Prussian Blue Fe

One of the most well-known mixed valence compounds is the pigment Prussian Blue which is a Class II compound. There has been considerable controversy as to whether Prussian Blue, which may be produced by the reaction, 

2Fe2(S04)3 + 3K4Fe(CN)6- Fe(III)4[Fe(II)(CN)6]3-t-6K2S04, and the related compound, Turnbull s Blue, produced by the reaction, 

FeCl3 + 3FeCl2 + 3K3Fe(CN)e Fe(III)4[Fe(II)(CN)4]3 + 9KCI, are the same or not. It has been possible to show by the use of Mossbauer spectroscopy that these two preparations give the same compound and that Prussian Blue and Turnbull s Blue are identical (Maer, Beasley, Collins Milligan, 1968 Bonnette Allen, 1971). 

The Mossbauer spectrum of Prussian Blue is very complex and the best understanding of its deconvolution into component spectra may be obtained by studying compounds selectively labelled with Fe and the Mossbauer isotope Fe. Bonnette Allen (1971) have been able to obtain Fe(III)4[Fe(II)(CN)6]3.xH20, Fe(III)4[56Fe(II)CN6]3.xH20 and 

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Ferric ferrocyanide [tetrairon(III) tris(hexakiscyanoferrate)(Prussian blue) Fe "4[Fe(CN)6]3 [14038-43-8]... 

Some of these derivatives, such as Prussian blue, are of considerable commercial importance on account of their characteristic deep colour. As a general rule, the derivatives which are devoid of colour contain iron in one stage of oxidation only within the molecule, whilst the coloured compounds possess divalent and trivalent atoms of iron respectively. It would appear, therefore, that the colour is in some way connected with the presence of similar atoms in more than one stage of oxidation. Thus, ferrous potassium ferrocyanide, Fe"K2[Fe (CN)6], is white, the iron atoms in the positive and negative radicles respectively being divalent. Upon oxidation, however, Prussian blue, Fe K[Fe (CN)e] is obtained, the iron atom of the negative radicle remaining divalent, whilst the positive iron ion is trivalent. 

Some mixed-valence systems are sufficiently stable to be isolated, such as Prussian Blue (Fe )4[Fe"(CN)6]3 or the much studied Creutz-Taube ion (p-pz)[Ru(NH3)5]2 (1), pz=pyrazine Flowever, in many (but not al ) cases they are less robust than their homovalent congeners, often being formed at inconveniently high or low redox potentials, or existing only in a very limited electrochemical potential range as quantified by a small comproportionation constant K. ... 

Prussian Blue. Reaction of [Fe(CN)3] with an excess of aqueous h on(Ill) produces the finely divided, intensely blue precipitate Pmssian Blue [1403843-8] (tetrairon(Ill) tris(hexakiscyanoferrate)), Fe4[Fe(CN)3]. Pmssian Blue is identical to Turnbull s Blue, the name which originally was given to the material produced by reaction of [Fe(CN)3] with excess aqueous h on(Il). The soHd contains or has absorbed on its surface a large and variable number of water molecules, potassium ions (if present in the reaction), and h on(Ill) oxide. The h on(Il) centers are low spin and diamagnetic h on(Ill) centers are high spin. Variations of composition and properties result from variations in reaction conditions. Rapid precipitation in the presence of potassium ion affords a colloidal suspension of Pmssian Blue [25869-98-1] which has the approximate composition KFe[Fe(CN)3]. Pmssian Blue compounds are used as pigments in inks and paints and its formation on sensitized paper is utilized in the production of blueprints. 

FIGURE 16.16 When potassium cyanide is added to a solution of iron(ll) sulfate, the cyanide ions replace the H.O ligands of the [Fe(H20), - + complex (left and produce a new complex, the hexacyanoferrate(ll) ion, Fe(CN)(l 4 (right). The blue color is due to the polymeric compound called Prussian blue, which forms from the cyanoferrate ion. 

Originally, compounds containing coordination complexes were given common names such as Prussian blue (KFe[Fe (CN)g ]), which is deep blue, or Reinecke s salt (NH4[Cr (NH3)2 (NCS)4]), named for its first maker. Eventually, coordination compounds became too numerous for chemists to keep track of all the common names. To solve the nomenclature problem, the International Union of Pure and Applied Chemistry (lUPAC) created a systematic procedure for naming coordination compounds. The following guidelines are used to determine the name of a coordination compound from its formula, or vice versa ... 

K+ is generally used as the reversibly intercalating ion since it leads to insoluble compounds for all the forms. In the mixed-valence Prussian Blue compound, Fe is in the high-spin state and coordinated octahedrally with the N ends of the cyaiudes, whereas Fe is low-spin and octahedrally coordinated with the C ends of the... 

Fig. 12. Configurational coordinate diagram of Prussian blue. Curve g gives the ground state Fe(III)-NC-Fe(II) Curve e gives the MMCT state Fe(II)-NC-Fe(III). The optical transition is indicated by E p, whereas Eo gives the energy difference between the two states. See also text (after data in Ref. [66])...

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). 

In the first experiment 10 ml of 500 ppm solutions of FeCl3 were sonicated for 15, 30, 45 and 60 min. To examine the reduction of Fe(III) to Fe(II), 0.1 ml of the sonicated sample was transferred to a 10 ml volumetric flask and mixed with 0.5 ml of 2,000 ppm K3[Fe(CN)6] solution before making up to the mark with distilled water. Final concentration of this sonicated sample in 10 ml of volumetric flask was 5 ppm. UV-vis absorbance at >.max 795 was recorded. Sonication reduced Fe3+ to Fe2+, which reacted with K3[Fe(CN)6], already added in the solution, to form blue colour due to prussian blue. Continuous sonication gradually increased the concentration and intensity of prussian blue complex, as is clear from the Table 10.1. 

Zboril R, Machala L, Mashlan M, Sharma V (2004) Iron(III) oxide nanoparticles in the thermally induced oxidative decomposition of prussian blue, Fe4[Fe(CN)6]3. Cryst Growth Design 4(6) 1317-1325... 

Extensive work has been carried out on microsensors built from electropolymerized nickel porphyrin films.328,329 Films of Prussian blue (Fe4[Fe(CN)6]3) 345 metal-salen complexes (M = Co, Fe, Cu, Mn)346 or the ferrocene-containing Nin-tetraaza[14] annulene (24),347 also exhibit interesting activity for NO electrooxidation and sensing. 

More recently, a new method of assembling multilayers of PB on surfaces has been described.110 In contrast to the familiar process of self-assembly, which is spontaneous and leads to single monolayers, directed assembly is driven by the experimenter and leads to extended multilayers. In a proof-of-concept experiment, the generation of multilayers of Prussian blue (and the mixed Fein/Run analog ruthenium purple) on gold surfaces by exposing them alternately to positively charged ferric cations and [Fe(CN)6]4- or [Ru(CN)f,]4 anions has been demonstrated.110... 

F. Herren, P. Fisher, A. Ludi, and W. Haig, Neutron difraction study of Prussian blue, Fe4[Fe(CN)6]3 xH20. Location of water molecules and long-range magnetic order. Inorgan. Chem. 19, 956-959 (1980). 

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