Iron oxide absorption coefficient

Inspection of the table shows that the quotient a/Wj e is in fact nearly constant that I changes much less rapidly than W e] and that the critical depth has doubled when the highest oxide is reached. All three conditions are reflections of the (positive) absorption effect that occurs in this binary system when iron is replaced by oxygen, which has a lower mass absorption coefficient. 

Opaque minerals like iron oxides are frequently examined in the reflectance mode - and usually give diffuse reflectance spectra. Reflectance spectra provide information about the scattering and absorption coefficients of the samples and hence their optical properties. The parameters of reflectance spectra may be described in four different ways (1) by the tristimulus values of the CIE system (see 7.3.3) (2) by the Kubelka-Munk theory and (3) by using the derivative of the reflectance or remission function (Kosmas et al., 1984 Malengreau et ak, 1994 1996 Scheinost et al. 1998) and, (4) more precisely, by band fitting (Scheinost et al. 1999). 

If over the region of interest, the scattering coefficient hardly varies with wavelength, the shapes of the remission spectrum and the absorption spectrum should be very similar. The relationship between the remission function and the reflectance spectrum is shown in Figure 7.2 left, and the Kubelka-Munk functions of the different iron oxides are illustrated in Figure 7.2, right. 

Figure 5. Absorption coefficient K for diffuse illumination as a function of the pigment volume concentration for three red iron oxide pigments...

The light absorption coefficient of black pigments determines their optical quality. Their color intensity and hiding power depend on the particle size and particle size distribution. The most important black pigments are carbon blacks, iron oxides (Section 3.1.1), and mixed metal oxides (Section 3.1.3). 

A filler such as titanium dioxide, having a refractive index much higher than that of the polymer, will therefore exhibit considerable scattering and will give white products, unless its absorption coefficient is high in which case dark, opaque composites will be produced (carbon black or black iron oxide are the ultimate in this instance). 

Fig. 6. Difference spectra between xanthine oxidase inactivated with various pyra-zolo [3, 4-d] pyrimidines and the native enzyme. The spectra are believed to represent the increase in absorption occurring when Mo(VI) of native enzyme is converted to Mo(IV) complexed with the inhibitors. Spectra were obtained by treating the enzyme with inhibitors in the presence of xanthine, then admitting air, so as to re-oxidize the iron and flavin chromophores. The extinction coefficients, de, are expressed per mole of enzyme flavin. Since some inactivated enzyme was present, extinction coefficients per atom of molybdenum of active enzyme will be about 30% higher than these values. (Reproduced from Ref. 33, with the permission of Dr. V. Massey.)...

A band of this type has been observed for an enzyme-substrate complex ES where the enzyme was represented by the oxidized form of peroxidase cytochrome c, cyt(Fe(III)) and the substrate was the reduced form of cytochrome c, cytj (Fe(II)) [298]. Indeed, on mixing the solution of cyt(Fe(I I)) and cytj (Fe(II)) there appeared a new absorption band with the absorption maximum at Emax = 1.4 eV, the extinction coefficient e = 0.35 M-1 cm-1, and the width a = 0.2 eV. This band was referred [298] to charge transfer via electron tunneling, [cyt(Fe(III))/ cyt, (Fe(II))] -> [cyt(Fe(II))/cytl(Fe(III))]. From a comparison of the data on the intensity of this band with the results of fluorescence measurements, the distance between the iron atoms Fe(III) and Fe (II) in the [cyt(Fe(III))/cyt1(Fe(II))] complex has been estimated to be R 15-20 A and the edge-to-edge tunneling distance Rt = 7 A. 

Properties Colorless, mobile liquid becomes yellowish under the action of light and air. Fruitlike odor (high dilution). Decomposed by water. Attacks brass but not iron (dry). D 1.742 (14C), bp 156C (decomposes), fp —65C, coefficient of thermal expansion 0.0011, vap d 6 (air = 1.29), volatility 20,000mg/m3(20C), vap press 2.29 mm Hg (21.5C). Soluble in alcohol, benzene, ether, and water. Derivation Chlorination of ethyl arsenious oxide. Hazard Toxic by ingestion, inhalation, and skin absorption strong irritant. 

The purple acid phosphatases can occur in two diferric forms—one as the tightly bound phosphate complex (characterized for the bovine and porcine enzymes) (45, 171, 203) and the other derived from peroxide or ferricyanide oxidation of the reduced enzyme (thus far accessible for only the porcine enzyme) (206). These oxidized forms are catalytically inactive. They are EPR silent because of antiferromagnetic coupling of the two Fe(IIl) ions and exhibit visible absorption maxima near 550-570 nm associated with the tyrosinate-to-Fe(III) charge-transfer transition. The unchanging value of the molar extinction coefficient between the oxidized and reduced enzymes indicates that the redox-active iron does not contribute to the visible chromophore and that tyrosine is coordinated only to the iron that remains ferric in agreement with the NMR spectrum of Uf, (45). 

The main absorption band of benzoquinones appears around 260 nm in nonpolar solvents and at 280 nm iu water. Extinction coefficients are 1.3-1.5 x 10 M Upon reduction to hydroquinones, a four times smaller band at 290 nm is found. The most important property of quinones and related molecules is the relative stability of their one-electron reduction products, the semiquinone radicals. The parent compound 1,4-benzoquinone is reduced by FeCl, ascorbic acid, and many other reductants to the semiquinone anion radical which becomes protonated in aqueous media (pk = 5.1). Comparisons of the benzaldehyde reduction potential with some of the model quinones given below show that carbonyl anion radicals are much stronger reductants than semiquinone radicals and that ortho- and para-benzoquinones themselves are even relatively strong oxidants comparable to iron(III) ions in water (Table 7.2.1). This is presumably caused by the repulsive interactions between two electropositive keto oxygen atms, which are separated only by a carbon-carbon double bond. When this positive charge can be distributed into neighboring n systems, the oxidation potential drops significantly (Lenaz, 1985). 

Figure 5 shows that the absorption spectrum of the tetrachloroferrate ion, with a characteristic maximum at 3150 A., is completely developed in such a solution. The dotted line is the spectrum of an oxidation mixture. The spectrum resembles that of a solution of FeC, but the characteristic maximum at 3600 A. appears to be shifted to 3580 A., and the extinction coefficient at the latter wavelength is lower than that at 3150 A. The result suggests that the principal iron (III) species in the oxidation mixtures is Cl3Fe( OCH3)". Figure 6 shows the effect of metallic chlorides on the spectrum of methanolic solutions of ferric chloride. 

The absorption spectrum of purified cytochrome b displayed maxima at 560 nm, 417 nm, and 278 nm in the air-oxidized form and at 561 nm, 530 nm, and 429 nm in the dithionite-reduced form (Fig. 13). The ratio of the absorbance of the protein part at 278 nm to the absorbance of the heme part at 417 nm was 0.72. Iron determination gave the following molar absorbance coefficients ... 

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