Electrodes indicator

We will study two broad classes of indicator electrodes. Metal electrodes described in this section develop an electric potential in response to a redox reaction at the metal surface. Ion-selective electrodes, described later, are not based on redox processes. Instead, selective binding of one type of ion to a membrane generates an electric potential. 

The most common metal indicator electrode is platinum, which is relatively inert—it does not participate in many chemical reactions. Its purpose is simply to transmit electrons to or from species in solution. Gold electrodes are even more inert than Pt. Various types of carbon are used as indicator electrodes because the rates of many redox reactions on the carbon surface are fast. A metal electrode works best when its surface is large and clean. To clean the electrode, dip it briefly in hot 8 M HN03 and rinse with distilled water. 

To find the cell voltage in Equation 15-1. we need to know [Ag+]  

Demonstration 15-1 is a great example of indicator and reference electrodes 

We see from the example that a silver electrode is also a halide electrode, if solid silver halide is present,6 If the solution contains AgCl(.v), we substitute [Ag+] = Jfsp/[C1 ] into Equation 15-1 to find an expression relating the cell voltage to [Cl  

The indicator electrode is the electrode that responds to the change in analyte activity. An ideal indicator electrode should be specific for the analyte of interest, respond rapidly to changes in activity, and follow the Nernst equation. There are no specific indicator electrodes, but there are some that show a high degree of selectivity for certain analytes. Indicator electrodes fall into two classes, 

A metal electrode of the first kind is just a metal wire, mesh, or sohd strip that responds to its own cation in solntion. Cn/Cn +, Ag/Ag+, Hg/Hg+, and Pb/Pb are examples of this type of electrode. There are significant problems encountered with these electrodes. They have poor selectivity, responding not only to their own cation but also to any other more easily reduced cation. Some metal surfaces are easily oxidized, giving erratic or inaccurate response unless the solution has been purged of air. Some metals can only be used in limited pH ranges because they will dissolve in acids or bases. Silver and mercury are the most commonly used electrodes of the first kind. 

A metal electrode of the second kind consists of a metal coated with one of its sparingly soluble salts (or immersed in a saturated solution of its sparingly soluble salt). This electrode responds to the anion of the salt. For example, a silver wire coated with AgCl will respond to changes in chloride activity because the chloride ion activity is controlled by the solubility of AgCl. The electrode reaction is AgCl(s) + e Ag(s) + Cl , with a potential = 0.222 V. The Nemst equation expression for the electrode potential at 25°C is = 0.222 - 0.05916 log[Cl ]. 

A metal electrode of the third kind uses two equilibrium reactions to respond to a cation other than that of the metal electrode. Ethylenediaminetetraacetic acid (EDTA) complexes many metal cations, with different stabilities for the complexes but a common anion (the EDTA anion) involved in the equilibria. A mercury electrode in a solution containing EDTA and Ca will respond to the Ca ion activity, for example. The complexity of the equilibria makes this type of electrode unsuitable for complex sample matrices. 

The last type of metallic electrode is the redox indicator electrode. This electrode is made of Pt, Pd, Au, or other inert metals and serves to measure redox reactions for species in solution 

The indicator electrode must have a stable and reproducible potential for the course of the measurement and should be able to respond in a Nemstian manner to varying conditions in the high-temperature aqueous environment. In other words, the activity of the dissolved species, a and the standard open-circuit potential, should be defined by measuring the open circuit potential between the indicator and reference electrodes and applying the Nemst equation (Equation (3.2)). 

Although a number of the indicator electrodes have been tested for operation over a wide range of temperatures, only the platinum/hydrogen, Pt(H2), and yttria-stabilized zirconia electrodes, with a mercury/mercury oxide electrochemical couple, YSZ(Hg/HgO), were found to be capable of operating in a Nemstian manner at temperatures up to 400 °C. 

The values of AfGn+ and AfG are zero at any temperature. Furthermore, ii sz(Hg/Hgo) is independent of the properties of the YSZ membrane so that for this electrode to operate effectively it only requires sufficient conductivity 

The Pt(H2) indicator electrode has been widely used for measiuing pHm in concentration cells housed in stirred Teflon-lined autoclaves (Palmer et al, 2001) and in a flowthrough design at temperatures below 300 °C (Sweeton et al, 1973). At temperatures above 300 °C the Pt(H2) indicator electrode was used in thermocells described in (Lvov et al., 1999, 2000b) and flow-through concentration cells (Sue et al., 2001). 

As stated previously, the reference electrode represents half of the complete system for potentio-metric measurements. The other half is the half at which the potential of analytical importance—the potential that is related to the concentration of the analyte—develops. There are a number of such indicator electrodes and analytical experiments that are of importance. 

The pH electrode consists of a closed-end glass tube that has a very thin fragile glass membrane 

The purpose of the silver-silver chloride combination is to prevent the potential that develops from changing due to possible changes in the interior of the electrode. The potential that develops is a membrane potential. Since the glass membrane at the tip is thin, a potential develops due to the fact that the chemical composition inside is different from the chemical composition outside. Specifically, it is the difference in the concentration of the hydrogen ions on opposite sides of the membrane that causes the potential—the membrane potential—to develop. There is no half-cell reaction involved. The Nernst equation is 

In addition, we can recognize that pH = -log [H+] and substitute this into the above equation  

The beauty of this electrode is that the measured potential (measured against a reference electrode) is thus directly proportional to the pH of the solution into which it is dipped. A specially designed voltmeter, called a pH meter, is used. A pH meter displays the pH directly, rather than the value of E. 

The guiding criterion for the choice of a working electrode is that it must be made of a redox-inert material, at least in the potential range of interest. 

In addition, one must choose the most appropriate geometrical form for such an electrode. The most common forms for fast voltammetric techniques are the planar geometry and the spherical (or hemispherical) geometry. In this regard, we have seen (Chapter 1, Section 4.2.2) that the simplest theoretical relationships describing the kinetics of electrode processes are valid under conditions of linear diffusion (even if we have briefly discussed also radial diffusion). 

The materials normally used in the construction of working electrodes are platinum, gold, mercury and carbon. However, there have been recent attempts to use more sophisticated materials such as superconductors (as will be discussed in Chapter 10, Section 1), but at moment, due to their poor chemical and mechanical properties, they are not very promising electrode materials. 

The accessible potential ranges for platinum and mercury electrodes, in the commonest organic solvents are reported in Table 1. It is noted that gold exhibits characteristics very similar to platinum. 

One can see immediately that the easy oxidation of mercury renders it of little use for anodic scans. Note that to construct a solid mercury electrode one can simply immerse a gold electrode in mercury for a few seconds. The amalgam that forms produces a mercury electrode much more manageable than the dropping electrode used in polarography. 

---

Experimentally, the aqueous iron(II) is titrated with cerium(IV) in aqueous solution in a burette. The arrangement is shown in Figure 4.6, the platinum indicator electrode changes its potential (with reference to a calomel half-cell as standard) as the solution is titrated. Figure 4.7 shows the graph of the cell e.m.f. against added cerium(IV). At the equivalence point the amount of the added Ce (aq) is equal to the original amount of Fe (aq) hence the amounts of Ce (aq) and Fe (aq) are also equal. Under these conditions the potential of the electrode in the mixture is ( - - f)/2 this, the equivalence point, occurs at the point indicated. 

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. 

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). 

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. 

The potential of the indicator electrode in a potentiometric electrochemical cell is proportional to the concentration of analyte. Two classes of indicator electrodes are used in potentiometry metallic electrodes, which are the subject of this section, and ion-selective electrodes, which are covered in the next section. 

If the copper electrode is the indicator electrode in a potentiometric electrochemical cell that also includes a saturated calomel reference electrode... 

Metallic indicator electrodes in which a metal is in contact with a solution containing its ion are called electrodes of the first kind. In general, for a metal M, in a solution of M"+, the cell potential is given as... 

If metallic electrodes were the only useful class of indicator electrodes, potentiometry would be of limited applicability. The discovery, in 1906, that a thin glass membrane develops a potential, called a membrane potential, when opposite sides of the membrane are in contact with solutions of different pH led to the eventual development of a whole new class of indicator electrodes called ion-selective electrodes (ISEs). following the discovery of the glass pH electrode, ion-selective electrodes have been developed for a wide range of ions. Membrane electrodes also have been developed that respond to the concentration of molecular analytes by using a chemical reaction to generate an ion that can be monitored with an ion-selective electrode. The development of new membrane electrodes continues to be an active area of research. 

Activity Versus Concentration In describing metallic and membrane indicator electrodes, the Nernst equation relates the measured cell potential to the concentration of analyte. In writing the Nernst equation, we often ignore an important detail—the... 

Faraday s law (p. 496) galvanostat (p. 464) glass electrode (p. 477) hanging mercury drop electrode (p. 509) hydrodynamic voltammetry (p. 513) indicator electrode (p. 462) ionophore (p. 482) ion-selective electrode (p. 475) liquid-based ion-selective electrode (p. 482) liquid junction potential (p. 470) mass transport (p. 511) mediator (p. 500) membrane potential (p. 475) migration (p. 512) nonfaradaic current (p. 512)... 

Substrate (compound to be determined) Enzyme (activity to be determined) Products (sensed species) Buffer and optimum PH Indicating electrode Range... 

Ey and E2 are the indicator electrodes. These may consist of a tungsten pair for a biamperometric end point for an amperometric end point they may both be of platinum foil or one can be platinum and the other a saturated calomel reference electrode. The voltage impressed upon the indicator electrodes is supplied by battery B (ca 1.5 volts) via a variable resistance Rs N records the indicator current. For a potentiometric end point Ey and E2 may consist of either platinum-tungsten bimetallic electrodes, or Ey may be an S.C.E. and E2... 

When applied on a macro scale — samples of 1 - 5 millimoles — generation rates of 100-500 milliamps are required parasitic currents may be induced in the indicator electrodes at currents in excess of about 10-20 mA consequently precise location of the equivalence point by amperometric methods is not trustworthy. 

If it is desired to use the biamperometric method for detecting the end point, then the calomel electrode and also the silver rod (if used) must be removed and replaced by two platinum plates 1.25 cm x 1.25 cm. The potentiometer (or pH meter) used to measure the e.m.f. must also be removed, and one of the indicator electrodes is then joined to a sensitive galvanometer fitted with a variable shunt. The indicator circuit is completed through a potential divider... 

Iodide. A 0.01 M solution of potassium iodide, prepared from the dry salt with boiled-out water, is suitable for practice in this determination. The experimental details are similar to those given for bromide, except that the indicator electrode consists of a silver rod immersed in the solution. The titration cell may be charged with 10.00 mL of the iodide solution, 30 mL of water, and 10 mL of the stock solution of perchloric acid + potassium nitrate. In the neighbourhood of the equivalence point it is necessary to allow at least 30-60 seconds to elapse before steady potentials are established. 

Apparatus. Use the apparatus of Section 14.7. The generator anode is of pure silver foil (3 cm x 3 cm) the cathode in the isolated compartment is a platinum foil (3 cm x 3 cm) bent into a half-cylinder. For the potentiometric end point detection, use a short length of silver wire as the indicator electrode the electrical connection to the saturated calomel reference electrode is made by means of an agar-potassium nitrate bridge. 

This procedure of using a single measurement of electrode potential to determine the concentration of an ionic species in solution is referred to as direct potentiometry. The electrode whose potential is dependent upon the concentration of the ion to be determined is termed the indicator electrode, and when, as in the case above, the ion to be determined is directly involved in the electrode reaction, we are said to be dealing with an electrode of the first kind . 

An element of uncertainty is introduced into the e.m.f. measurement by the liquid junction potential which is established at the interface between the two solutions, one pertaining to the reference electrode and the other to the indicator electrode. This liquid junction potential can be largely eliminated, however, if one solution contains a high concentration of potassium chloride or of ammonium nitrate, electrolytes in which the ionic conductivities of the cation and the anion have very similar values. 

One way of overcoming the liquid junction potential problem is to replace the reference electrode by an electrode composed of a solution containing the same cation as in the solution under test, but at a known concentration, together with a rod of the same metal as that used in the indicator electrode in other words we set up a concentration cell (Section 2.29). The activity of the metal ion in the solution under test is given by... 

As already stated, the indicator electrode of a cell is one whose potential is dependent upon the activity (and therefore the concentration) of a particular... 

Indicator electrodes for anions may take the form of a gas electrode (e.g. oxygen electrode for OH- chlorine electrode for Cl-), but in many instances consist of an appropriate electrode of the second kind thus as shown in Section 15.1, the potential of a silver-silver chloride electrode is governed by the chloride-ion activity of the solution. Selective-ion electrodes are also available for many anions. 

The indicator electrode employed in a potentiometric titration will, of course, be dependent upon the type of reaction which is under investigation. Thus, for an acid-base titration, the indicator electrode is usually a glass electrode (Section 15.6) for a precipitation titration (halide with silver nitrate, or silver with chloride) a silver electrode will be used, and for a redox titration [e.g. iron(II) with dichromate] a plain platinum wire is used as the redox electrode. 

Glass electrodes are now available as combination electrodes which contain the indicator electrode (a thin glass bulb) and a reference electrode (silver-silver chloride) combined in a single unit as depicted in Fig. 15.2(h). The thin glass bulb A and the narrow tube B to which it is attached are filled with hydrochloric acid and carry a silver-silver chloride electrode C. The wide tube D is fused to the lower end of tube B and contains saturated potassium chloride solution which is also saturated with silver chloride it carries a silver-silver chloride electrode E. The assembly is sealed with an insulating cap. 

To measure the e.m.f. the electrode system must be connected to a potentiometer or to an electronic voltmeter if the indicator electrode is a membrane electrode (e.g. a glass electrode), then a simple potentiometer is unsuitable and either a pH meter or a selective-ion meter must be employed the meter readings may give directly the varying pH (or pM) values as titration proceeds, or the meter may be used in the millivoltmeter mode, so that e.m.f. values are recorded. Used as a millivoltmeter, such meters can be used with almost any electrode assembly to record the results of many different types of potentiometric titrations, and in many cases the instruments have provision for connection to a recorder so that a continuous record of the titration results can be obtained, i.e. a titration curve is produced. 

A number of commercial titrators are available in which the electrical measuring unit is coupled to a chart recorder to produce directly a titration curve, and by linking the delivery of titrant from the burette to the movement of the recorder chart, an auto-titrator is produced. It is possible to stop the delivery of the titrant when the indicator electrode attains the potential corresponding to the equivalence point of the particular titration this is a feature of some importance when a number of repetitive titrations have to be performed. Many such instruments are controlled by a microprocessor so that the whole titration procedure is, to a large extent, automated. In addition to the normal titration curve, such instruments will also plot the first-derivative curve (AE/AV), the second-derivative curve (A2 E/AV2), and will provide a Gran s plot (Section 15.18). 

---