Electrodes system

The indicator electrodes for potentiometric measurements traditionally have been categorized into three separate classes. First-class electrodes consist of a metal immersed in a solution that contains the metal ion. These electrode systems provide a direct response to the ion or species to be measured  

Therefore, the primary electrode reaction includes the sensed species. Such electrodes give a direct response according to the Nemst equation for the logarithm of the activity of the species. The details of electrode fabrication and their characteristics are discussed in Chapter 5. An extensive authoritative treatment is provided by Ives and Janz.19 

Electrodes classified as second-class electrode systems are those in which the electrode is in direct contact with a slightly soluble salt of the electroactive species such that the potentiometric response is indicative of the concentration of the inactive anion species. Thus the silver/silver-chloride electrode system, which is representative of this class of electrodes, gives a potential response that is directly related to the logarithm of the chloride ion activity 

Because any potentiometric electrode system ultimately must have a redox couple (or an ion-exchange process in the case of membrane electrodes) for a meaningful response, the most common form of potentiometric electrode systems involves oxidation-reduction processes. Hence, to monitor the activity of ferric ion [iron(III)], an excess of ferrous iron [iron(II)] is added such that the concentration of this species remains constant to give a direct Nemstian response for the activity of iron(III). For such redox couples the most common electrode system has been the platinum electrode. This tradition has come about primarily because of the historic belief that the platinum electrode is totally inert and involves only the pure metal as a surface. However, during the past decade it has become evident that platinum electrodes are not as inert as long believed and that their potentiometric response is frequently dependent on the history of the surface and the extent of its activation. The evidence is convincing that platinum electrodes, and in all probability all metal electrodes, are covered with an oxide film that changes its characteristics with time. Nonetheless, the platinum electrode continues to enjoy wide popularity as an inert indicator of redox reactions and of the activities of the ions involved in such reactions. 

For many systems the gold electrode is as satisfactory as the platinum electrode. Both rhodium and palladium as well as carbon have been used for specialized systems as inert potentiometric electrodes. 

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Since it is not possible to measure a single electrode potential, one electrode system must be taken as a standard and all others measured relative to it. By international agreement the hydrogen electrode has been chosen as the reference ... 

Fig. 6. Electrode system for detecting high and low levels ia a conductive field.

Fig. 3. The three-electrode system. Terms are defined in text.

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. 

According to the definition, a passive technique is one for which no appHed signal is required to measure a response that is analytically usehil. Only the potential (the equiHbrium potential) corresponding to zero current is measured. Because no current flows, the auxiHary electrode is no longer needed. The two-electrode system, where the working electrode may or not be an ion-selective electrode, suffices. 

Another hmitation to be considered is the volume that the DEP force can affec t. This factor can be controlled by the design of electrodes. As an example, consider elec trodes of cylindrical geometry. A practical example of this would be a cylinder with a wire running down the middle to provide the two electrodes. The field in such a system is proportional to 1/r. The DEP force is then Fdep VlE I =< 1/r, so that any differences in particle polarization might well be masked merely by positional differences in the force. At the outer cyhnder the DEP force may even be too small to affect the particles appreciably. The most desirable electrode shape is one in which the force is independent of position within the nonuniform field. This fisomotive electrode system is shown in Fig. 22-33. 

Commercial instruments have either two or three electrodes. Also, there are different types of three-electrode systems. The apphcation and limitations of the instruments are largely dependent upon these elec trode systems. 

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. 

Ideally, one would prefer to compare anodic and cathodic potential limits instead of the overall ionic liquid electrochemical window, because difference sets of anodic and cathodic limits can give rise to the same value of electrochemical window (see Figure 3.6-1). However, the lack of a standard reference electrode system within and between ionic liquid systems precludes this possibility. Gonsequently, significant care must be taken when evaluating the impact of changes in the cation or anion on the overall ionic liquid electrochemical window. 

The earth electrode system must be designed to be capable of carrying without damage to the full earth fault current of the supply system. 

Potentiometric titration using a bright platinum-saturated calomel electrode system this can be used when the reaction involves two different oxidation states of a given metal. 

Pipette 25 mL of solution B into a 100 mL beaker mounted on a magnetic stirrer and add an equal volume of TISAB from a pipette. Stir the solution to ensure thorough mixing, stop the stirrer, insert the fluoride ion-calomel electrode system and measure the e.m.f. The electrode rapidly comes to equilibrium, and a stable e.m.f. reading is obtained immediately. Wash down the electrodes and then insert into a second beaker containing a solution prepared from 25 mL each of standard solution C and TISAB read the e.m.f. Carry out further determinations using the standards D and E. 

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. 

Titration assembly. The electrode system consists of a mercury electrode and a saturated calomel [or, in some cases, a mercury-mercury(I) sulphate] reference electrode, both supported in a 250 mL Pyrex beaker. Provision is made for magnetic stirring and the potential is followed by means of an electronic millivoltmeter or an auto-titrator. 

The use of a catalyst with oxidase enzyme is an example of the use of a combined enzyme system, which illustrates the wide potential offered by multi-enzyme electrode systems. Various enzymes can be arranged to work sequentially to transform quite complex substances and eventually produce a measurable concentration-dependent change, which is detected by the output signal and recorded for analysis. 

The basic instrumentation required for controlled-potential experiments is relatively inexpensive and readily available commercially. The basic necessities include a cell (with a three-electrode system), a voltammetric analyzer (consisting of a potentiostatic circuitry and a voltage ramp generator), and an X-Y-t recorder (or plotter). Modem voltammetric analyzers are versatile enough to perform many modes of operation. Depending upon the specific experiment, other components may be required. For example, a faradaic cage is desired for work with ultramicroelectrodes. The system should be located in a room free from major electrical interferences, vibrations, and drastic fluctuations in temperature. 

Chemically modified electrodes (CMEs) represent a modem approach to electrode systems. These rely on the placement of a reagent onto the surface, to impart the behavior of that reagent to the modified surface. Such deliberate alteration of electrode surfaces can thus meet the needs of many electroanalytical problems, and may form the basis for new analytical applications and different sensing devices. 

Chloride can be determined in AOS by potentiometric titration of a sample with silver nitrate after acidification with nitric acid. A silver/glass electrode system is used. 

Electrode System Etaa/V VS. SHE Atomic density/cm-2 Method References... 

Solvent0 Electrode System Eozi/V vs. aq. SHE vs. BBCr f Z Atomic density/cm 2 References... 

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