Ferrocyanide and Ferricyanide

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. 

Srikantan and Rao (79) reported that the ferrocyanide catalysis is not first order in peroxide as found by Kistiakowsky but much more complicated. No details of the experiments were given. They also studied the ferricyanide-peroxide reaction and found that in the dark there was a pronounced induction period after which the decomposition rate was first order in peroxide concentration. Illuminating for five minutes in bright sunlight removed this induction period but the subsequent first order rate is somewhat less than the original dark rate. They suggest that the induction period was the time taken to build up on 

These aquopentacyanoferrates and the corresponding ferrites have been prepared by Hofmann (80) and they were shown by Briggs (81) and Iimori (82) to be formed by the action of heat, acid, or light on the hexacyanides. Srikantan and Rao consider it possible for hydrogen peroxide to enter the coordination shell and for oxygen to be evolved by the reaction 

The removal of the induction period by illumination was attributed to colloidal Prussian blue which is known to be formed on prolonged illumination of ferricyanide. 

This is rapidly established from the left-hand side, and therefore the position is independent of the time of illumination. The reverse reaction occurs slowly in the dark and in the absence of hydrogen peroxide. Aquo-pentacyanoferrite formed in this way decomposes peroxide catalytically by a reaction which is not specified. At the same time it is oxidized by peroxide to aquopentacyanoferrate 

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Use the thermodynamic data of Appendix C to derive stability constants / 6 for the ferrocyanide and ferricyanide ions at 25 °C and infinite dilution. 

Emschwiller, G. Infrared Spectra of Ferrocyanides and Ferricyanides and Constitution of Prussian Blue. Compt. rend. 238, 1414 (1954). 

Potassium ferrocyanide and ferricyanide.— The mode s of preparation and the properties of potassium ferrocyanide, of potassium ferricyanide, and of similar salts are described in Vol. IX., Part II. 

Owing to the high charge of the anions, the effect of ionic strength is also large (E° = 0.46 V for 0.1 M solutions of both potassium ferrocyanide and ferricyanide), and the potential shift is in a direction opposite that observed for the aquo Fe(III)-Fe(II) couple because the more highly charged species is the reductant rather than the oxidant. 

Vanadium compounds (vanadic acid and vanadium chloride) have been proposed as substitutes for copper sulphide, but without much practical success. Potassium ferrocyanide and ferricyanide are also used to a certain extent. A mixture of these salts with aniline salt and potassium chlorate is printed, and the goods aged. The probable action is that the ferricyanide oxidises the aniline, and is continually regenerated from the ferrocyanide formed by the chloric acid present. Thus these salts play the part of oxygen carriers in a similar manner to the copper and vanadium compounds. In the opinion of technologists, the hlack produced by this process differs somewhat in its properties from that obtained with copper, but this may be ascribed to the presence of prussiaii blue in the former. In place of the aniline hydrochloride and potassium chlorate, a mixture of aniline sulphate and barium chlorate has recently been employed in black-printing. 

Fe(CN)5 /Fe(CN). Equimolal solutions of ferrocyanide and ferricyanide at pH 6.0 were found to impart Nemstian responses on platinum down to about 7x10 m in each valence. This corresponds to an exchange current of 8x10" amp cm". ... 

The calculations for the ferrocyanide and ferricyanide complex anions used parameters from electron-spin resonance and optical absorption spectra [23]. The net result is thatin[Fe"(CN)6] about one more electron charge resides on the ligands than in [Fe (CN)6] . In other words, when the paramagnetic... 

This procedure also destroys soluble ferrocyanides and ferricyanides. Altanatively, these can be precipitated as the ferric or ferrous salt, respectively, for possible landfill disposal. 

The substances contain the coordinated complexes ferrocyanide ion, Fe(CN)e, and ferricyanide ion, Fe(CN)e, respectively, and the ferrocyanides and ferricyanides of other metals are easily made from them. 

An interesting set of reactions involves the complex ions [Fe(CN)6] and [Fe(CN)6] . These ions are commonly called ferrocyanide and ferricyanide, respectively. Fe (aq) yields a dark blue precipitate called Prussian blue when treated with potassium ferrocyanide, K4[Fe(CN)6](aq), whereas Fe (aq) yields a similar blue precipitate, called Turnbull s blue, when treated with potassium ferricyanide, K3[Fe(CN)6](aq). Together, these two and similar related precipitates are known commercially as iron blue. Iron blue is used as a pigment for paints, printing inks, laundry bluing, art colors, cosmetics (eye shadow), and blueprinting. An additional sensitive test for Fe " (aq) is the formation of a blood-red complex ion with thiocyanate ion, SCN (aq). 

In Chapter 24, we will learn to write the systematic names for ferrocyanide and ferricyanide ions they are hexacyanidoferrate(II) and hexacyanidoferrate(III), respectively. 

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