Ferricyanide/ferrocyanide electrode

In redox electrodes an inert metal conductor acts as a source or sink for electrons. The components of the half-reaction are the two oxidation states of a constituent of the electrolytic phase. Examples of this type of system include the ferric/ferrous electrode where the active components are cations, the ferricyanide/ferrocyanide electrode where they are anionic complexes, the hydrogen electrode, the chlorine electrode, etc. In the gaseous electrodes equilibrium exists between electrons in the metal, ions in solution and dissolved gas molecules. For the half-reaction... 

Fleischmann M., Graves P.R., Robinson J., The Raman-spectroscopy of the ferricyanide ferrocyanide system at gold, beta-palladium hydride and platinum-electrodes, J. Electroanal. Chem. 1985 182 87-98. 

Blaedel, W.J. Schieffer, G.W. A hydrodynamic voltammetric study of the ferricyanide/ferrocyanide system with convective electrodes of platinum, gold, glassy carbon, carbon film, and boron carbide. J. Electroanal. Chem. Inter. Electrochem. 1977, 80, 259-271. 

The Hill reaction can be monitored by measuring the oxygen produced with an oxygen electrode or a Warburg manometer, or by spectrophotometry. The ferricyanide - ferrocyanide reduction can readily be measured by the last method ferrocyanide acceptor and spectrophotometry are therefore used to measure the Hill reaction inhibiting effect of herbicides (Hill, 1937, 1940, 1965). 

As an example, let s take the well-known reversible reaction of ferricyanide/ferrocyanide on an RRDE with Pt as ring and Pt as disk electrodes ... 

Fig. 1 Cyclic voltammogram at the water l,2-dichloroethane interface in the presence of ferricyanide/ferrocyanide coupie and ferrocene (Fc) at 10 mV. This experiment was performed with a four-electrode potentiostat, while the electrochemical cell can be represented as...

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 For this purpose, the natural electron acceptors of many oxidoreductases have been replaced by redox-active dyes or other reversible electron mediators.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... 

Fig. 6.10 Instantaneous currents with a rough potential staircase for ferricyanide/ferrocyanide 5 inM each in KCl 0.1 M. Overlaid heat pulses generate AT=78 K measured at the end of every pulse. Pt wire electrode 25 pm. Strokes at the abscissa mark potrmtial steps. Cathodic side left hand. For each step, the current versus time function is drawn. First half of every E step heating active second half heating switched off...

Fig. 6.12 Temperature-pulse voltammograms for reversible couples where the temperature coefficients of potential differ in sign at a platinum wire electrode 25 pm heated with 0.6 W. From left ferricyanide/ferrocyanide 5 mM each in KCl 0.1 M centre-, ferricyanide 2 mM in KCl 0.1 M right-, ferrocene 2 mM in acetonitrile/NBu4PF6 0.1 M. Reference electrodes left and centre bare cold platinum wire right. Ag/AgCl pseudo-reference electrode. The coefficient AEyJdT for ferricyanide/ferrocyanide is -1.6 mV/K, for ferrocene -1-0.8 mV/K...

Fig. 6.14 Temperature-pulse voltammograms at Pt electrode 25 pm heated with varied AC amplitudes. Staircase mode, heating pulses 0.1 s. Left Ferricyanide/ferrocyanide 5 mM each in KCl 0.1 M right lron(ll)/iron(lll)sulphate in 0.1 M sulphuric acid. (A) No heating (B-J) heating magnitude increasing in 0.1 Ams steps. Left starting with 0.3 Ams- fizht starting with O.lAnns-From [2], with permission...

A weighed amount of sample is dissolved in a mixture of propanone and ethanoic acid and titrated potentiometrically with standard lead nitrate solution, using glass and platinum electrodes in combination with a ferro-ferricyanide redox indicator system consisting of 1 mg lead ferrocyanide and 0.5 ml 10% potassium ferricyanide solution. The endpoint of the titration is located by graphical extrapolation of two branches of the titration plot. A standard solution of sodium sulfate is titrated in the same way and the sodium sulfate content is calculated from the amounts of titrant used for sample and standard. (d) Water. Two methods are currently available for the determination of water. 

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).]... 

In the reverse (cathodic) scan, ferricyanide ions remaining in the vicinity of the electrode surface are reduced to ferrocyanide ones. This process can be represented by the inverse of Eq. 2.1, with the voltammetric profile being interpreted from considerations similar to those made for the anodic peak. 

Figure 6. Dark cathodic currents at a p-GaP electrode in solutions of 0.05M potassium ferricyanide and 0.05M potassium ferrocyanide...

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. 

In electroanalysis, electrodes of millimeter dimensions are termed millielec-trodes, while the more recently developed very small area electrodes of micron dimensions are termed microelectrodes there are differences in properties beyond simply the change of dimension. Thus in millielectrode-scale experiments the enhancement of the diffusion-limited current plateau has been observed by a number of other workers—for example, in the reduction of methylviologen in aqueous acetonitrile [32], in the oxidation of bis(cyclopentadienyl) molybdenum dichloride in acetonitrile [33], as well as in several other studies on the aqueous ferrocyanide/ferricyanide couple using wire or disc millielectrodes to study diffu-sional phenomena [34—36], Typical values of the diffusion layer thickness of approximately 5 pm are found under ultrasound [35] in contrast to the normal value of approximately 500 pm in silent conditions. 

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. 

Shinbo et al. (1979) developed a potentiometric cytochrome b2 electrode. The change of the redox ratio of ferrocyanide/ferricyanide was indicated. The plot of the potential change versus the logarithm of lactate concentration yielded an S-shaped curve. 

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... 

The strength of the technique in this application is demonstrated by Fig. 51, which depicts the current-potential behaviour (calculated from the experimental data in Fig. 50) with reference to that of a reversible process. It is clear that the kinetic parameters of the ferrocyanide/ferricyanide couple, a typical reversible electrode reaction [163], can be measured. Vielstich and co-workers have suggested [99] that, with the condition I/IKV < 0.95 (over a reasonable potential range), the technique can be used to measure ks values up to 5 cm s"1 with an accuracy of around 10%. This... 

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