Hexacyanoferrate anions

Prussian Blue was tentatively deposited onto isolated polycations by the well-known layer-by-layer (or multiple sequential adsorption) technique. The stepwise adsorption of hexacyanoferrate anions and ferric (or ferrous) cations was actually effective in fabricating PB films [12], In this work, the first adsorption step of hexacyanoferrate anions along positively charged PMB chains were successful as confirmed by the increase of the chain thickness by approximately 0.7 nm (Fig. la). 

Sol-gels containing electroactive species have been used in the development of both amperometric and potentiometric electrodes. Films coated with anionic poly-(dimethyldiallylammonium chloride) (PDMDAAC) and cationic poly(vinylsulfonic acid) were used to concentrate Ru(bpy)3 + and the hexacyanoferrate anion, respectively, for use as amperometric electrodes [208a]. The detection limit by square-wave voltammetry improved by up to 50-fold compared with uncovered electrodes. In Figure 41, curve 1 corresponds to a bare graphite electrode, curve 2 to a sol-gel-covered electrode and curve 3 to a sol-gel-PDMDAAC-modified electrode after 10 min of exposure to Fe(CN)g. ... 

Ocypa, M. Michalska, A. Maksymiuk, K. Accumulation of Cu(II) cations in poly(3,4-ethylenediox54hiophene) films doped by hexacyanoferrate anions and its application in Cu +-selective electrodes with PVC based membranes. Electrochim. Acta 2006, 51, 2298-2305. 

Distinction should be made between the value of the geometrical and the electrochemical (active) areas of an electrode. The meaning of geometric area is obvious. The electrochemical area should be computed on the basis of the response to a benchmark species in one of the techniques discussed in the following. Once the diffusion coefficient of the species chosen, typically one partner of a reversible redox couple, such as the hexacyanoferrate anions in water or bis(cyclopentadienyl)iron (II)—ferrocene—in organic solvent, is known, the ratio between the measured current and the expected current density constitutes a reliable estimate of the electrochemical area. The dependence of this area value on the exact nature of the electroactive species may be discarded as a first approximation, once poisoning of the electrode and the occurrence of unknown complex electrode mechanisms can be excluded. 

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. 

The Stock Oxidation-Number System. Stock sought to correct many nomenclature difficulties by introducing Roman numerals in parentheses to indicate the state(s) of oxidation, eg, titanium(II) chloride for TiCl2, iron(II) oxide for FeO, titanium(III) chloride for TiCl, iron(III) oxide for Fe203, titanium(IV) chloride for TiCl, and iron(II,III) oxide for Fe O. In this system, only the termination -ate is used for anions, followed by Roman numerals in parentheses. Examples are potassium manganate(IV) for K2Mn02, potassium tetrachloroplatinate(II) for K PtCl, and sodium hexacyanoferrate(III) for Na3Fe(CN)3. Thus a set of prefixes and terminations becomes uimecessary. 

N-Heterocyclics la 252 lb 268 Hexachlorocyclohexane lb 227 Hexachlorocyclohexane isomers lb 211 Hexacyanoferrate(II) anions lb 307 Hexacyanoferrate(ni) anions lb 307 n-Hexadecanol esters, alkaline hydrolysis la 63... 

These possess the dangerous reactions of the CN ion, or cyano group. The hexacyanoferrate (III) anion also has oxidising properties. Finally, it is thought to produce an extremely unstable acid in certain conditions. These salts also produce hydrogen cyanide, which is highly toxic, in an acid medium (see p.334). 

When it is heated above 196°C, the mixture of this compound with chromium trioxide combusts. When it is submitted to friction or impact, the same mixture detonates violently. The same happens if it is heated with sodium nitrite. Nitrite gives rise to a detonation with potassium hexacyanoferrate (II) too. The dangerous site of these complex anions is the cyano group. 

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. 

The other three degenerate stretching modes of hexacyanoferrate (right side in Fig. 9.35b) involve the iron atom together with four CN groups relative to the rest of the anion and have together, according to (9.13), the composition factor... 

Particular cases are potassium selective potentiometric sensors based on cobalt [41] and nickel [38, 42] hexacyanoferrates. As mentioned, these hexacyanoferrates possess quite satisfactory redox activity with sodium as counter-cation [18]. According to the two possible mechanisms of such redox activity (either sodium ions penetrate the lattice or charge compensation occurs due to entrapment of anions) there is no thermodynamic background for selectivity of these sensors. In these cases electroactive films seem to operate as smart materials similar to conductive polymers in electronic noses. 

In the complex [Co(NH3)6]Cl3, the cation is [Co(NH3)6]3+, and it is named first. The coordinated ammonia molecules are named as ammine, with the number of them being indicated by the prefix hexa. Therefore, the name for the compound is hexaamminecobalt(III) chloride. There are no spaces in the name of the cation. [Co(NH3)5C1]C12 has five NH3 molecules and one CN coordinated to Co3+. Following the rules just listed leads to the name pentaamminechlorocobalt(III) chloride. Potassium hexacyanoferrate(III) is K3[Fe(CN)6j. Reinecke s salt, NH4[Cr(NCS)4(NH3)2], would be named as ammonium diamminetetrathiocyanatochro mate (III). In Magnus s green salt, [Pt(NH3)4][PtCl4], both cation and anion are complexes. The name of the complex is tetraammineplatinum(II) tetrachloroplatinate(II). The compound [Co(en)3](N03)3 is named as tris(ethylenediamine)cobalt(III) nitrate. 

Unfortunately, many compounds contain bonds that are a mixture of ionic and covalent. In such a case, a formal charge as written is unlikely to represent the actual number of charges gained or lost. For example, the complex ferrocyanide anion [Fe(CN)6]4- is prepared from aqueous Fe2+, but the central iron atom in the complex definitely does not bear a +2 charge (in fact, the charge is likely to be nearer +1.5). Therefore, we employ the concept of oxidation number. Oxidation numbers are cited with Roman numbers, so the oxidation number of the iron atom in the ferrocyanide complex is +11. The IUPAC name for the complex requires the oxidation number we call it hexacyanoferrate (II). 

Iron(III) very readily forms complexes, which are commonly 6-coordinate and octahedral. The pale violet hexaaquo-ion [Fe(H20)6]3+ is only found as such in a few solid hydrated salts (or in their acidified solutions), for example Fe2(S04)3.9H20. Fe(C104)3.10H20. In many other salts, the anion may form a complex with the iron(III) and produce a consequent colour change, for example iron(III) chloride hydrate or solution, p. 394. Stable anionic complexes are formed with a number of ions, for example with ethanedioate (oxalate), C204, and cyanide. The redox potential of the ironll ironlll system is altered by complex formation with each of these ligands indeed, the hexacyanoferrate(III) ion, [Fe(CN)6]3. is most readily obtained by oxidation of the corresponding iron(II) complex, because... 

The oxidative behaviour of glycolaldehyde towards hexacyanoferrate(III) in alkaline media has been investigated and a mechanism proposed, which involves an intermediate alkoxide ion. Reactions of tetranitromethane with the luminol and luminol-peroxide radical anions have been shown to contribute substantially to the tetranitromethane reduction in luminol oxidation with hexacyanoferrate(III) in aerated aqueous alkali solutions. The retarding effect of crown ethers on the oxidation of triethylamine by hexacyanoferrate(III) ion has been noted. The influence of ionic strength on the rate constant of oxidation of ascorbic acid by hexacyanofer-rate(III) in acidic media has been investigated. The oxidations of CH2=CHX (where X = CN, CONH2, and C02 ) by alkaline hexacyanoferrate(III) to diols have been studied. ... 

Kinetic studies of hexacyanoferrate(III) oxidations have included the much-studied reaction with iodide and oxidation of the TICI2 anion, of hydrazine and hydrazinium, and of phenylhydrazine and 4-bromophenylhydrazine. These last reactions proceed by outer-sphere mechanisms, and conform to Marcus s theory. Catalyzed [Fe(CN)g] oxidations have included chlororuthenium-catalyzed oxidation of cyclohexanol, ruthenium(III)-catalyzed oxidation of 2-aminoethanol and of 3-aminopropanol, ruthenium(VI)-catalyzed oxidation of lactate, tartrate, and glycolate, and osmium(VIII)-catalyzed oxidation of benzyl alcohol and benzylamine. ... 

Such cyanide complexes are also known for several other metals. All the fer-rocyanide complexes may be considered as the salts of ferrocyanic acid H4Fe(CN)e and ferricyanide complexes are that of ferricyanic acid, H3Fe(CN)e. The iron-cyanide complexes of alkali and alkaline-earth metals are water soluble. These metals form yellow and ruby-red salts with ferro-cyanide and ferricyanide complex anions, respectively. A few of the hexa-cyanoferrate salts have found major commercial applications. Probably, the most important among them is ferric ferrocyanide, FeFe(CN)e, also known as Prussian blue. The names, formulas and the CAS registry numbers of some hexacyanoferrate complexes are given below. Prussian blue and a few other important complexes of this broad class of substances are noted briefly in the following sections ... 

To prepare metal hexacyanoferrate films, very frequently the following procedure was followed first a film of the respective metal, for example, cadmium [79], copper [80], silver [81], or nickel [82, 83] was elec-trochemically plated on the surface of a platinum electrode, and that was followed by chemical oxidation of the metal film in a solution of K3[Fe(CN)6], leading to the formation of the metal hexacyanoferrates. The same method has been used to produce films of nickel hexacyanoruthen-ate and hexacyanomanganate using the appropriate anions [83]. It is also possible to perform the oxidation of the deposited metals in solutions containing hexacyano-ferrate(II) by cyclic oxidation/reduction of the latter. In a similar way, films of copper heptacyanonitrosylferrate have been deposited [84]. 

Zinc hexacyanoferrate was supported on the surface of Ti(IV) oxide grafted on a silica gel [476]. Electrochemical behavior of this electrode was dependent on alkaline metal cations and acetate anion concentrations. 

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