The Reference Electrode

The Kelvin-Zisman method makes it possible to monitor the variations in a sample s work function, provided the reference electrode s work function is stable under the chosen experimental conditions. It is therefore important to choose the right reference electrode depending on the gas atmosphere and the temperature. The materials most commonly employed as a reference are metals, metal oxides, glass-coated metals and graphite. 

Metals, especially noble metals, have the advantage of being able to support treatments at high tempeiatnres. Tungsten is a good reference in a vacuum, but it cannot be used in the presence of a gas. 

Noble metals are therefore often used under gaseous conditions, either becanse they do not react with gases, or because they produce a stable compound. 

The latter case mostly concerns platinum and gold, which can be used as a reference at temperatures reaching 400°C and under oxygen if they have been subjected to oxygen treatment at 400°C. 

Regarding glass-coated metals, recall the example of molybdenum, which is a stable reference nnder a hydrocaibon-air mix at temperatures below 300°C. 

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Fig. V-17. Schematic diagram for the apparatus for measurement of Vobs (see text). The vibrating reference electrode is positioned close to the surface of a AgN03 solution in which there is an Ag electrode, which, in turn, is in electrical contact with the reference electrode. (From Ref. 196.)...

Electrochemical methods may be classified into two broad classes, namely potentiometric metiiods and voltannnetric methods. The fonner involves the measurement of the potential of a working electrode iimnersed in a solution containing a redox species of interest with respect to a reference electrode. These are equilibrium experiments involving no current flow and provide themiodynamic infomiation only. The potential of the working electrode responds in a Nemstian maimer to the activity of the redox species, whilst that of the reference electrode remains constant. In contrast, m voltannnetric methods the system is perturbed... 

Figure Bl.28.8. Equivalent circuit for a tliree-electrode electrochemical cell. WE, CE and RE represent the working, counter and reference electrodes is the solution resistance, the uncompensated resistance, R the charge-transfer resistance, R the resistance of the reference electrode, the double-layer capacitance and the parasitic loss to tire ground.

Potentiometric measurements are made using a potentiometer to determine the difference in potential between a working or, indicator, electrode and a counter electrode (see Figure 11.2). Since no significant current flows in potentiometry, the role of the counter electrode is reduced to that of supplying a reference potential thus, the counter electrode is usually called the reference electrode. In this section we introduce the conventions used in describing potentiometric electrochemical cells and the relationship between the measured potential and concentration. 

Also, by convention, potentiometric electrochemical cells are defined such that the indicator electrode is the cathode (right half-cell) and the reference electrode is the anode (left half-cell). 

Potentiometric electrochemical cells are constructed such that one of the half-cells provides a known reference potential, and the potential of the other half-cell indicates the analyte s concentration. By convention, the reference electrode is taken to be the anode thus, the shorthand notation for a potentiometric electrochemical cell is... 

Standard Hydrogen Electrode The standard hydrogen electrode (SHE) is rarely used for routine analytical work, but is important because it is the reference electrode used to establish standard-state potentials for other half-reactions. The SHE consists of a Pt electrode immersed in a solution in which the hydrogen ion activity is 1.00 and in which H2 gas is bubbled at a pressure of 1 atm (Figure 11.7). A conventional salt bridge connects the SHE to the indicator half-cell. The shorthand notation for the standard hydrogen electrode is... 

Glass membrane pH electrodes are often available in a combination form that includes both the indicator and the reference electrode. The use of a single electrode greatly simplifies the measurement of pH. An example of a typical combination electrode is shown in Figure 11.12. 

Sources of Error. pH electrodes are subject to fewer iaterfereaces and other types of error than most potentiometric ionic-activity sensors, ie, ion-selective electrodes (see Electro analytical techniques). However, pH electrodes must be used with an awareness of their particular response characteristics, as weU as the potential sources of error that may affect other components of the measurement system, especially the reference electrode. Several common causes of measurement problems are electrode iaterferences and/or fouling of the pH sensor, sample matrix effects, reference electrode iastabiHty, and improper caHbration of the measurement system (12). 

Other problems occur in the measurement of pH in unbuffered, low ionic strength media such as wet deposition (acid rain) and natural freshwaters (see Airpollution Groundwatermonitoring) (13). In these cases, studies have demonstrated that the principal sources of the measurement errors are associated with the performance of the reference electrode Hquid junction, changes in the sample pH during storage, and the nature of the standards used in caHbration. Considerable care must be exercised in all aspects of the measurement process to assure the quaHty of the pH values on these types of samples. 

Combination electrodes have increased in use and are a consoHdation of the glass and reference electrodes in a single probe, usually in a concentric arrangement, with the reference electrode compartment surrounding the pH sensor. The advantages of combination electrodes include the convenience of... 

Samples that contain suspended matter are among the most difficult types from which to obtain accurate pH readings because of the so-called suspension effect, ie, the suspended particles produce abnormal Hquid-junction potentials at the reference electrode (16). This effect is especially noticeable with soil slurries, pastes, and other types of colloidal suspensions. In the case of a slurry that separates into two layers, pH differences of several units may result, depending on the placement of the electrodes in the layers. Internal consistency is achieved by pH measurement using carefully prescribed measurement protocols, as has been used in the determination of soil pH (17). 

The question of what happens when an electrical signal is appHed to an electrochemical ceU needs to be answered with respect to the three components of the ceU the working electrode, the reference electrode, and the sample itself. 

The three-electrode system serves two important purposes. Because the reference electrode carries no current, but merely measures a potential relative to the working electrode, its stabiUty is not unduly influenced by the electrolysis. Furthermore, because it is placed close to the working electrode the measured potential difference is more nearly representative of the tme potential difference between the working electrode and the sample solution. This latter is the significant quantity in electro analysis. 

The reference electrode contributes heavily to the economics of electroanalytical chemistry. Companies that sell and service electroanalytical instmmentation are few in number and small in size, or they are parts of much larger companies. One suppHer of electroanalytical instmmentation is Princeton AppHed Research Corp. (PARC) of Princeton, Newjersey. PARC is a subsidiary of EG G Instmments, Inc. Among the many suppHers of ion-selective electrodes are Orion (Boston, Massachusetts), Corning (Corning, New York), and Ingold (Wilmington, Massachusetts). Brinkmann Instmments, Inc. (Westbury, New York) is a useful suppHer of titration equipment. 

B is the potential of the reference electrode, without whose identification the potential U is undefined. Potentials are conveniently calculated against a standard reference value. Section 3.2 contains further details on reference electrodes and conversion factors. Section 3.3 describes practical methods for measuring potential in the case of flowing currents. 

The potential dependence of the velocity of an electrochemical phase boundary reaction is represented by a current-potential curve I(U). It is convenient to relate such curves to the geometric electrode surface area S, i.e., to present them as current-density-potential curves J(U). The determination of such curves is represented schematically in Fig. 2-3. A current is conducted to the counterelectrode Ej in the electrolyte by means of an external circuit (voltage source Uq, ammeter, resistances R and R") and via the electrode E, to be measured, back to the external circuit. In the diagram, the current indicated (0) is positive. The potential of E, is measured with a high-resistance voltmeter as the voltage difference of electrodes El and E2. To accomplish this, the reference electrode, E2, must be equipped with a Haber-Luggin capillary whose probe end must be brought as close as possible to... 

In addition, the temperature dependence of the diffusion potentials and the temperature dependence of the reference electrode potential itself must be considered. Also, the temperature dependence of the solubility of metal salts is important in Eq. (2-29). For these reasons reference electrodes with constant salt concentration are sometimes preferred to those with saturated solutions. For practical reasons, reference electrodes are often situated outside the system under investigation at room temperature and connected with the medium via a salt bridge in which pressure and temperature differences can be neglected. This is the case for all data on potentials given in this handbook unless otherwise stated. 

Point (a) only concerns simple metal electrodes and needs to be tested for each case. Point (b) is important for the measuring instrument being used. In this respect, polarization of the reference electrode leads to less error than an ohmic voltage drop at the diaphragm. Point (c) has to be tested for every system and can result in the exclusion of certain electrode systems for certain media and require special measures to be taken. 

The first term in Eq. (3-27) represents the voltage drop between the reference electrode over the pipeline and the pipe surface. The second term represents the potential difference AU measured at the soil surface (ground level) perpendicular (directly above) to the pipeline. Average values of the values measured to the left and right of the pipeline are to be used (see Fig. 3-24) [2]. In this way stray IR components can be eliminated. The third term comprises the current densities where, in the switched-off state of the protection installation, there is a cell current J. In the normal case J = 0 and also correspondingly AU f = 0 as well as = t/ ff On... 

The principle of the measurement is described with the help of Fig. 2-7 [50]. Potential measurement is not appropriate in pipelines due to defective connections or too distant connections and low accuracy. Measurements of potential difference are more effective. Figure 3-24 contains information on the details in the neighborhood of a local anode the positions of the cathodes and reference electrodes (Fig. 3-24a), a schematic representation of the potential variation (Fig. 3-24b), and the derived values (Fig. 3-24c). Figure 2-8 should be referred to in case of possible difficulties in interpreting the potential distribution and sign. The electrical potentials of the pipeline and the reference electrodes are designated by... 

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