Types of Reference Electrodes

Another class of reference electrodes, often called indicator electrodes, are electrodes in direct contact with the solution. The most common among these is the reversible hydrogen electrode, formed by bubbling hydrogen over a large-area platinized Pt electrode in the test solution. This electrode is reversible with respect to the hydronium ion, serving, in effect, as a pH indicator electrode. Similarly, 

The advantage of indicator electrodes is that they always measure the reversible potential with respect to the ion being studied, regardless of its concentration in solution. Their disadvantage is that they must be prepared for each experiment and often end up being less stable and less reliable than commercial reference electrodes. Also, being in intimate contact with all ingredients in the test solution, they can be contaminated, either by impurities or by components of the test solution, such as 

---

It is clear that to ensure adequate protection of a structure under cathodic protection it is necessary to measure its electrode potential. This can only be achieved by using a reference electrode placed in the same environment as the structure and measuring the e.m.f. of the cell so formed. Since the electrode potentials of different types of reference electrode vary, it is clear that the measured e.m.f. will also vary according to the particular reference electrode used. It follows that the potential measured must always be recorded with respect to the reference electrode deployed, which must always be stated. 

Various types of reference electrodes have been considered in Section 20.3, and the potentials of these electrodes and their variation with the activity of the electrolyte are listed in Table 21.7, Chapter 21. It is appropriate, however to point out here that the saturated calomel electrode (S.C.E.), the silver-silver chloride electrode and the copper-copper sulphate electrode are the most widely used in corrosion testing and monitoring. 

Reference electrodes. There are two types of reference electrodes (see the scheme in Section 1.3.1) (a) those constructed as a reference type and (b) those used as a reference type both types fulfil the requirement of a constant reference potential by either being non-polarizable or becoming non-polarized during the measurememt. 

Fig. 3.8 Various types of reference electrode vessels (A) laboratory-type calomel electrode (B), (C) portable calomel electrodes, (D) a Ag/AgCl micropipette with a KC1 electrolyte immobilized with agar in the tip of the capillary... 

Part—III exclusively treats Electrochemical Methods invariably and extensively used in the analysis of pharmaceutical substances in the Official Compendia. Two important methods, namely potentiometric methods (Chapter 16) deal with various types of reference electrodes and indicator electrodes, automatic titrator besides typical examples of nitrazepam, allopurinol and clonidine hydrochloride. Amperometric methods (Chapter 17) comprise of titrations involving dropping-mercury electrode, rotating—platinum electrode and twin-polarized microelectrodes (i.e., dead-stop-end-point method). 

Certain types of reference electrodes are found to be more suitable than others when determining the pH of solutions containing H2S. Recommendations regarding various electrodes are given in the text of the paper. 

Next we discuss four types of reference electrodes hydrogen, calomel, silver-silver chloric, and mercury-mercurous sulfate electrodes. 

V°rev = 1.229V is the standard state reversible potential for the water splitting reaction and Vaoc is the anode potential at open circuit conditions. Term Vmeas-Vaoc arises from the fact that Voc represents the contribution of light towards the minimum voltage needed for water splitting potential (1.229V) and that the potential of the anode measured with respect to the reference electrode Vmeas has contributions from the open circuit potential and the bias potential applied by the potentiostat (i.e. Vmeas= Vapp+Vaoc). The term Vmeas-Vaoc makes relation (3.6.16) independent of the electrolyte pH and the type of reference electrode used. Thus the use of V°rev in relation (3.6.16) instead of VV or V°hz as in the case of relation (3.6.15) is justified. 

The reference electrodes used in non-aqueous systems can be classified into two types. One type uses, in constructing a reference electrode, the same solvent as that of the solution under study. The other type is an aqueous reference electrode, usually an aqueous Ag/AgCl electrode or SCE. Some reference electrodes are listed in Table 6.2 and are briefly discussed below. For other types of reference electrodes used in non-aqueous solutions, see Ref. [4],... 

Several types of reference electrodes are convenient for use in analytical electrochemistry. The use of high-input-impedance operational amplifiers in the reference electrode inputs of potentiostats ensures that very low levels of current are drawn from the reference electrode (see Chap. 6). This permits the use of reference electrodes that do not have to contain a large number of redox equivalents in order to ensure a constant reference potential and are therefore very small. Three reference-electrode designs that are convenient for use in analytical electrochemistry are shown in Figure 9.4. Saturated calomel and silver-silver chloride (of various concentrations of chloride) are among the most common commercially available or conveniently fabricated reference electrodes. 

There are three types of reference electrodes discussed reference electrodes of the first kind, reference electrodes of the second kind, and redox reference electrodes. The first two are used with potentiometric chemical sensors, whereas the last one helps us to get around the difficult problem of comparing potentials in different solvents. There is also a pseudo-reference electrode that does not have a stable, defined, reproducible potential. It serves only as the signal return to satisfy the condition of closing the electrical circuit (see Section 5.2). Because the liquid junction always causes some leakage of the internal solution, electrodes of the first kind are particularly affected. 

Another form of redox reference electrode is similar to the electrode of the first kind. In this case the inert metal (e.g., Pt, Au, or C) is used as the inner electrode and a stable and soluble redox couple is placed inside the inner reference electrode compartment. A normal liquid junction is used in this type of reference electrode. Unlike the electrode of the first kind, the redox reference electrode is relatively immune to changes in concentration inside the reference electrode compartment because it is the ratio of the reduced/oxidized form of redox couple that determines the potential and not the absolute concentrations. However, redox reference electrodes are sensitive to changes of concentration of oxygen and other redox species. 

As each type of reference electrode has its own potential with respect to the SHE, the measured potential values must frequently be converted. The conversion from a value measured with one type of reference to the value for another reference is a simple operation. This is illustrated in the example below and in Fig. 2.1. 

Reference electrode potentials change with temperature. Both electrochemical reactions (Nernstian thermodynamics) and chemical solubilities, e.g. of the inner reference electrode solution, are affected. Accordingly, the temperature coefficient, dE/dT (mV °C4), varies from one type of reference electrode to another. To minimise errors in potential readings the coefficient should be low and at least known. Examples of temperature coefficients are given in Table 2.2. 

Various types of reference electrodes are used for permanent embedment in concrete. Some fall into the category of double junction electrodes of the second kind and are therefore reference electrodes in the true sense. Others are simply a piece of metal or another material put into the concrete. Although stable and accurate, SCE is not used for permanent embedment in concrete, mainly because it contains a liquid metal, which makes it difficult to manufacture in a rugged form. In addition, environmental reasons make it undesirable for permanent use in the field (poisonous mercury and mercury compounds). 

Before casting the concrete, the reference electrode must be firmly fixed at the depth of interest - usually at the depth of the first layer of steel reinforcement. The reference electrode is normally held in place with plastic ties attached to the steel. Depending on the type of reference electrode, care is taken to ensure that the concrete completely envelopes the electrode and that no ionically conductive or metallic part of the electrode comes into direct contact with the steel. 

Table 11.1 A cross-section of the different types of reference electrodes that have been used by various researchers in a range of different...

Reference electrodes, as their name suggests, are used to give a value of potential to which other potentials can be referred in terms of a potential difference—potentials can only be registered as differences with respect to a chosen reference value. Thus, a good reference electrode3,6 needs to have a potential that is stable with time and with temperature and which is not altered by small perturbations to the system—that is, by the passage of a small current. There are three types of reference electrode ... 

Phillips (19) formed a reference electrode by coating a noble metal with a Langmuir-Blodgett multilayer of calcium palmitate and used it to detect the presence of a polar vapor in dry atmospheres by its adsorption on an uncoated metal electrode. Water vapor and oxygen were carefully excluded from the surrounding atmosphere, since either gas penetrated the multilayer coating on the reference electrode. This report describes the preparation, properties, and several applications of a more general and useful type of reference electrode. 

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. 

The external type of reference electrode is connected to the membrane via a liquid electrolyte bridge, such as a sulphuric acid solution, as shown in Figure 5.45. Compared with the internal reference electrode configuration, the external type is easier to use in a normal PEM fuel cell set-up because it needs minimal modifications. However, attention must also be paid to ensure that the liquid electrolyte has good contact with the membrane and does not flow into the cell. Furthermore, the use of a liquid electrolyte in an acid bridge can induce non-uniform hydration and a proton concentration gradient in the membrane, therefore interfering with the fuel cell electrodes. 

Several types of reference electrodes may be used. The hydrogen electrode has been shown to behave reversibly in HF [217,218] and has been employed as reference electrode both in electroanalytical and preparative work [218], and mercury fluoride [251,312] and copper fluoride [213,306] have also been used as such. A comparison of these reference electrodes pointed to the hydrogen electrode (H2/Pd) as the most convenient [313]. 

Particular types of the ISFET are very diverse, since there is a possibility of widely varying the membrane composition, the type of reference electrode, the mode of operation (e.g., an additional polarizing voltage can be used), etc. 

The chloride ion contamination problem can be avoided by using reference electrodes that do not contain these ions. Examples are the sulfate and oxide types of reference electrodes in Table 6.1. 

The second, third, and fourth terms on the right-hand side of Eq. (13-135) are constants which depend only on T, p, and the type of reference electrode used. The quantity l however, is a function of the composition of solution X. Consequently Eq contains a variable quantity, which can neither be measured nor calculated exactly. 

A few types of reference electrodes are used for potential mapping, mainly silver/ silver chloride (Ag/AgCl) or copper/copper sulfate (CSE). They differ in their standard potential, which is the potential difference to the standard hydrogen electrode (SHE). Standard potentials of these reference electrodes are given in Table 16.1, together with some other types used as embedded probes in concrete (Chapter 17). 

The book edited by Ives and Janz [1 ] and more recently that by Bard, Parsons, and Jordan [2] contain both theoretical and practical aspects related to reference electrodes. Preparation, application and limitations of various types of reference electrodes such as the hydrogen electrode, the calomel and other mercury-mercurous salt electrodes, the silver-silver halide electrodes, and sulfide and sulfate electrodes are covered and general reference to these excellent critical reviews is recommended. 

Equation (23.6) tells us that although the standard value of a redox potential depends upon the type of reference electrode being used, it is otherwise a characteristic quantity for the redox pair because of its direct dependency upon the electron potential. Just like the electron potential, it represents a measure of... 

Cite two types of reference electrodes of experimental use, and specify the redox couple involved. 

---