Voltammetry indicator electrodes

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)... 

In linear sweep voltammetry, a rapidly changing ramp potential is applied to the indicator electrode. The current increases to a maximum... 

The so-called indicator electrodes must be considered as microelectrodes, which means that the active surface area is very small compared with the volume of the analyte solution as a consequence, the electrode processes cannot perceptibly alter the analyte concentration during analysis in either non-faradaic potentiometry or faradaic voltammetry. 

Before mentioning some more literature data on non-aqueous voltammetry, we suggest on the basis of our previous discussions that the choice of the experimental conditions used in the techniques must be a compromise between a sufficient solubility of the analyte in the solution, an ample redox potential range of the solvent, a suitable type of indicator electrode and adequate conductance of the solution with supporting electrolyte added. In this connection Fig. 4.20 may be a useful guide. 

The step 2 product was evaluated in a 100-mM acetonitrile solution of tetrabutylam-monium hexalluorophosphate at scan rates of 25, 50, 100, 200, and 400 mV/s. The peak current for the reductive process was found to scale linearly with the scan rate suggesting that the step 2 product had adhered to the surface of the electrode. Cyclic voltammetry indicated a highest occupied molecular orbital (HOMO) of 4.7 eV with an electrochemical band gap of 1.65 eV. Differential pulse voltammetry gave rise to a HOMO of 4.67 eV. 

Polarography and Voltammetry Both methods are the same in that current-potential curves are measured. According to the IUPAC recommendation, the tenn polarography is used when the indicator electrode is a liquid electrode whose surface is periodically or continuously renewed, like a dropping or streaming mercury electrode. When the indicator electrode is some other electrode, the term voltammetry is used. However, there is some confusion in the use of these terms. 

Indicator Electrode and Working Electrode In polarography and voltammetry, both terms are used for the microelectrode at which the process under study occurs. 

Voltammetry is a term used to include all the methods that measure current-potential curves (voltammograms) at small indicator electrodes other than the DME [6], There are various types of voltammetric indicator electrodes, but disk electrodes, as in Fig. 5.17, are popular. The materials used for disk electrodes are platinum, gold, graphite, glassy carbon (GC), boron-doped diamond8, carbon paste, etc. and they can be modified in various ways. For electrode materials other than mercury, the potential windows are much wider on the positive side than for mercury. However, electrodes of stationary mercury-drop, mercury-film, and mercury-pool are also... 

Positive feedback iR compensation in three-electrode measurements As described in Section 5.3, the influence of iR-drop is serious in two-electrode polarography or voltammetry. The influence is eliminated considerably with three-electrode instruments, if the tip of the reference electrode is placed near the surface of the indicator electrode. However, there still remains some iR-drop, which occurs by the residual resistance at... 

The characteristics of redox reactions in non-aqueous solutions were discussed in Chapter 4. Potentiometry is a powerful tool for studying redox reactions, although polarography and voltammetry are more popular. The indicator electrode is a platinum wire or other inert electrode. We can accurately determine the standard potential of a redox couple by measuring the electrode potential in the solution containing both the reduced and the oxidized forms of known concentrations. Poten-tiometric redox titrations are also useful to elucidate redox reaction mechanisms and to obtain standard redox potentials. In some solvents, the measurable potential range is much wider than in aqueous solutions and various redox reactions that are impossible in aqueous solutions are possible. 

For other techniques other kinds of data are given Instead. For chronopotentiometry these Include the area of the indicator electrode and the current or current density for stationary-electrode voltammetry they include the area of the indicator electrode and the scan rate for cyclic voltammetry they include the starting and reversal potentials and the area of the indicator electrode, and so on. The list of abbreviations must always be consulted regarding the units of the quantities given In this column. In the space that was available for these purposes it is quite Impossible to give a full description of the experimental conditions, but art attempt has been made to give an accurate Idea of their nature. 

Another specialized form of voltammetry involves the use of either a rotated-disk or a ring-disk indicating electrode. With this type of electrode the current is directly proportional to the square root of the rate of rotation if it is a diffusion-controlled process. To obtain complete adherence to the square-root relationship, a hydrodynamically sound design for the electrode is essential.43 Figure 3.12 illustrates the geometric features that have been found to give reliable performance for rotation rates as high as 10,000 rpm. 

The goal of this volume is to provide (1) an introduction to the basic principles of electrochemistry (Chapter 1), potentiometry (Chapter 2), voltammetry (Chapter 3), and electrochemical titrations (Chapter 4) (2) a practical, up-to-date summary of indicator electrodes (Chapter 5), electrochemical cells and instrumentation (Chapter 6), and solvents and electrolytes (Chapter 7) and (3) illustrative examples of molecular characterization (via electrochemical measurements) of hydronium ion, Br0nsted acids, and H2 (Chapter 8) dioxygen species (02, OJ/HOO-, HOOH) and H20/H0 (Chapter 9) metals, metal compounds, and metal complexes (Chapter 10) nonmetals (Chapter 11) carbon compounds (Chapter 12) and organometallic compounds and metallopor-phyrins (Chapter 13). The later chapters contain specific characterizations of representative molecules within a class, which we hope will reduce the barriers of unfamiliarity and encourage the reader to make use of electrochemistry for related chemical systems. 

Figure 1. Principal circuit of high temperature cell for voltammetry investigations under excess gas pressure 1 - high-temperature stainless steel box 2 -quartz box 3 - crucible and country electrode 4 — indicated electrode 5 - reference electrode 6 - thermocouple 7 - Pt lead wire for crucible 8 - water cooling for cell cover 9 -valve of pressure release in cell 10 - hose coupling 11 - gas control valves 12 - intermediate gas container (filling volume -2 liters) 13 - gauge-pressure manometer.

Peak potential — is a term used in -> voltammetry for the potential of the working (indicator) electrode at which the peak current is attained. In the cyclic voltammogram of a reversible redox couple, anodic and cathodic peak p. are separated by 2.2RT/nF, which is considered as a diagnostic feature and gives a possibility to determine the formal p. as a mid-peak p. For irreversible couples, quantitative relations exist, interconnecting peak p. and the rate constant of the -> electron transfer. 

Electrochemical analytical techniques are some of the oldest in chemistry and can be divided into potentiometry, voltammetry and conductimetry. They are most important as detectors after chromatographic separations and as chemical and biological sensors. They generally involve the use of electrodes that are housed in electrochemical cells. All electrochemical cells contain two electrodes but some have three. The first electrode is the actual working electrode (also called a sensing or indicator electrode) and the second is a combined reference electrode and auxiliary (counter) electrode. If there are three electrodes, the reference and counter electrodes are separate. 

Budyina and Marinin [130] have described methods based on anodic voltammetry for the determination of lonol (2,6-di- -butyl-p-cresol) and quinol in polyester acrylates. To determine lonol the sample is dissolved in 25 ml of acetone and a portion (10 ml) is treated with 2.5 ml of acetone and 5 ml of methanol and diluted to 25 ml with a solution 0.1 M in lithium chloride and 0.02 M in sodium tetraborate. A polarogram is recorded with a graphite-rod indicator electrode. To determine quinol, the sample (1 to 3 g) is dissolved in 80 ml of methanol or methanohacetone (1 1) and the solution is diluted to 100 ml with the lithium chloride - sodium tetraborate solution. A polarogram is recorded under the same conditions. Concentrations are determined by the addition method. The values versus the SCE) are 0.25 V for lonol and 0.16 V for quinol. 

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