Prussian Blue and Its Derivatives

McKenzie and Marken examined the electrochemical behavior of hydrous RuO  

Fujishima and coworkers reported a method to electrochemically deposit IrO,. NPs at BDD electrodes [87]. The deposition process was based on preparation of a solution containing hydrolysis products of IrClg and oxalate, followed by anodic electrodeposition of IrO from this solution onto an anodically pretreated BDD electrode. 

They showed that for conditions under which only limited deposition was allowed to occur it was possible to deposit about 30 nm diameter IrO N Ps that were relatively homogeneously distributed across the surface. They examined these deposits for hydrogen-peroxide oxidation and observed significant oxidative current at potentials as low as + 0.35 V at pH 7. 

Jayalakshmi and coworkers have prepared NPs of SnO, SnS and ZnS in order to study their capacitance behavior [88-90]. The metal oxide and metal sulfide NPs were prepared using hydrothermal methods. After immobilization in PIGE electrodes, their electrochemical properties were examined. Capacitance values in the 4—15 Fg were reported for SnS. Comparable values were reported for ZnS. 

Prussian Blue (PB) has been known for many decades, originally because of its wide use as a pigment and later because of its interesting redox properties [91]. PB is comprised of Fe(II) and Fe(III) centers, CN ligands and K counterions. It has a 

PB also can be oxidized to give first a green material, Berlin Green (BG), in the partially oxidized state, and then finally a yellow material, Prussian Yellow (PY), in the fully oxidized state. This oxidation typically occurs between 0.8 and 1.0 V vs. SCE. The complete oxidation of Prussian Blue to Prussian Yellow is shown in the following equation. 

In aqueous solutions containing K+ counterions this oxidation typically occurs near 0.8 V vs. SCE. Other counterions besides K+ may be incorporated into PB and its derivatives, changing the energetics of the various redox transitions. The ability to reversibly oxidize or reduce all of a given type of metal center in P B and its derivatives endows PB with behavior similar to that described above for many other electroactive NPs. 

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PB and its derivatives are of interest for a variety of reasons, the most important of which is its electrochromism [93]. In addition, it is an electrocatalyst for several different types of substrates, notably hydrogen peroxide, as will be seen below. Synthesis of nanopartides of Prussian Blue is relatively straightforward. It relies on many of the prindples of colloid chemistry, and produces ionically stabilized colloidal solutions (Figure 4.7). As a consequence, the electrochemical behavior of PB N Ps has been examined by several groups. In this section, we discuss the behavior of P B N Ps immobilized at electrodes. 

Prussian blue, in the form of printers inks, artists colors, and paints, soon flooded the market. It also stimulated interest in other potentially useful substances that might be derived from the potash, iron, and animal-residue mixtures. One of these, known as red prussiate of potash, did turn out to be very useful. When combined with ferric ions, it didn t produce a dramatic color until it was exposed to direct sunlight. Then it turned blue. Prussian blue. The discovery revolutionized archi-... 

While bifunctionality is known for the halogens and many pseudohalogens, it is most pronounced for cyanide and influences the structures, properties and chemistry of many of its derivatives. Cyanide bridges were present in the first recorded synthetic inorganic complex, Prussian blue (ca. 1700), and cyanide linkage isomers were often proposed in the old literature but reasonable evidence for the existence of linkage isomers and the structure of Prussian blue is very recent. 

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. 

Properties.—An aqueous solution of sodium nitroprusside deposits Prussian blue on exposure to light. In the presence of alkali sulphides— as, for example, ammonium sulphide—it yields a beautiful purple colour, which is very characteristic, and so sensitive that the presence of 0 0000018 gram of hydrogen sulphide in 0 004 c.c. can easily be detected.2 Ammonium hydroxide does not hinder the colour formation, but caustic alkalies destroy it. It gradually fades on standing, in consequence of oxidation of the sulphide to sulphite. The composition of the purple substance is uncertain, but Hofmann 3 suggests the formula Na3[Fe(CN)5(0 N.SNa)], since, by the action of thio-urea, CS(NH2)2, upon sodium nitroprusside, he obtained the complex derivative Na3[Fe(CN)5(0 N.SCNH.NH2)], as a carmine-red powder, closely similar to the substance under discussion.4... 

The system was first applied for development of chemosensors for gaseous hydrogen chloride. Polyaniline, and its copolymers with different derivates of aniline were used. Then a similar approach was tested in the author s group for optimization of amperometric biosensors for glucose based on electrocatalytical detection of hydrogen peroxide. A pigment Prussian blue was used as an electrocatalyst for decomposition of this product of enzymatic oxidation of... 

It cannot be expected that a structural model derived purely from X-ray powder data would provide a complete and reliable description of the actual structure. One, and probably the most important feature of the model of Keggin and Miles, however, seems to be beyond any doubt namely the linear arrangement M- N—C—M —C—N—M- - along the edge of the unit cell. The unit cell constants of a wide variety of Prussian blue analogs have been determined. All the lattice constants measured so far are between 9.9 and 10.9 A. Since the C—N distance is known to be close to 1.14 A (16), the differences in the cell constants directly reflect the differences in the distances N and M —C. 

The redox chemistry of the Prussian blue family (Table 7) has attracted considerable attention. The generation of thin films of Prussian blue has led to studies of its mediation in electron transfer reactions and of the electrochemical processes involved in its deposition and redox reactions. This work has been spurred by its electrochromic properties which have been used in prototype electronic display devices based, for example, on Prussian blue modified Sn02 electrodes. A recent review deals with the electrochemistry of electrodes modified by depositing thin films of PB and related compounds on them. Interestingly, true Prussian blue is somewhat difficult to process and modern iron blue pigments such as Milori blue are derived from the oxidation of rlin white Fe(NH4)2[Fe(CN)e] to give iron(III) ammonium ferrocyanides. 

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