Prussian blue oxidation with

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. 

Polynuclear transition metal cyanides such as the well-known Prussian blue and its analogues with osmium and ruthenium have been intensely studied Prussian blue films on electrodes are formed as microcrystalline materials by the electrochemical reduction of FeFe(CN)g in aqueous solutionThey show two reversible redox reactions, and due to the intense color of the single oxidation states, they appear to be candidates for electrochromic displays Ion exchange properties in the reduced state are limited to certain ions having similar ionic radii. Thus, the reversible... 

Prussian blue (iron hexaferrocyanate) came into contact with ethylene oxide at 20 C. The reaction was very violent and the residue formed combusted spontaneously in air. 

The industrial production of Prussian blue is based on the reaction in aqueous solution of sodium hexacyanoferrate(n), Na4Fe(CN)6, with iron(n) sulfate, FeS04-7H20 in the presence of an ammonium salt, which results initially in the formation of the colourless insoluble iron(n) hexa-cyanoferrate(n) (Berlin white). Prussian blue is generated by subsequent oxidation with a dichromate or chlorate. 

The fact that Prussian blue is indeed ferric ferrocyanide (Fe4in[Fen(CN)6]3) with iron(III) atom coordinated to nitrogen and iron(II) atom coordinated to carbon has been established by spectroscopic investigations [4], Prussian blue can be synthesized chemically by the mixing of ferric (ferrous) and hexacyanoferrate ions with different oxidation state of iron atoms either Fe3+ + [Fen(CN)6]4 or Fe2+ + [Fem(CN)6]3. After mixing, an immediate formation of the dark blue colloid is observed. However, the mixed solutions of ferric (ferrous) and hexacyanoferrate ions with the same oxidation state of iron atoms are apparently stable. 

A cyclic voltammogram of a Prussian blue-modified electrode is shown in Fig. 13.2. In between the observed two sets of peaks the oxidation state, which is correspondent to the Prussian blue itself, occurs. Its reduction is accompanied with loss of... 

A particular interest for clinical applications was a possibility for detection of dopamine by its oxidation on nickel [19], cobalt [65], and osmium [66] hexacyanofer-ates. Except for oxidation of dopamine, cobalt and osmium hexacyanoferrates were active in oxidation of epinephrine and norepinephrine. For clinical analysis it is also important to carry out the detection of morphine on cobalt [67] and ferric [68] hexacyanoferrates, as well as the detection of oxidizable amino acids (cystein, methionine) by manganous [69] and ruthenium [70] hexacyanoferrate-modified electrodes. In general, oxidation of thiols was first shown for Prussian blue [71] and nickel hexacyanoferrate [72], This approach has been used for the detection of thiols in rat striatum microdialysate [73], Alternatively, the detection of thiocholine with Prussian blue was employed for pesticide determination in acetylcholine-esterase test [74],... 

In contrast to a variety of oxidizable compounds, only a few examples for the detection of strong oxidants with transition metal hexacyanoferrates were shown. Among them, hydrogen peroxide is discussed in the following section. Except for H202, the reduction of carbon dioxide [91] and persulfate [92] by Prussian blue-modified electrode was shown. The detection of the latter is important in cosmetics. It should be noted that the reduction of Prussian blue to Prussian white occurs at the lowest redox potential as can be found in transition metal hexacyanoferrates. 

It is important to compare the catalytic properties of Prussian blue with known hydrogen peroxide transducers. Table 13.2 presents the catalytic parameters, which are of major importance for analytical chemistry selectivity and catalytic activity. It is seen that platinum, which is still considered as the universal transducer, possesses rather low catalytic activity in both H202 oxidation and reduction. Moreover, it is nearly impossible to measure hydrogen peroxide by its reduction on platinum, because the rate of oxygen reduction is ten times higher. The situation is drastically improved in case of enzyme peroxidase electrodes. However, the absolute records of both catalytic activity... 

As mentioned earlier, potassium ferrocyanide reacts with Fe to produce Prussian blue. On the other hand, reaction with Fe2+ first gives a white precipitate of K2Feii[Feii(CN)6], which can readily oxidize in the air forming Prussian blue. 

A GCE coated with a film of Prussian Blue (Fe4[Fe(CN)6]3) mimics HRP as catalyst for selective electrochemical reduction of H2O2 in the presence of O2. A careful deposition method has to be followed to prevent leakage of the oxidized form of the pigment from the coating into the solution. Amperometric determinations in phosphate buffer at pH 6.0, with an apphed potential of —50 mV vs. SCSE, shows Unearity in the 0.1 to 100 p,M range. ... 

As shown in Fig. 2.5, the cyclic voltammograms for Prussian blue attached to paraffin-impregnated graphite electrodes (PIGEs) in contact with aqueous electrolytes exhibit two well-defined one-electron couples. Prussian blue crystals possess a cubic structure, with carbon-coordinated Fe + ions and nitrogen-coordinated Fe + ions, in which potassium ions, and eventually some Fe + ions, are placed in the holes of the cubes as interstitial ions. The redox couple at more positive potentials can be described as a solid-state process involving the oxidation of Fe + ions. Charge conservation requires the parallel expulsion of K+ ions [77] ... 

The electrochemical processes involving Prussian blue and organic dyes studied above can be taken as examples of solid state redox processes involving transformation of a one solid compound into another one. This kind of electrochemical reactions are able to be used for discerning between closely related organic dyes. As previously described, the electrochemistry of solids that are in contact with aqueous electrolytes involves proton exchange between the solid and the electrolyte, so that the electrochemical reaction must in principle be confined to a narrow layer in the external surface of the solid particles. Eventually, however, partial oxidative or reductive dissolution processes can produce other species in solution able to react with the dye. 

Tha orude solution of prussiate of potassa, and even the prussiate purified by a single crystallization, still contain, as has been stated, more or less carbonate of potassa, so that, besides the prussian blue, there is precipitated at the same time yellow oxide of iron, which deteriorates its tint. To obviate tills inconvenience, the carbonate of potassa is neutralized by sulphuric acid, which may he added cither to the solution of prussiate of potassa or to that of the protosnlphate of iron. In the manufacture of the common Berlin blues, the carbonate of potassa is neutralized by alum thoro results a precipitate of alumina, which mixes intimately with the prussian blue, and greatly augjnente... 

Ruthenium trichloride catalyzes the production of oxygen at a platinum anode404 via higher oxidation state species, whilst one of the most effective catalysts for oxygen or chlorine evolution is formed by reduction of a solution containing [Fe2(S04)3] and K3[Ru(CN)6].404 This leads to a film on the electrode of ruthenium purple, Fe4[Ru(CN)6]3, with a structure analogous to that of Prussian blue. Oxygen can then be produced at 0.2 V vs. SCE.404... 

Hydrochloric acid should not be used for acidifying the alkaline solution since the yellow colour, due to the iron(m) chloride formed, causes the Prussian blue to appear greenish. For the same reason, iron(in) chloride should not be added -as is frequently recommended a suffident concentration of iron(m) ions is produced by atmospheric oxidation of the hot alkaline solution. An alternative, sensitive, test for cyanide ion is the formation of a deep purple coloration when the fusion solution is treated with an alkaline p-nitrobenzaldehyde-o-dinitrobenzene reagent. 

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