Electrode reference —.

Reference electrodes are used in the measurement of potential [see the explanation to Eq. (2-1)]. A reference electrode is usually a metal/metal ion electrode. The electrolyte surrounding it is in electrolytically conducting contact via a diaphragm with the medium in which the object to be measured is situated. In most cases concentrated or saturated salt solutions are present in reference electrodes so that ions diffuse through the diaphragm into the medium. As a consequence, a diffusion potential arises at the diaphragm that is not taken into account in Eq. (2-1) and represents an error in the potential measurement. It is important that diffusion potentials be as small as possible or the same in the comparison of potential values. Table 3-1 provides information on reference electrodes. 

Equations (2-13), (2-23), and (2-28) can be combined to give the following equation for the migration of ions  

The mobility u-can be taken from Table 2-2 for dilute solutions and is proportional to the diffusion coefficient D. It follows, for the transport of anions and cations of an n-n valent salt, that 

The indices A and C correspond to anion and cation. In the stationary state and Wg are equal because of the charge balance. It follows from Eqs. (3-2a) and (3-2b) and Eq. (2-13) that  

The error due to diffusion potentials is small with similar electrolyte solutions (cj = C2) and with ions of equal mobility (/ Iq) as in Eq. (3-4). This is the basis for the common use of electrolytic conductors (salt bridge) with saturated solutions of KCl or NH4NO3. The /-values in Table 2-2 are only applicable for dilute solutions. For concentrated solutions, Eq. (2-14) has to be used. 

The purpose of the reference electrode is to provide the second electrode of an electrical cell whose potential is measured for the determination of pH. It must have a stable and reproducible potential to which the glass electrode potential may be referenced. The reference electrode completes the circuit by contacting the sample solution through a liquid junction. It is this liquid junction that is most often a problem in pH measurement and will be discussed in detail in Section 3.2.3. 

The internal element appears as a grayish-white cylindrical pack with shiny mercury at the top of the element, if it is a calomel internal. This mercury-mercurous chloride half cell provides a potential of 244 mV versus the normal hydrogen electrode at 25°C if it is surrounded by saturated potassium chloride filling solution. It is important that this element be kept wet and uncontaminated in order to provide a stable and reproducible potential. With use, the pack may show some separation within the element tube, but this usually does not cause error or deviation of its potential. 

A standard silver-silver chloride reference electrode provides 199 mV versus the normal hydrogen electrode if it is surrounded by a filling solution saturated with both potassium chloride and silver 

Since it is important to keep the internal element wet and surrounded by a filling solution of known concentration, manufacturers of reference electrodes ship the electrode filled with filling solution. In order to prevent leakage of the filling solution through 

The reference electrode is also sold with a cap containing solution over the junction. This is to keep the junction wet and permeable at all times. It is important that the junction should be kept wet in order to function properly. (See Section 3.5.) 

A major factor in the selection of a reference electrode is the reference potential established. If the reference potential is close to the potential at the sensing electrode, the cell voltage will be small and thus tend to be more stable than a large voltage. For example, the oxygen partial pressure established by a Pd + PdO reference electrode 

Reference electrode performance also includes the response kinetics. For example, 

There are three requirements that a good reference potential must satisfy. It must be stable, reversible, and reproducible. In this context, stable means that it will not change when the composition of the sample changes. Reversible means that it will 

These systems are much used as reference electrodes since, because of the low solubility product of the salt, the potential is very stable. Other examples are Ag AgCl Cl- and, for alkaline solution, Hg HgO OH . 

This type of electrode is a source or sink of electrons, permitting electron transfer without itself entering into the reaction, as is the case for the first or second type of electrodes. For this reason they are called redox or inert electrodes. In reality the concept of an inert electrode is idealistic, given that the surface of an electrode has to exert an influence on the electrode reaction (perhaps small) and can form bonds with species in solution (formation of oxides, adsorption, etc.). Such processes give rise to non-faradaic currents (faradaic currents are due to interfacial electron transfer). This topic will be developed further in subsequent chapters. 

The first redox electrode materials to be used were the noble metals, namely gold and platinum, and also mercury. At present this designation includes many types of material such as glassy carbon, different types of graphite, and semiconductor oxides, so long as a zone of potential is employed where surface reactions involving the electrode material do not occur. 

Electrodes that cannot be grouped into the above categories, e.g. modified electrodes (see Chapter 14). 

This classification is useful mainly for electrodes of the first and second types. The great majority of electrodes are, however, of the third or fourth types. 

Although electrode potentials are referred to the SHE, one uses in most cases electrodes of a second kind. They are very stable and reproducible and simple to use because they avoid the use of hydrogen gas. The potentials of these electrodes are usually determined by the solubility of a weakly soluble reaction product. 

The solubility product of Hg2Cl2 has been determined with Equation 1.101, Ecai = 0.268V and E° = 0.796 V to be Kg = 1.27 10- . 

Some Commonly Used REs and Their Standard Potentials E /V 

Source Handbook of Chemistry and Physics, CRC Press, Cleveland, OH, 1977-1978. 

Impedance spectroscopy is predestined to separate the contributions of bulk and electrodes to the overall electrical properties of a solid and can thus be employed to investigate the kinetics of electrochemical reactions. However, the relaxation fre- 

Miniaturized working electrodes offer a possibility to cope with these difficulties Owing to their much larger electrochemically active areas, macroscopic counterelectrodes lead to voltage drops which are often negligible compared to the overpotential of a microelectrode. A reference electrode is therefore not necessary to get rid of the influence of the counter-electrode. As an example of an electrochemical 

It can be a further advantage of microelectrodes that they often increase the electrode resistance to bulk resistance ratio Rei/Rbuik- This is so because Re 1 frequently scales with the inverse area of the electrode, whereas the bulk resistance between a circular microelectrode and a counter-electrode is proportional to the inverse microelectrode diameter dme (see Sec. 4.1). Hence Rei/Rb iik oc im results and the importance of the bulk resistance decreases with decreasing microelectrode diameter. This is particularly helpful in order to investigate electrode polarization phenomena below the detection limit in experiments using macroscopic electrodes. (The reduced importance of the electrolyte resistance is also one of the reasons for ultramicroelectrodes to be applied in liquid electrochemistry [33, 34].) 

Even though the normal hydrogen electrode (NHE) is the best known and internationally accepted reference electrode, it is difficult to construct and handle, rendering it of little practical use. 

For non-aqueous solvents, the two most common reference electrodes are  

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 

The ideal reference electrode must provide a stable potential so that any change in Fceii is attributed to the indicator electrode, and, therefore, to a change in the analyte s concentration. In addition, the ideal reference electrode should be easy to make and to use. Three common reference electrodes are discussed in this section. 

Reference electrode based on the reduction ofH aq) to H2( ) at a Pt electrode that is, 

Reference electrode based on the reduction of Hg2Cl2 to Hg in an aqueous solution saturated with KCl that is, 

Calomel Electrodes Calomel reference electrodes are based on the redox couple between Hg2Cl2 and Hg (calomel is a common name for Hg2Cl2). 

Proper cell design is essential to reduce the uncertainty of the interpretation. Reference electrodes can be used to isolate the impedance of cell components, well-defined convective systems can be employed to queintify the role of mass transfer, and electrode configurations can be selected to minimize the role of current and potential distributions across the surface of the electrodes. 

As discussed in Section 5.7, the potential drop across an electrochemical cell can be expressed as the sum of contributions 

Under these conditions, the indicator electrode can provide unambiguous information about ionic activities in the cell solution. In most analytical work, it is not necessary to know the actual value of the reference electrode potential—as long as it is constant— because Ecoaatant is determined using known standard solutions. 

The hydrogen electrode is the ultimate standard electrode not only for the determination of (relative) potentials, but also for the determination of pH values. Owing to the experimental difficulties associated with it, however, it is seldom used for routine measurements, but rather for the evaluation of secondary reference and pH electrodes such as the calomel reference electrode and the glass pH electrode. 

The hydrogen electrode consists essentially of a piece of platinum foil, electroplated ( platinized ) with a thin layer of finely divided platinum. This provides a catalytic surface on which the half-cell reaction 

Perhaps the most widely used reference electrode for electrochemical measurements is one form or other of the calomel electrode. This electrode consists of mercury, mercurous chloride (calomel), and a chloride-ion solution  

For accurate work, 0.1 A/ or 1 A/ KCl calomel electrodes may be used because they reach their equilibrium potential more rapidly, and have less temperature-dependence. Calomel electrodes with NaCl electrolyte have also found use. Table 2.1 gives the potentials of several common reference electrodes at selected temperatures. 

In electrochemical experiments, often the interest is on one of the electrode reactions. Since for measurement a complete cell with two electrodes must be used, it is common practice to employ a reference electrode as the other half of the cell. It is desired that the reference electrode should be easy to prepare and maintain and has stable potential. This is accomplished by using an electrode reaction involving a saturated solution of an insoluble salt of the ion. The common reference electrodes and potential with respect to the SHE are saturated calomel electrode (E = +0.242 V saturated), copper-copper(II) sulfate electrode (E = -0.314 V), and silver chloride electrode (E = 0.225 V saturated). The silver-silver chloride electrode is 

Typically, this electrode is silver wire coated with AgCl. The coating is done by making the silver the anode in an electrolytic cell containing HCl the Ag ions combine with Q ions as fast as they are formed at the silver surface. 

The calomel electrode has the following half-cell and reaction  

The thermod5mamic equilibrium of any other chemical or electrochemical reaction can be calculated in the same manner, provided the basic information is found. Table 4.7 contains the chemical description of most reference electrodes used in laboratories and field units, and Tables 4.8 and 4.9, respectively, contain the thermod5mamic data associated with the solid and soluble chemical species making these electrodes. Table 4.10 presents the results of the calculations performed to obtain the potential of each electrode at 60°C. 

In the field of bioimpedanee, by far the most important nonpolarizable electrode for stable DC potential measurement is the AgCl electrode. This is because all tissue liquids contain some Cl ions, and because the electrode can be made very small (e.g., with just a small chlorided silver wire). The half-cell potential relative to a standard hydrogen electrode at 25°C in an aqueous solution is +0.222 V. Under ideal eonditions sueh an eleetrode can be reproducible to 20 pV. Thus two equal electrodes in the same solution should have zero 

The most cited reference electrode is the platinum-hydrogen electrode, and electrode DC potentials are often given relative to such an electrode. It is an important electrode for absolute calibration, even if it is impractical in many applications. The platinum electrode metal is submerged in a protonic electrolyte solution, and the surface is saturated with continuously supplied hydrogen gas. The reaction at the platinum surface is a hydrogen redox reaction H2 2H (aq) + 2e, of course with no direct chemical participation of the noble metal. Remember that the standard electrode potential is under the condition pH = 0 and hydrogen ion activity 1 mol/L at the reference electrode. Thus the values found in tables must be recalculated for other concentrations. Because of the reaction it is a hydrogen electrode, but it is also a platinum electrode because platinum is the electron source or sink, and perhaps a catalyst for the reaction. 

Imagine a solution containing an electroactive species whose concentration we wish to measure. We construct a half-cell by inserting an electrode (such as a Pt wire) into the solution to transfer electrons to or from the species of interest. Because this electrode responds directly to the analyte, it is called the indicator electrode. The potential of the indicator electrode is E+. We then connect this half-cell to a second half-cell by a salt bridge. Hie second half-cell has a fixed composition that provides a known, constant potential, E. Because the second half-cell has a constant potential, it is called a reference electrode. The cell voltage ( = +- ) is the difference between the variable potential that reflects changes in the analyte concentration and the constant reference potential. 

The two half-reactions were 14-15 and 14-16, and the two electrode potentials were given in Equations 14-18 and 14-19. Hie cell voltage is the difference + - .  

The standard reduction potential for AgCl Ag is +0.222 V at 25°C. If the cell is samrated with KCl, the potential is +0.197 V. We will use this value for all problems 

Reference electrode maintains a fixed (reference) potential 

Voltage in Rgure 14-9 responds only to changes in the quotient Fe ]/ [Fe ]. Everything else is constant. 

As it is not possible to determine the absolute potential of an electrode, the electrode potential must always be referred to an arbitrary zero point, defined by the potential of a chosen reference electrode. Thus, it is very important always to quote the type of reference electrode used in electrochemistry. Differences in operating potentials reported in the literature are often attributable to the use of different reference electrodes. 

In order to measure the emf of a given half-cell, it is necessary to connect it with a second halfcell and measure the voltage produced by the complete cell. In general, the second half-cell serves as a reference cell and should be one with a known, stable electrode potential. Although the SHE serves to define the standard redaction potential, in practice, it is not always convenient to nse an SHE as a reference electrode. It is difficnlt to set up and control. Other, more convenient reference electrodes 

In order to measure the emf of a given half-cell, it is necessary to connect it with a second half-cell and measure the voltage produced by the complete cell. In general, the second halfcell serves as a reference cell and should be one with a known, stable electrode potential. Although the standard hydrogen electrode serves to define the standard reduction potential, in practice it is not always convenient to use an SHE as a reference electrode. It is difficult to set up and control. Other, more convenient reference electrodes have been developed. In principle, any metal-ion half-cell could be used under controlled conditions as a reference electrode, but in practice, many metals are unsatisfactory materials. Active metals, such as sodium and potassium, are subject to chemical attack by the electrolyte. Other metals, such as iron, are difficult to obtain in the pure form. With some metals, the ionic forms are unstable to heat or to exposure to the air. Also, it is frequently difficult to control the concentration of the electrolytes accurately. As a result, only a few systems provide satisfactory stable potentials. 

The characteristics of an ideal reference electrode are that it should have a fixed potential over time and temperature, long term stability, the ability to return to the initial potential after exposure to small currents (i.e., it should be reversible), and it should follow the Nemst equation. Two common reference electrodes that come close to behaving ideally are the saturated calomel electrode and the silver/silver chloride electrode. 

The saturated calomel electrode (SCE) is composed of metalhc mercury in contact with a saturated solution of mercurous chloride, or calomel (Hg2Cl2). A Pt wire in contact with the metallic Hg conducts electrons to the external circuit. The mercurous ion concentration of the solution is controlled through the solubility product by placing the calomel in contact with a saturated potassium chloride solution. It is the saturated KCl solution that gives this electrode the saturated name there are other calomel reference electrodes used that differ in the concentration of KCl solution, but all contain saturated mercurous chloride solution. A typical calomel electrode is shown in Fig. 15.5. The half-cell 

But Hg2Cl2(s) and Hg(l) are in standard states at unit activity therefore 

When the KCl solution is saturated and its temperature is 25°C, the concentration of chloride ion, [Cl ], is known and 

As mentioned previously, electroanalytical techniques that measure or monitor electrode potential utilize the galvanic cell concept and come under the general heading of potentiometry. Examples include pH electrodes, ion-selective electrodes, and potentiometric titrations, each of which will be described in this section. In these techniques, a pair of electrodes are immersed, the potential (voltage) of one of the electrodes is measured relative to the other, and the concentration of an analyte in the solution into which the electrodes are dipped is determined. One of the immersed electrodes is called the indicator electrode and the other is called the reference electrode. Often, these two electrodes are housed together in one probe. Such a probe is called a combination electrode. 

In electroanalytical chemistry, the unchanging reference is a half-cell that, at a given temperature, has an unchanging potential. There are two designs for this half-cell that are popular—the saturated calomel electrode (SCE) and the silver-silver chloride electrode. These are described below. 

FIGURE 14.4 Left, a drawing of a commercial saturated calomel electrode. Right, a photograph of a commercial saturated calomel electrode. 

The outer tube has a porous fiber tip, which acts as the salt bridge to the analyte solution and the other half-cell. A saturated solution of potassium chloride is in the outer tube. The saturation is evidenced by the fact that there is some undissolved KC1 present. Within the inner tube is mercury metal and a paste-like material known as calomel. Calomel is made by thoroughly mixing mercury metal (Hg) with mercurous chloride (Hg2Cl2), a white solid. When in use, the following half-cell reaction occurs  

Obviously the only variable on which the potential depends is [ CF]. The saturated KC1 present provides the [CF] for the reaction, and, since it is a saturated solution, [CF] is a constant at a given temperature represented by the solubility of KC1 at that temperature. If [CF] is constant, the potential of this halfcell, dependent only on the [CF], is therefore also a constant. As long as KC1 is kept saturated and the temperature kept constant, the SCE is useful as a reference against which all other potential measurements can be made. Its standard reduction potential at 25°C (see Table 14.1) is +0.2412 V. 

In electrochemical studies of fuel cell systems, two-electrode EIS measurements have been applied extensively. However, the results are often difficult to interpret due to superposition of the behaviours at both anode and cathode. In some cases, 

In general, the reference electrode should be (1) a reversible electrode that obeys the Nemst equation, (2) stable, and (3) able to respond quickly to changes in environmental conditions [63], The most commonly used reference electrodes for measuring the AC impedance spectra for fuel cells are the DHE (dynamic hydrogen electrode) and the RHE (reversible hydrogen electrode) [64, 65], 

Conventional reference electrodes consist of a solid reversible electrode and an aqueous electrolyte solution. To measure the individual contributions from the anode and the cathode of a PEM fuel cell, the electrolyte solution of the reference electrode must either be in direct contact with one side of the solid proton exchange membrane or be located in a separate compartment with electrical contact between the reference electrode and the solid membrane by means of a salt bridge [66], As a result, two different types of reference electrode configurations are employed for the study of fuel cells internal and external. 

Fuel cell researchers have also investigated other reference electrodes, such as a pseudo-reference electrode constructed by inserting a micro-sized carbon filament between two polymer electrolyte membranes [73], The main advantage of pseudoreference electrodes is their easy implementation, although one disadvantage is that their DC potential is unknown. However, this DC potential may not be that critical because EIS measurements mainly rely on the AC perturbation signal from which the impedance is calculated. 

Another alternative, an ionized air reference electrode (IAE), has been described by Foulkes et al. [66] for use with solid polymer electrolytes, in particular Nafion 425. Use of the IAE is achieved by establishing electrical contact with the solid electrolyte from a weak source of ionizing radiation to ionize an air gap. In contrast to conventional reference electrodes, the IAE is a liquid-free device and can make electrical contact with a solid polymer electrolyte without a salt bridge. It is also temperature independent. In view of these special characteristics, the IAE might provide a useful alternative to conventional reference electrodes for use with solid polymer electrolytes. 

Electrochemical Sensors, Biosensors and Their Biomedical Applications 

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There are two procedures for doing this. The first makes use of a metal probe coated with an emitter such as polonium or Am (around 1 mCi) and placed above the surface. The resulting air ionization makes the gap between the probe and the liquid sufficiently conducting that the potential difference can be measured by means of a high-impedance dc voltmeter that serves as a null indicator in a standard potentiometer circuit. A submerged reference electrode may be a silver-silver chloride electrode. One generally compares the potential of the film-covered surface with that of the film-free one [83, 84]. 

Incidentally, a quantity called the rational potential is defined as E for the mercury-water interface (no added electrolyte) so, in general, = E + 0.480 V if a normal calomel reference electrode is used. 

The standard states of Ag and of Ag (aq) have the conventional definitions, but there is an ambiguity in the definition of the standard state of e. Suppose that a reference electrode R is positioned above a solution of AgN03, which in turn is in contact with an Ag electrode. The Ag electrode and R are connected by a wire. Per Faraday, the processes are... 

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.)...

Cells need not necessarily contain a reference electrode to obtain meaningfiil results as an example, if the two electrodes in figure A2.4.12 are made from the same metal, M, but these are now in contact with two solutions of the same metal ions, M but with differing ionic activities, which are separated from each other by a glass frit that pennits contact, but impedes diffusion, then the EMF of such a cell, temied a concentration cell, is given by... 

Several designs for STM electrochemical cells have appeared in the literature [M]- hr addition to an airtight liquid cell and the tip insulation mentioned above, other desirable features include the incorporation of a reference electrode (e.g. Ag/AgCl in saturated KCl) and a bipotentiostat arrangement, which allows the independent control of the two working electrodes (i.e. tip and substrate) [ ] (figure BL19.11). 

Wlien an electrical coimection is made between two metal surfaces, a contact potential difference arises from the transfer of electrons from the metal of lower work function to the second metal until their Femii levels line up. The difference in contact potential between the two metals is just equal to the difference in their respective work fiinctions. In the absence of an applied emf, there is electric field between two parallel metal plates arranged as a capacitor. If a potential is applied, the field can be eliminated and at this point tire potential equals the contact potential difference of tlie two metal plates. If one plate of known work fiinction is used as a reference electrode, the work function of the second plate can be detennined by measuring tliis applied potential between the plates [ ]. One can detemiine the zero-electric-field condition between the two parallel plates by measuring directly the tendency for charge to flow through the external circuit. This is called the static capacitor method [59]. 

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... 

The apparatus consists of a tip-position controller, an electrochemical cell with tip, substrate, counter and reference electrodes, a bipotentiostat and a data-acquisition system. The microelectrode tip is held on a piezoelectric pusher, which is mounted on an inchwomi-translator-driven x-y-z tliree-axis stage. This assembly enables the positioning of the tip electrode above the substrate by movement of the inchwomi translator or by application of a high voltage to the pusher via an amplifier. The substrate is attached to the bottom of the electrochemical cell, which is mounted on a vibration-free table [, and ]. A number... 

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.

Figure C2.8.3. A tliree-electrode electrochemical set-up used for the measurement of polarization curves. A potentiostat is used to control the potential between the working electrode and a standard reference electrode. The current is measured and adjusted between an inert counter-electrode (typically Pt) and the working electrode.

Table 8.20 Potentials of Reference Electrodes in Volts as a Function of...

Table 8.21 Potentials of Reference Electrodes (in Volts) at 25°Cfor Water-...

The values of several additional reference electrodes at 25°C are listed ... 

In Section 8, the material on solubility constants has been doubled to 550 entries. Sections on proton transfer reactions, including some at various temperatures, formation constants of metal complexes with organic and inorganic ligands, buffer solutions of all types, reference electrodes, indicators, and electrode potentials are retained with some revisions. The material on conductances has been revised and expanded, particularly in the table on limiting equivalent ionic conductances. 

Finding the End Point Potentiometrically Another method for locating the end point of a redox titration is to use an appropriate electrode to monitor the change in electrochemical potential as titrant is added to a solution of analyte. The end point can then be found from a visual inspection of the titration curve. The simplest experimental design (Figure 9.38) consists of a Pt indicator electrode whose potential is governed by the analyte s or titrant s redox half-reaction, and a reference electrode that has a fixed potential. A further discussion of potentiometry is found in Chapter 11. 

The potential of the working electrode, which changes as the composition of the electrochemical cell changes, is monitored by including a reference electrode and a high-impedance potentiometer. 

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). 

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... 

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