Ring-disk

Figure 59.6 Typical laminated disk-ring couplings...

Laminated disk-ring couplings (material-flexing) Bellows couplings (material-flexing)... 

The UMEs used in bioarrays can be divided into three types disk, ring, and strip electrodes. The theory of the disk, ring, and strip UMEs has been extensively studied [97-100], Due to the edge effect, the profile of the mass diffusion to the ultramicroelectrode surface is three dimensional, and can significantly enhance the mass transportation in comparison to the conventional large electrode with one-dimensional mass transportation. The steady-state measurement at a planar UME can be expressed as... 

Recently, Mottola [98] reported a sensor based on the disk-ring principle previously developed by Kamin and Wilson [99], and Wang and Lin [100]. Unlike Mottola s design, its forerunners involved no stationary ring electrode or rotation of the reactor part in addition, their reactor/electrode was located at the cell bottom. In Mottola s assembly, a product of an enzyme-catalysed reaction at a bioreactor rotated at a constant speed was hydrodynamically transported to a stationary ring electrode, where it was electrochemically monitored. The sample was transported to the detection imit by an tm-... 

Figure 5 Current versus potential for rotating disk-ring...

In a plot of /disk//ring against co 1/2,7disk//ring will have the ratio 1 /A, and the plot will be a straight line parallel to the of1/2 axis. 

Figure 7.104 shows the plot of the value of J vs. S obtained for different potentials on the bare iron region from the data plotted in Fig. 7.103. A straight line obtained with an intercept much greater than 1 indicates that Oz reduction on reduced iron proceeds by the direct four-electron reaction pathway. Formation of HjOj as an intermediate in the consecutive reaction pathway is less than 1 % of the total reduction current Conversely, in the potential region corresponding to passive iron, the slope, S, of the /disk//ring plot is zero, and the intercept J = (l/N) indicates that 02 reduction on passive Fe is a two-electron process in which is the product, and not an intermediate, of the reaction.

As volatile levels drop further, yielding very concentrated polymer solutions, the viscosity increases to a level that requires rotary equipment for forward pumping of the solution, which imparts surface renewal and often entraps vapor bubbles, for improved mass and heat transfer as well. There is a wide variety of rotary equipment available, from advanced ribbon devolatilizers, vertical-cone devolatilizers, and disk-ring devolatilizers for moderately viscous solutions, to single and twin screw devolatilizers and thin-film... 

Figure 41. (A) RRDE experiment recorded on an iron phthalocyanine catalyst deposited on a carbon disk. Ring potential 1.2 V vs. RHE. Scan rate 5 mV s. (B) Variation of the percentage of water, X(H20) and hydrogen peroxide, XCHiOi) in the product of reaction. (C) Total number of electrons exchanged for the reduction of one molecule of oxygen.

Figure 15. Rotating disk-ring electrode measurements for O2 reduction on Ru(1010) in 0.1 M HCIO4. Sweep rate 20 mV s ring potential E = 1.2 V disc area 0.282 cm rotation rates are indicated in the graph. The insert shows the logy Kjd-j) vs. E plot obtained for 1600 rpm. Reprinted from Copyright (2002) with permission from Elsevier.

All rate constants for this complex reaction have been determined from these measurements [163, 165]. H and Bi adlayers cause similar catalytic effects. Tafel plots obtained from rotating disk-ring measurements show slopes of —120 mV for Au/Pb and Au/Bi systems, while a slope of —55 mV vras found for Au/Tl. The —120 mV slope, as observed for bare... 

Due to the defined hydrodynamic conditions, the diffusion-convection process near the disk/ring electrode surface can be calculated based on the experiment conditions and the measured data, providing the information of the electrode kinetics as a function of the applied potential, which has been described in Chapter 5. 

Schone and Wiesbeck [684] proposed using two working electrodes (disk-ring) with electronic compensation of the solution resistance and frequency analyzer without a potentiostat. The potentiostat was used only for slow dc polarization of the working electrodes. 

Much of the progress in the investigation of possible parallel-consecutive courses of the oxygen reduction reaction is due to the application of the disk-ring method proposed by Frumkin and Nekrasov. The hydrodynamic theory of this method has been reviewed by various authors. Mueller... 

Later, after substantial rotating disk-ring data were available for O2 reduction on various electrode materials, Bagotskii et proposed a... 

The analysis of the parallel-series reactions according to this scheme has been developed for the rotating disk-ring method by Bagotskii et al, The equations relating the ratio of currents at the disk Id) and the ring Ir) to the electrode rotation rate o> are as follows ... 

Figure 16. Rotating disk-ring electrode data for O2 reduction on gold in O.lMNaOH at 25°C. IdUr vs. at various disk potentials. Au ring potentiostated at f n = 11 V. iV = 0.43.

On platinum and platinum family metals, the O2 reduction appears to occur by the parallel mechanism with the direct four-electron reduction to water predominant in both acid and alkaline electrolyte in the absence of impurities. Evidence to support this conclusion is to be found in the influence of surface chemisorbed oxygen species (Pt-OH or Pt-O), adsorbed anions, and other adsorbed species on the extent to which the O2 reduction proceeds to water and to H2O2. In rotating disk-ring electrode studies, the intercepts in the IdIIb vs. plots have been found to be greater than unity and... 

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