Prussian blue structure

Despite the prevalence of the Prussian blue structure compounds of lower symmetry are known in the heavy metal ferrocyanide systems (Weiser, Milligan and Bates, 1942). Also, Rigamonti (1937) and Rock and Powell (1964) report non-cubic examples of copper and zinc ferro-cyanides. 

On the basis of the foregoing generalization, it is reasonable to postulate that the simple transition metal cyanides with six or less d electrons will adopt the Prussian blue structure. However, metal cyanides with seven or more d electrons will crystallize in less symmetric structures. At the present there are insufficient data to thoroughly check this proposal, but the few known structures lend support to the idea. For example, ferrous cyanide (ferrous ferrocyanide) and ferric cyanide (ferric ferri-cyanide) crystallize with the Prussian blue structure while nickel and zinc cyanides do not. 

The most significant developments in the chemistry of a well-known1 [Cr(CN)6]3- complex are related to its use in the preparation of molecular magnets. The synthesis of a room-temperature magnetic material with a Prussian blue structure, Vno.42Vnio.58[Crm(CN)6]o.86 2.8H20 (Tc = 315 K),... 

Prussian blue films can be reduced to the colourless form, called Everitt s salt (KFe4[Fe (CN)g]3 or K2FeFe (CN)g), or oxidized to the yellow form called Prussian yellow (Fe4[Fe (CNigJaCl or KFeFe (CNlgCl). These electrochemical processes can be easily detected by cyclic voltammetry of Prussian blue films in KCl solutions. The reduction process for the soluble Prussian blue structure has been described as "... 

The ideal composition of Prussian blue is Fe(III)4[Fe(II)(CN)g]3.15H2O. The crystal structure is cubic. All Fe(III) lattice sites are occupied, whereas those of Fe(II) are only 75% occupied. At low temperatures the paramagnetic Fe(III) ions order ferromagnetically (T = 5.6 K). Finally we note that upon replacing Fe(II) by Ru(II) or Os(II) the color properties are not drastically influenced. 

Further ligands that can be bonded by different atoms include OCN- and NG2. Cyanide ions always are linked with their C atoms in isolated complexes, but in polymeric structures as in Prussian blue they can be coordinated via both atoms (Fe—C=N—Fe). 

The crystalline structure of Prussian blue was first discussed by Keggin and Miles on the basis of powder diffraction patterns [5] and then has been determined more precisely... 

In the literature the term soluble Prussian blue introduced by Keggin and Miles [5] to determine the KFeFe(CN)6 compound is still widely used. However, it is important to note, that the term soluble refers to the ease with which the potassium ion can be peptized rather than to the real solubility of Prussian blue. Indeed, it can be easily shown by means of cyclic voltammetry that the stability of Prussian blue films on electrode supports is nearly independent of their saturation by potassium cations. Moreover, Itaya and coworkers [9] have not found any appreciable amount of potassium ions in Prussian blue, which makes doubtful structures like KFeFe(CN)6. Thus, the above equation fully describes the Prussian blue/Prussian white redox reaction. 

The Prussian blue/Prussian white redox activity with potassium as the countercation is observed in cyclic voltammograms as a set of sharp peaks with a separation of 15-30 mV. These peaks, in particular the cathodic one, are similar to the peaks of the anodic demetallization. Such a set of sharp peaks in cyclic voltammograms correspond to the regular structure of Prussian blue with homogeneous distribution of charge and ion transfer rates throughout the film. This obvious conclusion from electrochemical investigations was confirmed by means of spectroelectrochemistry [10]. 

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... 

Prussian blue-based nano-electrode arrays were formed by deposition of the electrocatalyst through lyotropic liquid crystalline [144] or sol templates onto inert electrode supports. Alternatively, nucleation and growth of Prussian blue at early stages results in nano-structured film [145], Whereas Prussian blue is known to be a superior electrocatalyst in hydrogen peroxide reduction, carbon materials used as an electrode support demonstrate only a minor activity. Since the electrochemical reaction on the blank electrode is negligible, the nano-structured electrocatalyst can be considered as a nano-electrode array. 

Fig. 13.5. As seen, a conventional Prussian blue film is of polycrystalline structure, however, the layer covers the surface completely.

The analytical performance of Prussian blue-modified electrodes in hydrogen peroxide detection were investigated in a flow-injection system equipped with a wall-jet cell. Nano-structured Prussian blue-modified electrodes demonstrate a significantly decreased background, which results in improved signal-to-noise ratio. 

Nano-structuring also results in a decreased detection limit. Since the latter has different explanations in analytical literature, we define it as the lower limit of the linear calibration range. For nano-structured Prussian blue the detection limit was found to be of 1 X 10 9 mol L 1 (Fig. 13.6). 

J.E Keggin and F.D. Miles, Structure and formulae of the Prussian blue and related compounds. Nature 137, 577-578 (1936). 

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