Iron hydrolysis constant

The solubility and the hydrolysis constants enable the concentration of iron that will be in equilibrium with an iron oxide to be calculated. This value may be underestimated if solubility is enhanced by other processes such as complexation and reduction. Solubility is also influenced by ionic strength, temperature, particle size and by crystal defects in the oxide. In alkaline media, the solubility of Fe oxides increases with rising temperature, whereas in acidic media, the reverse occurs. Blesa et al., (1994) calculated log Kso values for Fe oxides over the temperature range 25-300 °C from the free energies of formation for hematite, log iCso fell from 0.44 at 25 °C to -10.62 at300°C. 

In most cases such solubility diagrams are calculated from solubility constants which have been evaluated for acidic or basic solutions. The results obtained by extrapolating such data to other regions should be regarded with much reserve. Figure 4 shows that for iron (III) hydroxide the observed solubility is considerably higher than that expected from solubility and hydrolysis constants. At log [H+] = —8.1, the calculated value is log[Fe(OH)2 ] = —10.85, whereas the observed value is log [FetotnI] = —7, which is approximately the iron concentration in sea... 

TABLE 10.3. Iron(III) Hydrolysis Constants from Selected Literature Sources ... 

Millero (1985) based his kinetic model on the assumption that the first step in the sequential reduction of Oa to H20 is rate-limiting. Since the hydrolysis constants of ferrous iron are p/Cf =9.5 and pp 20.6, we may approximate the speciation of Fe(II) in these laboratory experiments the following way ... 

Equilibrium and rate constants for the hydrolysis and chloride complexation of Fe(III) and Fe(II) ions are necessary in a detailed study of iron redox chemistry. Table I lists an internally consistent set of values for the relevant equilibrium constants. Their accuracy is discussed later in the context of a brief sensitivity analysis of the data. The rates of iron hydrolysis and chloride complexation reactions are also mentioned. 

Indeed, numerous experimental studies have been performed to study the evolution of the enviromnent in a crevice. Most data were obtained in actively corroding artificial crevices or pits, either by sampling the solution or by direct pH and Cr concentration measurements. Table 1 summarizes significant results that confirm the foregoing trends. In crevice solutions, the drop of pH depends on the hydrolysis constants of the metal cations. On stainless alloys, chromium and molybdenum are considered to be the cause of the very low, sometimes negative, pH observed. Iron, nickel, and aluminiun exhibit much less acidic hydrolysis reactions and the pH values in the crevices are higher values of 3 to 5 are reported for iron, values of 3 to 4 for the aluminum alloys. 

The extent of hydrolysis of (MY)(n 4)+ depends upon the characteristics of the metal ion, and is largely controlled by the solubility product of the metallic hydroxide and, of course, the stability constant of the complex. Thus iron(III) is precipitated as hydroxide (Ksal = 1 x 10 36) in basic solution, but nickel(II), for which the relevant solubility product is 6.5 x 10 l8, remains complexed. Clearly the use of excess EDTA will tend to reduce the effect of hydrolysis in basic solutions. It follows that for each metal ion there exists an optimum pH which will give rise to a maximum value for the apparent stability constant. 

The second-order redox reaction, giving rise to the rate constant k2, is accompanied also by loss of the iron(II) complex by hydrolysis, which leads to the /tj term. The latter can be more accurately measured in the absence of Tl(III). The kinetics of substitution of many square-planar complexes conform to behavior (c), see Sec. 4.6. It is important to note that an intercept might be accurately defined and conclusive only if low concentrations of B are used. In the base catalyzed conversion... 

Structures have been determined for [Fe(gmi)3](BF4)2 (gmi = MeN=CHCF[=NMe), the iron(II) tris-diazabutadiene-cage complex of (79) generated from cyclohexanedione rather than from biacetyl, and [Fe(apmi)3][Fe(CN)5(N0)] 4F[20, where apmi is the Schiff base from 2-acetylpyridine and methylamine. Rate constants for mer fac isomerization of [Fe(apmi)3] " were estimated indirectly from base hydrolysis kinetics, studied for this and other Schiff base complexes in methanol-water mixtures. The attenuation by the —CH2— spacer of substituent effects on rate constants for base hydrolysis of complexes [Fe(sb)3] has been assessed for pairs of Schiff base complexes derived from substituted benzylamines and their aniline analogues. It is generally believed that iron(II) Schiff base complexes are formed by a template mechanism on the Fe " ", but isolation of a precursor in which two molecules of Schiff base and one molecule of 2-acetylpyridine are coordinated to Fe + suggests that Schiff base formation in the presence of this ion probably occurs by attack of the amine at coordinated, and thereby activated, ketone rather than by a true template reaction. ... 

Figure 2 Diagrammatic summary of selected structural, substituent, and solvent effects on rate constants (kj, at 298 K) for base hydrolysis of low spin iron(II)-diimine complexes. Ligand abbreviations not appearing in the list at the end of this chapter are apmi = (73) with = Me BOH cage = (78) with X = OH ...

Such equilibria and their stability constants are summarized in Table 9.1. The total concentration of dissolved iron (Fe-r) at any pH is given by the sum of the concentrations of the free metal iron and all the soluble hydrolysis species, i. e. 

Dang, M.-Z. Rancourt, D.G. Dutrizac J.E. La-marche, G. Provencher, R. (1998) Interplay of surface conditions, particle size, stoichiometry, cell parameters, and magnetism in synthetic hematite-like materials. Hyperfme Interactions 117 271-319 Daniele, P.G. Rigano, C. Sammartano, S. Zeland,V. (1994) Ionic strength dependence of formation constants - XVIII. The hydrolysis of iron(III) in aqueous KNOj solutions.Ta-lanta41 1577-1582... 

Metastability of Hydrolyzed Iron (III) Solutions The low solubility of ferric hydroxide has been alluded to in the Introduction. Feitknecht and Michaelis (29) have observed that aU ferric perchlorate solutions to which base has been added are unstable with respect to eventual precipitation of various forms of hydrated ferric oxides. In 3 M NaC104 at 25° C the two phase system reaches an apparent equilibrium after 200 hours, according to Biedermann and Schindler (6), who obtained a reproducible solubility product constant for ferric hydroxide at varying degrees of hydrolysis. It appears that many of the solutions used in the equilibrium studies of Hedstrom (9) and Biedermann (22) were metastable, and should eventually have produced precipitates. Nevertheless, since the measured potentials were reversible, the conclusions reached about the species present in solution remain valid. 

These solutions have been examined in sedimentation velocity runs on the analytical ultracentrifuge (31). Beyond 0.5 base equivalent per mole of iron a fairly narrow sedimentation peak developed. The sedimentation coefficient, 7 1 S, was essentially constant up to 2.5 base equivalents per mole of iron, although the area under the peak increased with increasing degree of hydrolysis. Apparently, then, hydrolysis of ferric nitrate beyond the reversible equilibrium region produces increasing amounts of a fairly discrete high polymer whose size is constant. 

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