Oxidation of Fe

In corrosion, adsorbates react directly with the substrate atoms to fomi new chemical species. The products may desorb from the surface (volatilization reaction) or may remain adsorbed in fonning a corrosion layer. Corrosion reactions have many industrial applications, such as dry etching of semiconductor surfaces. An example of a volatilization reaction is the etching of Si by fluorine [43]. In this case, fluorine reacts with the Si surface to fonn SiF gas. Note that the crystallinity of the remaining surface is also severely disrupted by this reaction. An example of corrosion layer fonnation is the oxidation of Fe metal to fonn mst. In this case, none of the products are volatile, but the crystallinity of the surface is dismpted as the bulk oxide fonns. Corrosion and etching reactions are discussed in more detail in section A3.10 and section C2.9. 

In aqueous solution, the oxidation of Fe can be conceived as a reaction of two aquo-complexes of the fomi... 

Let us consider tire oxidation of Fe(s) to Fe (solvated), which can be described by tire following reaction sequence [36, 40] ... 

In this titration the analyte is oxidized from Fe + to Fe +, and the titrant is reduced from CryOy to Cr +. Oxidation of Fe + requires only a single electron. Reducing CryOy, in which chromium is in the +6 oxidation state, requires a total of six electrons. Conservation of electrons for the redox reaction, therefore, requires that... 

Since the current due to the oxidation of H3O+ does not contribute to the oxidation of Fe +, the current efficiency of the analysis is less than 100%. To maintain a 100% current efficiency the products of any competing oxidation reactions must react both rapidly and quantitatively with the remaining Fe +. This may be accomplished, for example, by adding an excess of Ce + to the analytical solution (Figure 11.24b). When the potential of the working electrode shifts to a more positive potential, the first species to be oxidized is Ce +. 

End Point Determination Adding a mediator solves the problem of maintaining 100% current efficiency, but does not solve the problem of determining when the analyte s electrolysis is complete. Using the same example, once all the Fe + has been oxidized current continues to flow as a result of the oxidation of Ce + and, eventually, the oxidation of 1T20. What is needed is a means of indicating when the oxidation of Fe + is complete. In this respect it is convenient to treat a controlled-current coulometric analysis as if electrolysis of the analyte occurs only as a result of its reaction with the mediator. A reaction between an analyte and a mediator, such as that shown in reaction 11.31, is identical to that encountered in a redox titration. Thus, the same end points that are used in redox titrimetry (see Chapter 9), such as visual indicators, and potentiometric and conductometric measurements, may be used to signal the end point of a controlled-current coulometric analysis. For example, ferroin may be used to provide a visual end point for the Ce -mediated coulometric analysis for Fe +. 

Coulometric Titrations Controlled-current coulometric methods commonly are called coulometric titrations because of their similarity to conventional titrations. We already have noted, in discussing the controlled-current coulometric determination of Fe +, that the oxidation of Fe + by Ce + is identical to the reaction used in a redox titration. Other similarities between the two techniques also exist. Combining equations 11.23 and 11.24 and solving for the moles of analyte gives... 

Mixed oxides of Fe(IV) can be prepared by heating iron(III) oxide with a metal oxide or hydroxide in oxygen at elevated temperatures. These black compounds have general formulas M FeO, M monovalent, or M2Fe04, M divalent, but do not contain discrete [FeOJ" ions. They are readily decomposed by mineral acids to iron(III) and oxygen. 

The pale blue tris(2,2 -bipyridine)iron(3+) ion [18661-69-3] [Fe(bipy)2], can be obtained by oxidation of [Fe(bipy)2]. It cannot be prepared directiy from iron(III) salts. Addition of 2,2 -bipyridine to aqueous iron(III) chloride solutions precipitates the doubly hydroxy-bridged species [(bipy)2Fe(. t-OH)2Fe(bipy)2]Cl4 [74930-87-3]. [Fe(bipy)2] has an absorption maximum at 610 nm, an absorptivity of 330 (Mem), and a formation constant of 10. In mildly acidic to alkaline aqueous solutions the ion is reduced to the iron(II) complex. [Fe(bipy)2] is frequentiy used in studies of electron-transfer mechanisms. The triperchlorate salt [15388-50-8] is isolated most commonly. 

The unstable pale blue-green bis(2,276, 2 -terpyridine)iron(3+) ion [47779-99-7], [Fe(terpy)2], has been obtained by oxidation of [Fe(terpy)2]. It is very unstable with respect to reduction by solvent and ligand dissociation. The perchlorate salt [2153642-5] has been reported. 

Fig. 1. The H. C. Starck pyrometallurgical upgrading process of Ta—Nb-containing tin slags where the soHd vertical bars represent electrodes. The product of (a) electrothermic reduction of Sn slag goes to (b) oxidation of Fe alloy 20—60 Ta + Nb and then to (c) electrothermic reduction of oxidized Fe ahoy.

The oxidation of Fe(CN)g to Fe(CN)g by peroxidisulfate, SjOg", can be monitored spectrophotometrically by observing die increase in absorbance at 420 nm, D 2o well-mixed batch system. Assume dial die kinetic scheme is ... 

Mixed oxides of Fe such as M FeO and M FeO can be prepared by heating Fe203 with the appropriate oxide or hydroxide in... 

Further similarity with Mn may be seen in the fact that the vast majority of the compounds of Fe are high-spin. Only ligands such as bipy and phen (already mentioned) and CN , which are high in the spectrochemical series, can induce spin-pairing. The low-spin [Fe(CN)g] , which is best known as its red, crystalline potassium salt, is usually prepared by oxidation of [Fe(CN)6]" with, for instance, CI2. It should be noted that in [Fe(CN)6] the CN ligands are sufficiently labile to render it poisonous, in apparent contrast to [Fe(CN)6]" , which is kinetically more inert. Dilute acids produce [Fe(CN)5(H20)] , and other pentacyano complexes are known. 

In summary for non-metal transfer situations chemical thermodynamics is a useful guide to probable behaviour. The transfer of a non-metal, X, dissolved in a molten metal, M to another metal M", will depend on the relative free energies of formation of M X and M X (see Section 7.6). Thus sodium will give up oxygen to Zr, Nb, Ti and U, as the free energy of oxide formation of these metals is greater than that for sodium on the other hand, sodium will remove oxygen from oxides of Fe, Mo and Cu unless double oxides are formed. 

Oxidation of Fe by H+. Finely divided iron in the form of steel wool reacts with hydrochloric... 

Stoichiometrically, the total quantity of electricity passed is exactly the same as it would have been if the Fe(II) ions had been directly oxidised at the anode and the oxidation of Fe(II) proceeds with 100 percent efficiency. The equivalence point is marked by the first persistence of excess Ce(IV) in the solution, and may be detected by any of the methods described above. The Ce3+ ions added to the Fe(II) solution undergo no net change and are said to act as a mediator. 

Ammonium carbamate NH2Fe2(OH)4(CO3)2-H20 Hydrated basic ferric The transport of pre-boiler corrosion debris to the boiler section will includes these minerals. Also the oxides of Fe, Cu, Ni, Zn, and Cr as... 

A diffuse-layer minimum in C,E curves has not been found with electrodes kept 3 min at E = -0.74 V, i.e., at a potential close to the rest potential of Fe.728 Complete cathodic reduction at <<-0.74 V (SCE) is not achieved since a diffuse-layer minimum is not found for cathodically reduced electrodes. This effect has been explained by the oxidation of Fe. According to impedance data, strong specific adsorption of Cl anions at renewed Fe electrodes occurs since a very large shift of Eosq takes place going from KF to KC1 solutions. 

Transport in solution or aqueous suspension is the major mechanism for metal movement from the land to the oceans and ultimately to burial in ocean sediments. In solution, the hydrated metal ion and inorganic and organic complexes can all account for major portions of the total metal load. Relatively pure metal ores exist in many places, and metals from these ores may enter an aquatic system as a result of weathering. For most metals a more common sequence is for a small amount of the ore to dissolve, for the metal ions to adsorb onto other particulate matter suspended in flowing water, and for the metal to be carried as part of the particulate load of a stream in this fashion. The very insoluble oxides of Fe, Si, and A1 (including clays), and particulate organic matter, are the most important solid adsorbents on which metals are "carried."... 

The kinetics of the oxidation of Fe(CN)6 - by permanganate have been examined in phosphate buffers over the pH range 1.6-6.3. The stoichiometry... 

RATE PARAMETERS FOR THE OXIDATION OF Fe(ll) BY VARIOUS EDTA AND HEDTA... 

Wood and Higginson " have made a detailed study of the kinetics of oxidation of Fe(ll) by a number of complexes of Co(IIl) with ethylenediaminetetraacetic acid (H4Y = EDTA) and hydroxyethylethylenediaminetriacetic acid (H3YOH = HEDTA). Rate data and activation parameters are quoted (Table 21) for the... 

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