Reference systems, potentiometry

When voltammetry measurements are made in nonaqueous solvents, the problems of an adequate reference electrode are compounded. Until the 1960s the most common reference electrode was the mercury pool, because of its convenience rather than because of its reliability. With the advent of sophisticated electronic voltammetric instrumentation, more reliable reference electrodes have been possible, especially if a three-electrode system is used. Thus, variation of the potential of the counter electrode is not a problem if a second non-current-canying reference electrode is used to monitor the potential of the sensing electrode. If three-eleetrode instrumentation is used, any of the conventional reference electrodes common to potentiometry may be used satisfactorily. Our own preference is a silver chloride electrode connected to the sample solution by an appropriate noninterfering salt bridge. The one problem with this system is that it introduces a junction potential between the two solvent systems that may be quite large. However, such a reference system is reproducible and should ensure that two groups of workers can obtain the same results. 

Both direct potentiometry and the potentiometric titration method (see next Section) require the measurement of emf between an indicator electrode system and a reference electrode system, the two comprising a cell system. Despite the fact that potentials are usually referred to the standard hydrogen electrode (SHE) for tabulation purposes, they can be expressed relative to any acceptable reference system. In practice, the common reference electrodes are saturated KCl calomel (SCE), 3M KCl calomel or 3M KCl Ag/AgCl. In many present-day applications, the indicator and reference electrodes are combined in a single electrode system called a "combination electrode". 

A schematic diagram of a typical pH electrode system is shown in Fig. 10.1. The cell potential, i.e. the electromotive force, is measured between a pH electrode and a reference electrode in a test solution. The pH electrode responds to the activity or concentration of hydrogen ions in the solution. The reference electrode has a very stable half-cell potential. The most commonly used reference electrodes for potentiometry are the silver/silver chloride electrodes (Ag/AgCl) and the saturated calomel electrodes (SCE). 

Equation 2.16 shows that potentiometry is a valuable method for the determination of equilibrium constants, ffowever, it should be borne in mind that the system should be in equilibrium. Some other conditions, which are described below, also need to be fulhlled for use of potentiometry in any application. The basic measurement system must include an indicator electrode that is capable of monitoring the activity of the species of interest, and a reference electrode that gives a constant, known half-cell potential to which the measured indicator electrode potential can be referred. The voltage resulting from the combination of these two electrodes must be measured in a manner that minimises the amount of current drawn by the measuring system. This condition includes that the impedance of the measuring device should be much higher than that of the electrode. 

Potentiometry has found extensive application over the past half-century as a means to evaluate various thermodynamic parameters. Although this is not the major application of the technique today, it still provides one of the most convenient and reliable approaches to the evaluation of thermodynamic quantities. In particular, the activity coefficients of electroactive species can be evaluated directly through the use of the Nemst equation (for species that give a reversible electrochemical response). Thus, if an electrochemical system is used without a junction potential and with a reference electrode that has a well-established potential, then potentiometric measurement of the constituent species at a known concentration provides a direct measure of its activity. This provides a direct means for evaluation of the activity coefficient (assuming that the standard potential is known accurately for the constituent half-reaction). If the standard half-reaction potential is not available, it must be evaluated under conditions where the activity coefficient can be determined by the Debye-Hiickel equation. 

Measurements can be done using the technique of redox potentiometry. In experiments of this type, mitochondria are incubated anaerobically in the presence of a reference electrode [for example, a hydrogen electrode (Chap. 10)] and a platinum electrode and with secondary redox mediators. These mediators form redox pairs with Ea values intermediate between the reference electrode and the electron-transport-chain component of interest they permit rapid equilibration of electrons between the electrode and the electron-transport-chain component. The experimental system is allowed to reach equilibrium at a particular E value. This value can then be changed by addition of a reducing agent (such as reduced ascorbate or NADH), and the relationship between E and the levels of oxidized and reduced electron-transport-chain components is measured. The 0 values can then be calculated using the Nernst equation (Chap. 10) ... 

Open-circuit potential (OCP) — This is the - potential of the - working electrode relative to the - reference electrode when no potential or - current is being applied to the - cell [i]. In case of a reversible electrode system (- reversibility) the OCP is also referred to as the - equilibrium potential. Otherwise it is called the - rest potential, or the - corrosion potential, depending on the studied system. The OCP is measured using high-input - impedance voltmeters, or potentiometers, as in - potentiometry. OCP s of - electrodes of the first, the second, and the third kind, of - redox electrodes and of - ion-selective membrane electrodes are defined by the - Nernst equation. The - corrosion po-... 

Potentiometry with ISEs is the electroanalytical technique most frequently used with continuous segmented configurations. Figure 5.15 shows an assembly designed for the simultaneous determination of sodium and potassium In animal urine [33], The system Is in fact composed of two distinct units for the determination of each analyte. The sample is aspirated and split prior to the peristaltic pump into two channels, which are subsequently mixed with an appropriate buffer (Tris/acetlc acid of pH 8.15 for sodium and diethylamine/acetlc acid for potassium) and de-aerated before they reached their respective selective electrodes. The reference electrode Is an ordinary one and Is connected to both streams, Into which are Introduced two platinum wires connected to the... 

Figure 15.6 An electrochemical measuring system for potentiometry. The indicator (sensing, working) electrode responds to the activity of the analyte of interest. The potential difference developed between the reference electrode and the indicator electrode is read out on the potentiometer (voltmeter). [Courtesy of Thermo Orion, Beverly, MA (www.thermoorion.com).]...

The important techniques for measuring the oscillations in the chemical reaction are the potentiometry. The advantage of this technique is that by using a bromide ion-sensitive electrode, the composition of bromide ions in the reaction system can be monitored easily. However, platinum electrode is susceptible to changes in the oxidation state of the metal-ion catalyst. The measured electrode potential can be used to monitor oscillations in [Br ] and [Mox]/[Mjed] with the help of a suitable reference electrode. 

For the study of the solvent effect, comparable equilibrium constants have to be determined in water and in solution made with non-aqueous solvents or solvent mixtures. Potentiometric (usually pH-metric) equilibrium measurements are used for this purpose in polyfunctional systems. The solvent effect makes the application of potentiometry somewhat difficult. The substitution of water by organic solvents results in changes of the autoprotolysis constant of the solvent changing the pH scale. The lower relative permittivity of the system favours association processes which have to be considered, e.g., in the determination of the ionic strength of the solution. Diffusion potentials at the liquid junctions connecting the galvanic cell with the reference electrode may falsify the measured data. 

Potentiometry is a powerful electrochemical method when the OCP is measured using a high-resistance electrometer. This method is a common tool for studying equilibrium systems in electrochemical science and engineering. In noneqnilibrium stndies, OCP measurement is nsnally the first step to find out if the cell of interest works properly. Note that dnring the potentiometric measurements, due to the absence of cnrrent in the circuit, any undesirable potential drops inside the phases are absent. In noneqnilibrinm electrochemical systan, the potential drop between working and reference electrodes (called IR drop) is a serious issue to be taken into account experimentally and/or theoretically. 

Indeed, while potentiometry refers to a static system, in which no reaction takes place, the current flow in controlled potential techniques is the cause of occurrence of a redox reactiOTi the electrode potential is imposed by an external source and eventually changes with time. Considering what happens at WE, reversibility... 

Potentiometry measures the difference in potential between two electrodes immersed in a solution. One of the electrodes probes the solution, while the other serves as a reference. The reference electrode has a constant and reproducible potential which is independent of its environmenL The potential of the probe electrode is the potential at the interface between the solid and liquid phases, where the oxidation and reduction reactions occur. For example, at the interface between a conducting wire and a redox system, there is an exchange of electrons between the wire and the compounds being oxidized and reduced. Equilibrium is achieved when the rates of oxidation and reduction are equal, and the composition of the solution surrounding the electrode is constant. The equilibrium potential is then given by the Nemst Law ... 

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