Ferrocyanide-ferricyanide redox system

II. The use of the zinc, ferrocyanide, ferricyanide redox system. Analy-... 

The ready reversibility of the ferrocyanide-ferricyanide redox system makes it a potential catalyst for the decomposition of hydrogen peroxide by the mechanism of compensating oxidation-reduction reactions. Moreover, the well-known facts that in acid solution ferrocyanide is oxidized to ferricyanide, whereas in alkaline solution the reverse reduction occurs, seem a good indication that at suitable pH s both reactions might occur to give catalytic decomposition. But from the investigations to date it would appear doubtful whether any such catalysis occurs to a measurable extent, and that what seems to be ready reactions of ferro- and ferricyanides are in fact those of partial hydrolysis products of these ions in which water molecules replace the cyanide ions in the coordination shell. 

Calcium Ion Sensor. Cyclic voltammograms (CV) of ferrocyanide/ferricyanide redox couple with the modified electrode were measured. The peak currents due to the reversible electrode reaction of a Fe(CN) /Fe(CN) system on a bare Pt electrode were almost completely suppressed by the coating witti the polyvinyl-polypeptide block copolymer. This indicates that the electrode was covered with the hydrophobic polymer and was insulated from redox active species. 

Cyclic voltammograms of ferrocyanide/ferricyanide redox couple with the bare and the modified electrodes are shown in Figure 8. The peak currents due to the reversible electrode reaction of a Fe(CN)5 /Fe(CN>5 system on the bare Au electrode were significantly suppressed by the treatment with the disulfide-modified DNA. In contrast, the treatment with unmodified DNA made no suppression, and that with 2-hydroxyethyl disulfide (HEDS) did only a slight as seen in Figure 8. These results indicate that the surface-anchored DNA blocks the electrochemical reaction of Fe(CN) with the underlying Au electrode, due to the electrostatic repulsion between the polyanionic DNA and the anionic redox couple ions. 

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. 

Figure 14. Dependence of transmembrane potential on proton concentration gradient in ferrocyanide-ferricyanide system (1). Potential sign is positive in part of chain with pH 7, buffer citrate-phosphate. The same dependence on membrane modified by chlorophyll in absence of redox system (2). Same dependence in presence of redox-system on nonmodified membrane.103...

Using the same tests as for the redox system ferrocyanide-ferricyanide we found that when redox reactions take place on both sides of the membrane a pH gradient is formed in the layer adjacent... 

At the anode the reverse of Eq. (2.31) occurs, keeping the concentration of the redox system and hence that of the reactant constant. To ensure that limiting current conditons are not reached at the counterelectrode, i.e., the anode, the ferrocyanide concentration is well above that of the ferricyanide. It is advisable, to ensure that the total anode area is greater than that of the cathode. One disadvantage of the ferri-ferrocyanide system is its sensitivity to light. Any transparent part of the equipment has to be shielded and made-up solutions stored in the dark. Even then, reagents should be renewed regularly. 

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... 

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