Electrode iridium

The mechanism of the oxygen reduction reaction is by no means as fully understood as the h.e.r., and a major experimental difficulty is that in acid solutions (pH = 0) E02/H20 = 1 23, which means that oxygen will start to be reduced at potentials at which most metals anodically dissolve. For this reason accurate data on kinetics is available only for the platinum metals. In the case of an iridium electrode at which oxygen reduction is relatively rapid, a number of reaction sequences have been proposed, of which the most acceptable appear to be the following ... 

The kinetics of the electrochemical oxidation of ammonia on platinum to dinitrogen in basic electrolytes has been extensively studied. In the widely supported mechanism originally suggested by Gerischer and Mauerer[ll], the active intermediate in the selective oxidation to N2 is a partly dehydrogenated ammonia adsorbate, NH2 ads or NHaatomic nitrogen adsorbate N ag, which is apparently formed at more positive potentials, is inactive toward N2 production at room temperature. Generally, only platinum and iridium electrodes exhibit steady-state N2 production at potentials at which no sur-... 

Moissan finally used as electrolyte a solution of dry potassium acid fluoride in anhydrous hydrofluoric acid. His apparatus consisted of two platinum-iridium electrodes sealed into a platinum U-tube closed with fluorspar screw caps covered with a layer of gum lac (42, 49, 59). The U-tube was chilled with methyl chloride, the gas now used in many modem refrigerators, to a temperature of —23°. 

The figure below shows a series of standard additions of Cu2+ to acidified tap water measured by anodic stripping voltammetry at an iridium electrode. The unknown and all standard additions were made up to the same final volume. 

Zhang Y, Wilson GS. Electrochemical oxidation of hydrogen peroxide on platinum and platinum/iridium electrodes in physiological buffer and its applicability to hydrogen peroxide-based biosensors. Journal of Electroanalytical Chemistry 1993, 345, 253-271. 

Ferrer, J. E. and Victori, L. (1994), Oxygen evolution reaction on the iridium electrode in basic medium studied by electrochemical impedance spectroscopy. Electrochim. Acta, 39(4) 581-588. 

Colloidal iridium may also be prepared by Bredig s method, which consists in sparking between iridium electrodes immersed in ice-cooled water. A current of 20 to 25 amperes at 220 volts gives satisfactory results.2 The colour of the hydrosol ranges from red to black, according to the method of preparation. A small current favours the formation of the black solution. 

F. Colom and A. De la Plaza, Oxide Film Formation on Iridium Electrodes in KCl-LiCl Melts Containing Alkaline Oxide, (Proceedings of the 27th Meeting of International Society of Electrochemistry, Zurich, 1978) Ext. Abstr., S.l, s.a. N161. 

Alternatively, air is passed through 1-pm PTFE membrane and 1% sodium bisulfite solution. The solution is treated with chromotropic and sulfuric acid mixture. The color development due to formaldehyde is measured by a visible spectrophotometer at 580-mn absorbance (NIOSH 1984, Method 3500). In polarography analysis, a Girard-T reagent is used. Formaldehyde forms a derivative that is analyzed by sampled direct current DC polarography (NIOSH 1984, Method 3501). Auel et al. (1987) reported a similar electrochemical analysis of industrial air using an iridium electrode backed by a gas-permeable fluorocarbon-based membrane. 

Figure 8. Cyclic voltammograms recorded for a hydrous-oxide-coated iridium electrode (40 mV s 1) in...

Bothwell, M. E., Soriaga, M. P. (1990). Reversible redox chemistry, hydrodesulfurization, and anodic oxidation of thiophenols chemisorbed at smooth polycrystalline iridium electrodes. Journal of Electroanalytical Chemistry, 295, 123—138. http //dx.doi.0rg/lO.lOl6/ 0022-0728(90)85010-3. 

The most recent version of the cell (Ho et al., 1994) consists of two coaxial 75% platinum-25% iridium electrodes coated with platinum black. As can be seen in Figure 4.1, one electrode is external platinum-iridium lined high-pressure vessel (6.35 cm long and 1 cm volume) and the other, a thin platinum wire, insulated by a non-porous sintered AI2O3 or Al203/Zr02 tube (for alkaline media), welded to a platinum-iridium cylinder located in the center. 

Figure 3.1. Electrochemical behaviors of various noble metal electrodes in sulfuric acid [23-26]. (Graphs (a) (c) reprinted from Journal of Electroanatytical Chemistry, 35, Rand DAJ, Woods R, A study of the dissolution of platinum, palladium, rhodium and gold electrodes in 1 M sulphuric acid by cyclic voltammetry, 209-18, 1972 (b) reprinted from J Electroanal Chem, 55(3), Rand DAJ, Woods R, Cyclic voltammetric studies on iridium electrodes in sulphuric acid solutions nature of oxygen layer and metal dissolution, 375-81, 1974 (d) reprinted from J Electroanal Chem, 89(1), Michell D, Rand DAJ, Woods R, A study of ruthenium electrodes by cyclic voltammetry and X-ray emission spectroscopy, 11-27, 1978 (e) reprinted from J Electroanal Chem, 39(2), Capon A, Parsons R, The effect of strong acid on the reactions of hydrogen and oxygen on the noble metals. A study using cyclic voltammetry and a new teflon electrode holder, 275-86, 1972, all with permission from Elsevier.)...

Rand DAJ, Woods R. Cyclic voltammetric studies on iridium electrodes in sulphuric acid solutions Nature of oxygen layer and metal dissolution. J Electroanal Chem... 

The electrocatalytic oxidation of D-glucose and related polyols on large platinum and gold electrodes was discussed in a symposium report, and the oxidation of glucose and polyols on nickel electrodes and on platinum, rhodium, and iridium electrodes under alkaline... 

In 1886 Ferdinand Frederic Henri Moissan, a student of Fremy, succeeded where his mentor had failed. (Moissan also had his work interrupted a number of times while he recovered from HF and F2 poisoning.) He finally isolated this furiously reactive element by electrolyzing a mixture of anhydrous hydrofluoric acid and potassium fluoride with platinum-iridium electrodes in a platinum vessel. He chilled the apparatus to reduce the activity of the resulting pale-yellow gas. Equation (18.4) summarizes his procedure, which was the only method of preparing fluorine for a century and remains the principal method even today ... 

Frazer EJ, Woods R (1979) The oxygen evolution reaction on cycled iridium electrodes. J Electroanal Chem 102 127-130... 

Mozota J, Conway BE (1983) Surface and bulk processes at oxidized iridium electrodes-1 monolayer stage and transition to reversible multilayer oxide film behavior. Electrochimica Acta 28 1-8... 

Fig. 8 Cyclic voltammogram for 4-mm iridium electrode in saline. Current increase is observed with prolonged voltage cycling...

Therefore, an electrode with a rougher surface would exhibit a larger surface area, and thus a higher capacitance. As seen in Fig. 9, three similarly sized electrodes made of titanium, platinum, and iridium exhibit similar impedance values when they are initially submerged. However, after the iridium electrode is subjected to repeated voltage cycles at the full voltage range of the water window (as in cyclic voltammetry), its impedance at lower frequencies decreases by up to 10 fold [17]. 

F. 9 Impedance spectroscopy of titanium, platinum, and iridium electrodes all are around 2.5 mm in size. Smaller electrodes are used here so the electrode impedances are above that for connection wires... 

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