes in reference electrode half-cell potentials, a three electrode potentiostat is often employed. In simple terms, the potentiostat appHes a voltage to the working electrode, which is measured versus a reference electrode via a zero current potentiometric type measurement, but the current flow is between the working electrode and a third electrode, called the counter electrode. Thus if reduction takes place at the working electrode, oxidation would occur at the counter electrode but no net reaction would take place at the surface of the reference electrode, since no current flows through this electrode. A potentiostat circuit is relatively simple to construct using modern operational amplifiers. 

Figure 16.6 Multiple-wheel electrode half-cell potential measuring instrument with computer-assisted data acquisition. Note the slight wetting of the concrete surface at the wheels in order to achieve a good electrolytic contact between reference electrode and concrete...

The measurement and reference electrode half-cell potentials in Equation 4-lj, which originate from an electrochemical reaction between the internal electrode element and fill, are of opposite sign and should ideally be equal so that their sum is zero. However, the half-cell potentials depend on the type of internal electrode, the internal fill concentration, and the electrode temperature. If the internal electrode element and fill are identical for the measurement and reference electrodes, then the change in half-cell potential with temperature will cancel out unless a temperature gradient exists. The measurement electrode normally has a silver-silver chloride internal electrode in a chloride buffer. The most common type of reference electrode also has a silver-silver chloride internal electrode and a... 

Our inability lo measure absolute potenliaHfyr half-cell processes is not a serious problem because relative half-cell potentials, measured versus a common reference electrode, arc just as useful. These relative potentials can be combined to give real cell potentials. In addition, they can be used toealculale equilibrium constants of oxidation-reduction processes. 

Measured redox voltages were taken as provided by the authors assuming that corrections of the mV reading for the potential of the reference electrode half cell had been performed. 

Principle. Corroding and passive rebars in concrete show a difference in electrical potential of up to 0.5 V, thus a macrocell generates and current flows between these areas (Chapter 8). The electric field coupled with the corrosion current between corroding and passive areas of the rebars (Figure 16.4) can be measured experimentally with a suitable reference electrode (half-cell) placed on the concrete surface, resulting in equipotential lines (potential field) that allow the location of corroding rebars at the most negative values [5-8]. 

Two metal electrodes placed into an electrolytic solution constitute what is called an electrochemical cell. A potential arises at the solution interface of each electrode due to the separation of charged ions across its double layer. This potential is known as the electrode half-cell potential. It cannot be measured directly since it requiies some reference potential for comparison. 

Potentiometric methods are based on the measurement of the potential of an electrochemical cell consisting of two electrodes immersed in a solution. Since the cell potential is measured under the condition of zero cmrent, usually with a pH/mV meter, potentiometry is an equilibrium method. One electrode, the indicator electrode, is chosen to respond to a particular species in solution whose activity or concentration is to be measured. The other electrode is a reference electrode whose half-cell potential is invariant. 

A schematic diagram of a typical pH electrode system is shown in Fig. 10.1. The cell potential, i.e. the electromotive force, is measured between a pH electrode and a reference electrode in a test solution. The pH electrode responds to the activity or concentration of hydrogen ions in the solution. The reference electrode has a very stable half-cell potential. The most commonly used reference electrodes for potentiometry are the silver/silver chloride electrodes (Ag/AgCl) and the saturated calomel electrodes (SCE). 

Measurement with the hydrogen electrode The SHE is the primary reference electrode, so other half-cell potentials are measured relative to its potential. In practice, if we wish to determine the value of E M then we construct a cell of the type... 

A reference electrode is defined as a constant-potential device . In other words, if we make a cell in which one half cell is the SHE and then measure the emf. we then simply employ the equation emf = Eright-hand side — ieft-hand side (equation (3.3)) to determine the other half-cell potential. 

When two interval scales are used to measure the amount of change in the same property, the proportionality of differences is preserved from one scale to the other. For example. Table 1.4 shows reduction potentials of three electrochemical half-cell reactions measured in volts with reference to the standard hydrogen electrode (SHE, E°) and in millivolts with reference to the standard silver-silver chloride electrode (Ag/AgCl, ). For the SHE potentials the proportion of differences between the intervals +0.54 to +0.80 and +0.34 to +0.80 is... 

If we could determine E° values for individual half-reactions, we could combine those values to obtain E° values for a host of cell reactions. Unfortunately, it s not possible to measure the potential of a single electrode we can measure only a potential difference by placing a voltmeter between two electrodes. Nevertheless, we can develop a set of standard half-cell potentials by choosing an arbitrary standard half-cell as a reference point, assigning it an arbitrary potential, and then expressing the potential of all other half-cells relative to the reference half-cell. Recall that this same approach was used in Section 8.10 for determining standard enthalpies of formation, A H°f. 

An improved electrochemical cell for in situ studies is presented in Figure 14.2. In this method a platinized Pt electrode located in the anode compartment serves as the reference electrode. This cell can be installed in a test station. Such a station can have facilities for temperature and pressure control, humidification of reactant gases (e.g., hydrogen and oxygen), gas flow rate measurement, and measurement of half-and single-cell potentials as a function of current density. Finally, electrical leads from this test station could be connected to, say, power supplies. 

Equation 2.16 shows that potentiometry is a valuable method for the determination of equilibrium constants, ffowever, it should be borne in mind that the system should be in equilibrium. Some other conditions, which are described below, also need to be fulhlled for use of potentiometry in any application. The basic measurement system must include an indicator electrode that is capable of monitoring the activity of the species of interest, and a reference electrode that gives a constant, known half-cell potential to which the measured indicator electrode potential can be referred. The voltage resulting from the combination of these two electrodes must be measured in a manner that minimises the amount of current drawn by the measuring system. This condition includes that the impedance of the measuring device should be much higher than that of the electrode. 

Reference half-cells The fact that individual half-cell potentials are not directly measurable does not prevent us from defining and working with them. Although we cannot determine the absolute value of a half-cell potential, we can still measure its value in relation to the potentials of other half cells. In particular, if we adopt a reference half-cell whose potential is arbitrarily defined as zero, and measure the potentials of various other electrode systems against this reference cell, we are in effect measuring the half-cell potentials on a scale that is relative to the potential of the reference cell. 

In thermodynamics, only energy differences are measurable absolute energies are not. Therefore, energies (or enthalpies or free energies) are defined relative to a reference state for which these quantities are arbitrarily set at 0 by international agreement. The same reasoning applies to half-cells Because only differences are measured, we are free to define a reference reduction potential for a particular half-cell and measure other half-cell reduction potentials relative to it. The convention used is to define %° for the half-cell reduction of Hilg) to H30 (d (j ) to be 0 at all temperatures, when the gas pressure at the electrode is 1 atm and the -iiO aq) concentration in solution is I M (Fig. 17.3). 

Electrochemical cells employed to carry out voltammet-ric or amperometric measurements can involve either a two or three electrode configuration. In the two electrode mode, the external voltage is applied between the working and a reference electrode, and the current monitored. Since the current must also pass through the reference electrode, such current flow can potentially alter the surface concentration of electroactive species that poises the actual half-cell potential of the reference electrode, changing its value by a concentration polarization process. For example, if an Ag/AgCl reference electrode were used in a cell in which a reduction reaction for the analyte occurs at the working electrode, then an oxidation reaction would take place at the surface of the reference electrode ... 

An electrode potential is a measure of the thermodynamics of a redox reaction. It may be expressed as the difference between two half-cell potentials, which by convention are measured against a hydrogen electrode. Tabulated values refer to standard conditions (ions at unit activity). 

The reference electrode, against whose known half-cell potential the electrode potential of the working electrode is measured. 

The previous concepts may be summarized by briefly reviewing the experimental procedures for determining the kinetic parameters, i0, Pox, and Pred. If a single half-cell reaction is involved, the equilibrium half-cell potential will be measured against some reference electrode. If the electrode is now connected to a potentiostat and the potential increased in the positive or oxidation direction, the upper solid curve of Fig. 3.11 will be plotted. If the potential is decreased, the lower solid curve will be plotted. The higher current-density linear sections of each curve are then extrapolated through the value of the equilibrium poten-... 

The mercurous sulfate [7783-36-0j/, Hg2S04, mercury reference electrode, (Pt)H2 H2S04(/ ) H S04(Hg), is used to accurately measure the half-cell potentials of the lead—acid battery. The standard potential of the mercury reference electrode is 0.6125 V (14). The potentials of the lead dioxide, lead sulfate, and mercurous sulfate, mercury electrodes versus a hydrogen electrode have been measured (24,25). These data may be used to calculate accurate half-cell potentials for the lead dioxide, lead sulfate positive electrode from temperatures of 0 to 55°C and acid concentrations of from 0.1 to 8m. 

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