Rotating voltammetric electrode

Smart and Freese [9] analyzed the chlorine dioxide content of different water samples with a rotating voltammetric electrode. In their measurements, the electrode potential was kept at +0.5 V versus an Ag/AgCl electrode and a rotation rate of 400 rpm was employed. Using the amperometric current, as analytical signal lower limit of determination less than 1 mg/dm could be achieved. 

Smart RB, Freese JW. 1982. Measuring chlorine dioxide with a rotating voltammetric membrane electrode. J Am Water Works Assoc 74(10) 530-531. 

The homogeneous catalysis method is suitable to measure rate constants over a very wide range, up to the diffusion limit. The lower limit is determined by interferences, such as convection, which occur at very slow scan rates. It is our experience that, unless special precautions are taken, scan rates below lOOmV/s result in significant deviations from a purely diffusion-controlled voltammetric wave. For small values of rate constants (down to 10 s ), other potentiostatic techniques are best suited, such as chronoamperometry at a rotating disk electrode UV dip probe and stopped-flow UV-vis techniques. ... 

Figure 27.15 Residual voltammetric response (i - E) for an Au rotated-disk electrode in 0.1 M NaOH. [Reproduced with permission from Ref. 31.]...

Finally, the pFI dependency of the current signals was investigated. Voltammetric curves were recorded obtained at a platinum rotating-disc electrode for different pFI values in the 11.65-12.95 range at constant electrode-rotation rate. These experiments were repeated at other sodium dithionite concentrations. It was found that the measured current in all three regions of the voltammetric waves did not vary with pH. 

The first voltammetric methods met are stationary voltammetries performed on a dropping mercury electrode (polarography) or on a solid rotating disk electrode. The limiting current measured is directly proportional to the concentration of the electroactive species in the solution. Experimental potential scan rate is lower than lOrnVs-1. 

The voltammetric oxidation of HOOH at a rotated-disk electrode yields a peaked anodic wave with a half-wave potential (Em) of + 2.1 V versus SCE. The maximum current (ilim) for HOOH oxidation at the rotated-disk electrode is directly proportional to the concentration of HOOH, and is consistent with a first-order diffusion-controlled process. 

Section 2 (Fig. 1, curves A and B), usually performed at the rotating platinum electrode (anode reactions) or the dropping mercury electrode (cathode reactions), should ideally suffice to define the electroactive species and determine its half-wave potential. It may be that systems in which acid-base equilibria exist are somewhat more laborious to study due to the necessity of recording voltammetric curves over a wide pH range, but in most cases the task can be accomplished with some effort. Once the voltammetric characteristics are known, it remains to carry out preparative constant potential electrolysis (cpe) at a suitable potential in order to make sure that the electroactive species is connected with the reaction of interest. 

Finite diffusion — Finite (sometimes also called -> limited) diffusion situation arises when the -> diffusion layer, which otherwise might be expanded infinitely at long-term electrolysis, is restricted to a given distance, e.g., in the case of extensive stirring (- rotating disc electrode). It is the case at a thin film, in a thin layer cell, and a thin cell sandwiched with an anode and a cathode. Finite diffusion causes a decrease of the current to zero at long times in the - Cottrell plot (-> Cottrell equation, and - chronoamperometry) or for voltammetric waves (see also - electrochemical impedance spectroscopy). Finite diffusion generally occurs at -> hydrodynamic electrodes. 

The polarographic behavior of the 1-oxides and 1,4-dioxides of pyrazine, 2,5-dimethylpyrazine, and tetramethylpyrazine at various pH values has been investigated. It was assumed that at lower pH values, the A -oxide group was reduced in its protonated form. In acid media the 1-oxides exhibited double waves, the first of which is attributable to the reduction of jV-oxide groups and the second to that of the pyrazine nucleus (production of 1,4-dihydro compounds). Reduction of both A -oxide groups of pyrazine-1,4-dioxide proceeded simultaneously (588). Half-wave potentials of the voltammetric oxidation and reduction of pyrazine mono- and di-A -oxides have been measured in dimethylformamide, and in acetonitrile by the technique of a rotating platinum electrode (750). 

It has been shown voltammetrically that oxidation of 1,4-dioxin and of its 2,3,5,6-tetraphenyl, benzo, and dibenzo derivatives takes place at a rotating platinum electrode in acetonitrile in single-electron steps with the intermediate formation of radical cations.The simplest dioxin cation-radical for which an ESR spectrum has been obtained is 79, for which hyperfine splittings are indicated in gauss.The radical was prepared by treatment with sulfuric acid of the dimer which forms spontaneously on the storage of acetoin. 

Curve (b) of Figure 4 shows the same silent system as curve (a) but now upon a contracted current scale, while curve (c) shows the effect of ultrasonic irradiation upon curve (b), scanned at the same rate and in the oxidation direction only. Note that curves (b) and (c) are on the same current scale, both taken from ref. 31. Ultrasound has produced a 10-fold increase in maximum current. The plateau shape shows a limiting current at the extreme of oxidation potential reflecting hydrodynamic control independent of the voltammetric sweep rate. (This shape is also seen in other voltammetric procedures, e.g. when using rotating disk electrodes or microelectrodes.) In Figure 4 curve (c) this limiting current is found to be inde-... 

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