Ferri-/ferrocyanide

A wide variety of enzymes have been used in conjunction with electrochemical techniques. The only requirement is that an electroactive product is formed during the reaction, either from the substrate or as a cofactor (i.e. NADH). In most cases, the electroactive products detected have been oxygen, hydrogen peroxide, NADH, or ferri/ferrocyanide. Some workers have used the dye intermediates used in classical colorimetric methods because these dyes are typically also electroactive. Although an electroactive product must be formed, it does not necessarily have to arise directly from the enzyme reaction of interest. Several cases of coupling enzyme reactions to produce an electroactive product have been described. The ability to use several coupled enzyme reactions extends the possible use of electrochemical techniques to essentially any enzyme system. 

Quinn et al. studied ET at micro-ITIES supported by micropipettes or microholes [16]. The studied systems involved ferri/ferrocyanide redox couple in aqueous phase and ferrocene, dimethylferrocene, or TCNQ in either DCE or o-nitrophenyl octyl ether. Sigmoidal, steady-state voltammograms were obtained for ET at the water-DCE interface supported at a micropipette. The semilogarithmic plot of E versus log[(/(j — /)//] was... 

Fig. 3. (a) Typical galvanostatic limiting-current curve for copper deposition at a copper disk in acidified CuS04 solution. The circles indicate the experimental curve. The solid curves were calculated using kinetic parameters as indicated, (b) Typical galvanostatic limiting current curve for ferricyanide reduction at a nickel electrode in equimolar ferri ferrocyanide solution with excess NaOH. [From Selman (S8).]... 

The current peaks observed when fast potential ramps are applied appear similar to the one shown in Fig. 8. The peak currents (triangles in Fig. 10) can be satisfactorily interpreted in terms of a pure-diffusion model (S10) in equimolar ferri-ferrocyanide solution,... 

Later measurements by Fouad and Gouda (F5), who used the ferri-ferrocyanide redox reaction, yielded a much lower coefficient in the free-convection correlation, Eq. (29a). However, uncertainty about the interfacial composition in free convection (S9a) may be responsible for the discrepancy observed in the results obtained with the redox system. Taylor and Hanratty (Tib) showed that the data of Fouad and Gouda could be... 

Microcylindrical electrodes are easier to constract and maintain than microdisk electrodes [37]. Mass transport to a stationary cylinder in quiescent solution is governed by axisymmetrical cylindrical diffusion. For square-wave voltammetry the shape and position of the net current response are independent of the extent of cyhn-drical diffusion [38]. The experiments were performed with the ferri-ferrocyanide couple using a small platinum wire (25 pm in diameter and 0.5 -1.0 cm in length) as the working electrode [37]. 

Pulsed-current techniques can furnish electrochemical kinetic information and have been used at the RDE. With a pulse duration of 10-4 s and a cycle time of 10-3 s, good agreement was found with steady-state results [144] for the kinetic determination of the ferri-ferrocyanide system [260, 261], Reduction of the pulse duration and cycle time would allow the measurement of larger rate constants. Kinetic parameter extraction has also been discussed for first-order irreversible reactions with two-step cathodic current pulses [262], A generalised theory describing the effect of pulsed current electrolysis on current—potential relations has appeared [263],... 

This value is maximized when EF reaches the flatband potential. Thus, a plot of incident light intensity versus open-circuit electrode potential is expected to saturate at the flatband potential, thereby allowing identification of this potential. A typical experiment of this sort is shown in Figure 28.3b for an n-CdSe electrode immersed in a ferri/ferrocyanide electrolyte. 

Figure 28.3 The flatband potential of a semiconductor can be established by measuring the photopotential of the semiconductor as a function of illumination intensity. In the dark (left), the semiconductor Fermi level and the redox potential of the electrolyte are equal, providing an equilibrium condition. However, illumination of the semiconductor (right) generates charge carriers that separate the Fermi level and the redox potential. The difference in these two parameters is the observed photovoltage as shown for an n-CdS electrode immersed in a ferri/ferrocyanide electrolyte (bottom). The measured photovoltage is observed to saturate at the flatband potential. In this case, a value of -0.2 V vs. SCE is obtained. Note that the photovoltage response yields a linear functionality at low light intensity with saturation behavior occurring as the flatband potential is approached.

The existence of surface states in general can lead to a variety of nonidealities in the output parameters associated with semiconductor-electrolyte junctions. Figure 28.6 provides the current-potential response for a photo-electrochemical cell containing a cadmium ferrocyanide-modified n-CdS electrode in an aqueous ferri/ferrocyanide electrolyte. Although open-circuit and... 

Figure 28.6 The current-potential response of an n-CdS/ferri/ferrocyanide cell under chopped irradiation in (488 nm, 40 mW/cm2). The ferri/ferrocyanide redox potential is marked in the diagram. Note that even when a dark cathodic current is present (solid line), a photocurrent is observed (spikes).

Eq. (1-4) shows that only two quantities are needed, the third one being determined from the two first. Eq. (1-4) was directly verified by measuring Zac, ZEHD G and ZEHd, p with the ferri/ferrocyanide system in KC1(M) at the half-wave potential [29]. Experimental Zac values were shown to be in good agreement with the calculated ones deduced from experimental ZEHd,g and ZEHD P values, and application of Eq. (1-4) written as Zac = -Zehd>g/zehd,p (see Fig. 1-1). 

At the present time, the theory of electrochemical impedance of electrodes with distributed potentials is not yet completed, and algorithms of parametrical and structural identification procedures are not available. In addition, the interpretation of the results is very complicated. For this reason, in this work we analyzed only the frequency characteristics of impedance s components in the modified electrode system. As a result, we obtained a set of response peculiarities in the frequency range under investigation. Rather low frequency dispersion was observed in a solution containing a ferri-ferrocyanide system for both active (Fig.3, curve 2) and reactive (Fig.4, curve 3) components. In our opinion, this fact confirms that the independent on frequency resistance of charge transfer determines the main contribution to the impedance. 

As a final example, we should perhaps look at the oxidation and reduction of the ferri/ferrocyanide couple, which is often used as a "test reaction" in electrode kinetics ... 

The second important feature of a metal-semiconductor composite approach is that the metal can function as a template for chemical or electrochemical deriva-tization to afford a film comprising of molecular redox-semiconductor (or even semiconductor-semiconductor) contacts. Figure 34 generically illustrates the occlusion electrosynthesis approach for preparing M-TiO composite films and a subsequent derivatization with ferri/ferrocyanide to afford the corresponding metal hexacyanoferrate (MHCF)-Ti02 counterparts [424]. 

We now describe the conditions for obtaining the net difference spectrum for the Q->Qa" change alone. The chemical intervention needed to obtain the net difference spectrum for the Qa >Qa is the addition of DCMU to a Tris-treated sample so that the system is blocked not only before Z but also after Q, as shown below. Then a ferri-/ferrocyanide couple [2.5 mM Fe Cy/2.5 mM Fe Cy] is added to provide additional chemical intervention ... 

Fig. 9 shows the titration results for the following samples chloroplast lamellae and TSF-1 particles, both measured at 820 nm, and the CPI complex measured at 820 as well as 703 nm. Each sample was titrated oxidatively (starting with 100 pM ferrocyanide and adding ferricyanide to a maximum concentra tion of 10 mM) and reductively (starting with 1-5 mM ferricyanide and adding ferrocyanide to a maximum concentration of 10 mM). The titration is a plot of the light-induced AA V5. the actual redox-potential of the medium or the ferri-/ferrocyanide ratio as shown in Fig. 9. The plot of the data points clearly show that the titration was completely reversible and that P700 was in redox equilibrium with the ferri-/ferro-cya-nide couple. The solid line is the theoretical Nernst curve for a one-electron transition and the data points agree well with the theoretical course. The titration curve for both the chloroplast lamellae and the TSF-1, as well as D144 (data not shown here), yielded an value of+492 mV.

There have been fewer studies in electrochemistry where the flow is known but the boundary-layer approach is inapplicable. One example has been recently analyzed and compared with experiment. In this case, mass transfer to a line electrode or an array of line electrodes in the presence of an oscillatory shear flow was treated. A finite-volume approach was used for the numerical analysis and a ferri/ferrocyanide redox couple was used to measure the mass-transfer rate. The studies show that boundary-... 

The redox reactions for supported BLMs containing vinylferrocene as an electron mediator have been investigated using cyclic voltammetry. The results have shown the following, (i) Ferrocene can be very easily immobilized in the lipid bilayer on the surface of a metallic wire (s-BLM) system. This demonstrates that the s-BLM system offers a novel approach to electrode modification by simple immobilization of compounds within BLM. (ii) Ferrocene in a BLM increases the sensitivity to the potassium ferri/ferrocyanide ion by about two orders of magnitude in comparison to that of the platinum electrode [79]. 

FIG. 17 Reversal of the photocurrent sign upon replacing the electron donor DCMFc (a) by the electron acceptor TCNQ (b) in the presence of the porphyrin heterodimer ZnTMPyP-ZnTPPS at the water-DCE interface. The strong back electron-transfer features in the photoreduction of TCNQ were diminished upon addition of an equimolar ratio of ferri/ferrocyanide acting as supersensitizer in the aqueous phase (b). The mechanism of supersensitization is described in Fig. 11. From the potential relationship between these redox couples (Fig. 4), these phenomena can be regarded as interfacial photosynthetic processes as defined in Fig. 3(b). (Reprinted with permission from Ref. 87. Copyright 1999 American Chemical Society and from Ref. 166 with permission from Elsevier Science.)... 

Figure 16 shows the theoretically predicted influence of the normalised frequency, f, on the phase angle between current and voltage, , along with experimental verification [70] using the ferri-ferrocyanide reversible couple... 

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