Oxidation of hexacyanoferrate

Tripotassium hexakiscyanoferrate [13746-66-2] K2[Fe(CN)g], forms anhydrous red crystals. The crystalline material is dimorphic both orthorhombic and monoclinic forms are known. The compound is obtained by chemical or electrolytic oxidation of hexacyanoferrate(4—). K2[Fe(CN)g] is soluble in water and acetone, but insoluble in alcohol. It is used in the manufacture of pigments, photographic papers, leather (qv), and textiles and is used as a catalyst in oxidation and polymerisation reactions. 

Kinetic and mechanistic studies of hexacyanoferrate(II) reductions include those of hypochlorite, of peroxodisulfate (for which activation volumes were determined), and of trart5-[Co(salen)(H20)2] (salen = A,A -ethylenebis(salicylideneaminate), (37)). Peroxonitrite oxidation of hexacyanoferrate(II) is first-order in peroxonitrite, zeroth-order in hexacyanoferra-te(II). The inference that the rate-limiting step in this reaction is decomposition of the oxidant is supported by activation volume data - the value of A for oxidation of hexacyanoferrate(II), -fllcm moU lies within the range established for peroxonitrite decomposition but is very different from the value of AF, -7cm moU ... 

This photochemical mode is observed in the case of electron-rich metal centres ligated by poor electron acceptors cyanide complexes, eg [Fen(CN),J4, [Run(CN)6]4 , [Moiv(CN)8]4, and [ WIV(CN)S]4 are known as the most efficient in the eMlv production. A typical reaction is photo-oxidation of hexacyanoferrate(II) ... 

The most important cyano complex of Fe111 is the red hexacyanoferrate(III) anion, [Fe(CN)6]3. This is usually prepared, as the potassium salt, by the oxidation of hexacyanoferrate(II), [Fe(CN)6]4, with chlorine. [Fe(CN)6]3 has a magnetic moment of 2.25 BM at 300 K in accordance with the presence of one unpaired electron as required for a 2T J ground state (Figure 1) with appreciable orbital contribution.21... 

Because the H atom and the OH radical possess only small absorptions in the UV region [56], the first rate constants for their reactions with a variety of substrates were obtained by following the formation of an absorbing product or by using the competition kinetic method [60]. A prime example of the use of this method is the study of the rate of oxidation of hexacyanoferrate(II) by "OH as a function of the pH of the aqueous solution [61]. By following the formation of hexacyanoferrate(III) at 420 nm, Rabani and Matheson [61] measured the value of and found it to decrease sharply with increasing pH at pH > 12. 

The oxidation of hexacyanoferrate(n) by [PaOg] " is strongly inhibited by CN, and a substitution process is again involved [reaction (18)]. Nucleophilic sub-... 

The oxidation of hexacyanoferrate(II) by nitrous acid involves an encounter-controlled reaction between [HFe(CN)6] and the nitrosonium ion. " More conventional nitrosations have also received much attention, as have the use of specific nitrosating agents and the transfer of NO" groups between molecules. The rate constant for the reaction of nitrous acid and methanol (O-nitrosation) is significantly lower than that expected for a diffusion-controlled reaction. Solvent-jump relaxation techniques have been used in the study of the NOCl— 1-butanol reaction in CCI4—HO AC mixtures. 

Xiong, L., Batchelor-McAuley, C., Ward, K. R. et al. 2011. Voltammetry at graphite electrodes The oxidation of hexacyanoferrate (11) (ferrocyanide) does not exhibit pure outer-sphere electron transfer kinetics and is sensitive to pre-exposure of the electrode to organic solvents. J. Electroanal. Chem. 661 144—149. 

Cassidy IF, Vos JG (1987) Polymer-modified electrodes. Part V. The use of hydrodynami-cally modulated rotating-disk electrodes in the study of the mediated oxidation of hexacyanoferrate(4-) at ruthenium-containing polymer-modified electrodes. J Electroanal Chem Interfacial Eiectrochem 218 341-345... 

Drastic acceleration of the oxidation of hexacyanoferrate(II) in solvents, strong electron donors... 

Oscillating chemiluminescence has been observed in the H2O2/KSCN/ CuS04/NaOH/luminol system there are two types of oscillation, and the low-intensity mode may involve the reaction of superoxide with luminol. A simplified mechanism for the methylene-blue/HS /02 oscillation in a CSTR has been proposed. Experimental and modeling structures of oscillation in the C102/l2/malonic acid system show that the oscillations are not due to autocatalysis but to self-inhibition in the ClOf/I reaction. A study of the oxidation of hexacyanoferrate(II) by bromine is concluded to involve the formation of Brf as an intermediate, formed in the first step. The aerobic oxidation of NADH catalyzed by horse radish peroxidase... 

Manganese(II) can be titrated directly to Mn(III) using hexacyanoferrate(III) as the oxidant. Alternatively, Mn(III), prepared by oxidation of the Mn(II)-EDTA complex with lead dioxide, can be determined by titration with standard iron(II) sulfate. 

Hexa.cya.no Complexes. Ferrocyanide [13408-63 ] (hexakiscyanoferrate-(4—)), (Fe(CN) ) , is formed by reaction of iron(II) salts with excess aqueous cyanide. The reaction results in the release of 360 kJ/mol (86 kcal/mol) of heat. The thermodynamic stabiUty of the anion accounts for the success of the original method of synthesis, fusing nitrogenous animal residues (blood, horn, hides, etc) with iron and potassium carbonate. Chemical or electrolytic oxidation of the complex ion affords ferricyanide [13408-62-3] (hexakiscyanoferrate(3—)), [Fe(CN)g] , which has a formation constant that is larger by a factor of 10. However, hexakiscyanoferrate(3—) caimot be prepared by direct reaction of iron(III) and cyanide because significant amounts of iron(III) hydroxide also form. Hexacyanoferrate(4—) is quite inert and is nontoxic. In contrast, hexacyanoferrate(3—) is toxic because it is more labile and cyanide dissociates readily. Both complexes Hberate HCN upon addition of acids. 

Ana.lytica.1 Methods. Various analytical methods involve titration with oxidants, eg, hexacyanoferrate (ferricyanide), which oxidize dithionites to sulfite. lodimetric titration to sulfate in the presence of formaldehyde enables dithionite to be distinguished from sulfite because aldehyde adducts of sulfite are not oxidized by iodine. Reductive bleaching of dyes can be used to determine dithionite, the extent of reduction being deterrnined photometrically. Methods for determining mixtures of dithionite, sulfite, and thiosulfates have been reviewed (365). Analysis of dithionite particularly for thiosulfate, a frequent and undesirable impurity, can be done easily by Hquid chromatography (366). 

Because of the time and expense involved, biological assays are used primarily for research purposes. The first chemical method for assaying L-ascorbic acid was the titration with 2,6-dichlorophenolindophenol solution (76). This method is not appHcable in the presence of a variety of interfering substances, eg, reduced metal ions, sulfites, tannins, or colored dyes. This 2,6-dichlorophenolindophenol method and other chemical and physiochemical methods are based on the reducing character of L-ascorbic acid (77). Colorimetric reactions with metal ions as weU as other redox systems, eg, potassium hexacyanoferrate(III), methylene blue, chloramine, etc, have been used for the assay, but they are unspecific because of interferences from a large number of reducing substances contained in foods and natural products (78). These methods have been used extensively in fish research (79). A specific photometric method for the assay of vitamin C in biological samples is based on the oxidation of ascorbic acid to dehydroascorbic acid with 2,4-dinitrophenylhydrazine (80). In the microfluorometric method, ascorbic acid is oxidized to dehydroascorbic acid in the presence of charcoal. The oxidized form is reacted with o-phenylenediamine to produce a fluorescent compound that is detected with an excitation maximum of ca 350 nm and an emission maximum of ca 430 nm (81). 

Important organic applications are to the determination of quinine and the vitamins riboflavin (vitamin B2) and thiamine (vitamin Bj). Riboflavin fluoresces in aqueous solution thiamine must first be oxidised with alkaline hexacyanoferrate(III) solution to thiochrome, which gives a blue fluorescence in butanol solution. Under standard conditions, the net fluorescence of the thiochrome produced by oxidation of the vitamin Bj is directly proportional to its concentration over a given range. The fluorescence can be measured either by reference to a standard quinine solution in a null-point instrument or directly in a spectrofluorimeter.27... 

Such free radicals may be stabilized by binding to proteins. Redox reactions may also occur between ionic species, for example the oxidation of reduced cytochrome c by hexacyanoferrate (ferricyanide) ions. 

Until now examples for catalytic reactions involving ferrates with iron in the oxidation state of -l-3 are very rare. One example is the hexacyanoferrate 8-catalyzed oxidation of trimethoxybenzenes 7 to dimethoxy-p-benzoquinones 9/10 by means of hydrogen peroxide which was published by Matsumoto and Kobayashi in 1985 [2]. Using hexacyanoferrate 8 product 9 was favored while other catalysts like Fe(acac)3 or Fe2(S04)3 favored product 10 (Scheme 2). The oxidation is supposed to proceed via the corresponding phenols which are formed by the attack of OH radicals generated in the Fe/H202 system. 

Copper nitrate reacts with sodamide and ammonia by forming explosive copper amides. The oxidising properties of this nitrate have led to violent detonations with ammonium hexacyanoferrates heated to 220 C in the presence of water traces, or dry at the same temperature, but in the presence of an excess of hexacyanoferrate. These accidents illustrate once more the incompatibility between compounds with a cyano group (or cyanide anion) and oxidants. An accident also occurred with a potassium hexacyanoferrate. 

Nickel hexacyanoferrate (NiHCF) films can be prepared by electrochemical oxidation of nickel electrodes in the presence of hexacyanoferrate(III) ions,141 or by voltammetric cycling of inert substrate electrodes in solutions containing nickel(II) and hexacyanoferrate(III) ions.142 NiHCF films do not possess low-energy intervalent CT bands, however, when deposited on ITO they are observed to reversibly switch from yellow to colorless on electroreduction.143... 

Electrocatalysis in oxidation has apparently first been shown for ascorbic acid oxidation by Prussian blue [60] and later by nickel hexacyanoferrate [61]. More valuable for analytical applications was the discovery in the early 1990s of the oxidation of sulfite [62] and thiosulfate [18, 63] at nickel [62, 63] and also ferric, indium, and cobalt [18] hexacyanoferrates. More recently electrocatalytic activity in thiosulfate oxidation was shown also for zinc [23] hexacyanoferrate. Prussian blue-modified electrodes allowed sulfite determination in wine products [64], which is important for the wine industry. 

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],... 

Nitric oxide (NO) and nitrite were found to be oxidized by Prussian blue and indium hexacyanoferrate-modified electrodes [75-77], For pharmaceutical application oxidation of isoprenaline [78] and vitamin B-6 [79] at cupric hexacyanoferrate-modified electrodes was shown. 

Cobalt hexacyanoferrates and Prussian blue have shown high activity in oxidations of hydrazine and hydroxylamine [80-82], Electrocatalytic activity in this reaction has also been found for nickel [83] and manganese [69] hexacyanoferrates. 

Ten years ago oxidation of NADH [84, 85] seems not to be important because of the other more powerful electrocatalysts [86-89], A possibility for oxidation of guanine even in DNA [90] with cobalt hexacyanoferrate, on the contrary, seems to be more apposite. 

S.-M. Chen, Electrocatalytic oxidation of thiosulfate by metal hexacyanoferrate film modified electrodes. J. Electroanal. Chem. 417, 145-153 (1996). 

D.M. Zhou, H.X. Ju, and H.Y. Chen, Catalytic oxidation of dopamine at a microdisc platinum electrode modified by electrodeposition of nickel hexacyanoferrate and Nafion. J. Electroanal. Chem. 408, 219-223 (1996). 

S.F. Wang, M.A. Jiang, and X.Y. Zhou, Electrocatalytic oxidation of ascorbic acid on nickel hexacyanoferrate film modified electrode. Gaodeng Xuexiao Huaxue Xuebao 13, 325-327 (1992). 

X.Y. Zhou, S.F. Wang, Z.P. Wang, and M. Jiang, Electrocatalytic oxidation of thiosulfate on a modified nickel hexacyanoferrate-film electrode. Fresenius J. Anal. Chem. 345, 424-427 (1993). 

P. Wang, X.Y. Jing, W.Y. Zhang, and G.Y. Zhu, Renewable manganous hexacyanoferrate-modified graphite organosilicate composite electrode and its electrocatalytic oxidation of L-cysteine. J. Solid State Electrochem. 5, 369-374 (2001). 

E. Casero, F. Pariente, and E. Lorenzo, Electrocatalytic oxidation of nitric oxide at indium hexacyanoferrate film-modified electrodes. Anal. Bioanal. Chem. 375, 294—299 (2003). 

S.M. Golabi and F. Noor-Mohammadi, Electrocatalytic oxidation of hydrazine at cobalt hexacyanoferrate-modified glassy carbon, Pt and Au electrodes. J. Solid State Electrochem. 2, 30-37 (1998). 

C.X. Cai, H.X. Ju, and H.Y Chen, Catalytic oxidation of reduced nicotinamide adenine dinucleotide at a microband gold electrode modified with nickel hexacyanoferrate. Anal. Chim. Acta 310, 145-151 (1995). 

A. Abbaspour and M.A. Mehrgardi, Electrocatalytic oxidation of guanine and DNA on a carbon paste electrode modified by cobalt hexacyanoferrate films. Anal. Chem. 76, 5690-5696 (2004). 

The method based on oxidation of dioximes is illustrated by the oxidation of l,l,4,4-tetramethoxy-2,3,5,6-tetrahy-droximinocyclohexane by an alkaline solution of potassium hexacyanoferrate(m) to give a mixture of isomers 284 and 285 (Equation 57) <1997CHE471>. 

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