Ferricyanide electrode potentials

The distance decay constant / (see below) in Miller et al. s original study was 0.9 per CH2, using ferricyanide and iron(IH) hexahydrate [44]. In a later study which accounted more thoroughly for double layer effects, 2 was determined to be 1 eV for kinetically facile redox probes such as ferricyanide, 1.3 eV for Ru-hexamine and 2.1 eV for iron(III) hexahydrate. With a better understanding of the redox probe behavior, f was found to be 1.08 + 0.20 per CH2 and independent of the redox couple and electrode potential [96]. Pre-exponential factors were also extracted from the Tafel plots. The edge-to-edge rate constants (extrapolated) are approximately 10 -10 s for all redox probes, which is reasonable for outer-sphere electron transfer. The pre-exponential factors are 5 x lO s [96]. 

Aromatic amines are activators of horseradish peroxidase. Kulys and Vidziunaite (1983) adsorbed HRP together with GOD on a carbon electrode and crosslinked the enzymes with glutaraldehyde to assemble a sensor for aromatic amines. H2O2 is produced in the presence of glucose, acting as cosubstrate in the HRP-catalyzed oxidation of ferrocyanide. The ferricyanide formed was reduced back to ferrocyanide at an electrode potential of +10 mV vs SCE, the current being limited by the... 

A way to reduce interferences by cooxi-dizable sample constituents is by keeping the applied electrode potential as low as possible. Therefore, a reaction partner is chosen to be electrochemically indicated that is converted at low potential. For this purpose, the natural electron acceptors of many oxidoreductases have been replaced by redox-active dyes or other reversible electron mediators. Among them are the ferricyanide/ferrocyanide couple, V-methylphenazinium sulfate, fer-rocenes, and benzoquinone. With these mediators an electrode potential around -1-200 mV can be applied, which decreases... 

Termination of the plateau at a sufficiently high overpotential. The potential at which a consecutive electrode reaction sets in (e.g., hydrogen evolution in cathodic reactions) is determined by the composition of the electrolyte (specifically, the pH) and by the nature and state of the electrode surface (hydrogen overpotential). The reduction of ferricyanide in alkaline solution on nickel also provides a better-defined plateau in this respect than the deposition of copper in acid solution. 

Experimental results obtained at a rotating-disk electrode by Selman and Tobias (S10) indicate that this order-of-magnitude difference in the time of approach to the limiting current, between linear current increases, on the one hand, and the concentration-step method, on the other, is a general feature of forced-convection mass transfer. In these experiments the limiting current of ferricyanide reduction was generated by current ramps, as well as by potential scans. The apparent limiting current was taken to be the current value at the inflection point in the current-potential curve. 

The sharpness of Prussian blue/Prussian white redox peaks in cyclic voltammograms can be used as an indicator of the quality of Prussian blue layers. To achieve a regular structure of Prussian blue, two main factors have to be considered the deposition potentials and the pH of initial growing solution. As mentioned, the potential of the working electrode should not be lower then 0.2 V, where ferricyanide ions are intensively reduced. The solution pH is a critical point, because ferric ions are known to be hydrolyzed easily, and the hydroxyl ions (OH-) cannot be substituted in their... 

Fig. 18b. 7. (a) Chronoamperogram showing the response due to a triple pulse 500-0-500 with a 3 mm diameter glassy carbon working electrode in 2.0 mM Potassium Ferricyanide in 0.1 M KC1. No current was recorded for the initial potential, 500 mV, where no faradaic reduction took place, (b) The same solution, except with a 10 pm diameter Pt working electrode. Current was recorded for the initial potential at 500 mV for 0-4000 ms where no faradaic reduction took place. Note the magnitude of current scale. 

The experimental / w/2f (E E ) and (2sw — (E — E ) curves of the FcC()SH C4SH mixed monolayer at a disc gold electrode of 100 pm diameter in a solution 1.0 M NaCICU, obtained for different values of the square wave pulse amplitude and a fixed ferricyanide concentration 10 mM, are plotted in Fig. 7.58. It can be seen that whereas the peak height of the charge-potential curves increases with sw until charge plateau for sw > 110 mV is obtained, the anodic limit region remains unaffected, in line with Fig. 7.57b and Eq. (7.150). From the measurement of the charge plateau for sw = 130 mV, the value... 

An example of the size of the impurity effects that may arise is shown in Fig. 1, which gives the electrode kinetics for the ferro-ferricyanide reaction on three different zinc oxide single crystals of varying conductivity. Each of the crystals was in excess of 99.999% pure. As can be seen, each crystal gives a linear Tafel plot under cathodic bias. However, the exchange currents, i.e, the extrapolations back to the reversible potential (+. 19 volts), differ by a factor of about 1000 and... 

There is nothing in the foregoing discussion that restricts it to reactions at the cathode or to ions it holds, in fact, for any electrode process, either anodic, i.e., oxidation, or cathodic, i.e., reduction, using the terms oxidation and reduction in their most general sense, in which the concentration of the reactant is decreased by the electrode process, provided the potential-determining equilibrium is attained rapidly. The fundamental equation (10) is applicable, for example, to cases of reversible oxidation of ions, e.g., ferrous to ferric, ferrocyanide to ferricyanide, iodide to iodine, as well as to their reduction, and also to the oxidation and reduction of non-ionized substances, such as hydroquinone and qui-none, respectively, that give definite oxidation-reduction potentials. 

FIGURE 6.22. Current-time transients at -2.0V on /j-Si(lOO) in 2M KOH at 45°C measured for an oxide-free chemically etching electrode when 3mM ferricyanide was added to the solution at t = 0 measured after anodic oxidation at 0.0 V when, at t = 0, the potential was stepped back to -2.0 V in a solution containing 3mM ferricyanide. (Reprinted from Bressers et 1995, with permission from Elsevier Science.)... 

One typical example of this behavior is the voltammogram of the ferro/ferricyanide couple (test reaction) that at carbon electrodes is less reversible than at noble metal electrodes. The kinetics of the test reaction in 1 M aqueous KCl was used as the reference to compare its electrochemical behavior on different carbon electrodes [20]. This electrochemical reaction occurs via an outer sphere mechanism and its rate depends on the electrolyte composition and can be increased by appropriate treatment of carbon electrodes, for instance, by application of a high current potential routine to electrodes of carbon fibers. Similar results have been obtained with glassy carbon surfaces that had been pretreated at 500°C under reduced pressure. An alternative activation method is based on careful electrode surface polishing [6]. 

Chen and Liu (1977) utilized the spontaneous oxidation of NADH by potassium ferricyanide for the construction of a potentiometric LDH electrode. The coupled reduction of ferricyanide ions to ferrocyanide ions results in a measurable electrochemical zero-current potential. The potential was found to be Nemstian in nature and directly proportional to the logarithm values of lactate concentration over the range 0.02 to 50 mmol/1. The response time was as high as 10 min. 

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