Reference electrodes important

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... 

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 Solution. The responses of working and reference electrodes to appHed voltages are important only because this information can be indicative of what goes on in the solution, or at the solution/electrode interface. The distinction between bulk (solution) and interfacial events is basically the distinction between chemical kinetics and charge transfer. 

Table 3-1 Data and application range of important reference electrodes...

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. 

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. 

This electrode is perhaps next in importance to the calomel electrode as a reference electrode. It consists of a silver wire or a silver-plated platinum wire, coated electrolytically with a thin layer of silver chloride, dipping into a potassium chloride solution of known concentration which is saturated with silver chloride this is achieved by the addition of two or three drops of 0.1M silver nitrate solution. Saturated potassium chloride solution is most commonly employed in the electrode, but 1M or 0.1 M solutions can equally well be used as explained in Section 15.1, the potential of the electrode is governed by the activity of the chloride ions in the potassium chloride solution. 

Values of the electrode potentials for the more common reference electrodes are collected in Table 15.1 together with an indication of the effect of temperature for the most important electrodes. 

Prepare an approximately 0.1 M silver nitrate solution. Place 0.1169 g of dry sodium chloride in the beaker, add 100 mL of water, and stir until dissolved. Use a silver wire electrode (or a silver-plated platinum wire), and a silver-silver chloride or a saturated calomel reference electrode separated from the solution by a potassium nitrate-agar bridge (see below). Titrate the sodium chloride solution with the silver nitrate solution following the general procedure described in Experiment 1 it is important to have efficient stirring and to wait long enough after each addition of titrant for the e.m.f. to become steady. Continue the titration 5 mL beyond the end point. Determine the end point and thence the molarity of the silver nitrate solution. 

An important step in measurements of electrode potentials is that of selecting a suitable reference electrode (RE). Reference electrodes with electrolytes of the same nature and same (or similar) composition as that at the working electrode are used... 

Potentiostatic conditions are realized with electronic potentiostats. The potential of the working electrode is monitored continuously with the aid of a reference electrode. When the potential departs from a set value, the potentiostat will adjust the current flow in the cell automatically so as to restore the original value of potential. Important characteristics of potentiostats are their rise time and the maximum currents which they can deliver to the cell. Modem high-quality potentiostats have rise times of 10 to 10 s. 

One important advantage of the polarized interface is that one can determine the relative surface excess of an ionic species whose counterions are reversible to a reference electrode. The adsorption properties of an ionic component, e.g., ionic surfactant, can thus be studied independently, i.e., without being disturbed by the presence of counterionic species, unlike the case of ionic surfactant adsorption at nonpolar oil-water and air-water interfaces [25]. The merits of the polarized interface are not available at nonpolarized liquid-liquid interfaces, because of the dependency of the phase-boundary potential on the solution composition. 

Before dealing with various important applications of the electrochemical series with some practical examples, a break is given here to present a more detailed elaboration on the hydrogen electrode, reference electrodes, and some of the theoretical and general aspects pertaining to electrode potentials and free energy changes involved with cell reactions. 

In addition to their use as reference electrodes in routine potentiometric measurements, electrodes of the second kind with a saturated KC1 (or, in some cases, with sodium chloride or, preferentially, formate) solution as electrolyte have important applications as potential probes. If an electric current passes through the electrolyte solution or the two electrolyte solutions are separated by an electrochemical membrane (see Section 6.1), then it becomes important to determine the electrical potential difference between two points in the solution (e.g. between the solution on both sides of the membrane). Two silver chloride or saturated calomel electrodes are placed in the test system so that the tips of the liquid bridges lie at the required points in the system. The value of the electrical potential difference between the two points is equal to that between the two probes. Similar potential probes on a microscale are used in electrophysiology (the tips of the salt bridges are usually several micrometres in size). They are termed micropipettes (Fig. 3.8D.)... 

Reference electrodes provide a standard for the electrochemical measurements. For potentiometric sensors, an accurate and stable reference electrode that acts as a halfcell in the measurement circuit is critical to providing a stable reference potential and for measuring the change in potential difference across the pH sensitive membrane as the pH concentration changes. This is especially important in clinical applications such as pH measurements in the blood, heart, and brain, where the relevant physiological pH range is restricted to a very small range, usually less than one unit. 

Any charge change occurring only between the reference electrode and the semiconductor is a candidate for a change of Ids. In particular one of the most important points is the surface potential at the oxide-solution interface (surface potentials between the CIM and the solution and the potential between the SiC>2 and the CIM, in the presence of a given CIM. The ISFET operation may be represented by the following changes-flow which may be considered as superimposed on the quiescent point determined by the reference electrode potential ... 

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