Ferrous ferric ratio

Mossbauer spectra have been measured for various tektites, as well as for both natural and synthetic iron-bearing silicate minerals. These results are reported and compared with other similar studies available in the literature. The ratios of the intensities of the appropriate Mossbauer lines have been used to determine the ferric-ferrous ratios where possible. The spectra of the ferrosilite-enstatite series of pyroxenes show four lines which are interpreted as two quadrupole split doublets, and the ratio of the intensities of these lines indicates the degree of ordering in filling the available metal ion sites. Similar studies on the fayalite-forsterite series of olivines are also reported. 

The data from all of these spectra are compared in Table II, which lists the quadrupole splitting and the isomer shift (relative to iron in palladium) in millimeters per second and the ferric-ferrous ratio (as obtained from the Mossbauer data, by comparing the areas of the two sets of lines). The variations in the quadrupole splitting, ranging from 1.84 to 2.08 mm./sec., are certainly outside the range of experimental error and must be attributed to actual variations in the different tektites studied. The only type of tektite for which we measured many diflFerent samples was the indochinite, and the quadrupole splitting and isomer shift of these samples were the same for all samples within experimental error. Hence, these two parameters would be useful in verifying the classification of different types of tektites. The difficulties involved in... 

Holloway J. R., Pan V., and Gudmundsson G. (1992) High-pressure fluid-absent melting experiments in the presence of graphite oxygen fugacity, ferric/ferrous ratio and dissolved CO2. Euro. J. Mineral. 4, 105—114. 

Dyar MD, Delaney JS, Sutton SR (2000) Advances in interpretation of Fe XANES pre-edge spectra, and resultant improvements in microanalysis of ferric/ferrous ratios on thin sections. 31st Ann Lunar Planet Sci Conf... 

Only the Fe(II) oxidation state for iron forms the colored complex. Hence, this selectivity provides the basis for quantitatively determining the ferric/ ferrous ratio that characterizes the dissolved Fe portion of the iron analysis. Several organic compounds that are readily soluble in water and easily reduce Fe(III) to Fe(II) are available and include hydroquinone, ascorbic acid, and hydroxylamine hydrochloride, among others. 

The estimate for excess ferric iron in hard rocks is problematic, as we do not have good global sampling of the middle and deep crust. The estimates compiled by Lecuyer and Ricard [15] extrapolate near surface ferric/ferrous ratios [equivalent to an excess of 3% Fe203 (by mass)] downward to 35 km depth. This may lead to an overestimate as the fraction of rocks that have been exposed to oxidation at the surface should decrease with depth. 

Since probe is imable to distinguish valence states e g. ferric/ferrous ratio, some complimentary technique (e.g. Mossbauer) to be used when needed. 

Feldstein, S.N., Lang, R.A., Vennemann, T., O Neil, J.R. (1996) Ferric-ferrous ratios, H2O contents and D/H ratios of phlogopite and biotite from lavas of different tectonic regimes. Contrib. Mineral Petrol, 126,51-66. 

Roth, C.B., Jackson, M.L. and Syers, J.K., 1969. Deferration effect on structural ferrous-ferric iron ratio and C.E.C. of vermiculites and soils. Clays Clay Miner., 17 253-264. 

Redox status (largely controlled by oxygen level, ferrous/ferric ion ratio)... 

The solution potential is then controlled by the ratio of ferric to ferrous ions. 

Ferrous chloride-hydrochloric acid mixtures catalyzed the thermal decomposition of sulphonyl azides in isopropyl alcohol to give occasionally almost quantitative yields of sulphonamide and acetone, and the molar ratio of azide consumed to ferric chloride formed was typically of the order of 20 to 1 21>. 

The mechanism of iron-initiated superoxide-dependent lipid peroxidation has been extensively studied by Aust and his coworkers [15-18]. It was found that superoxide produced by xanthine oxidase initiated lipid peroxidation, but this reaction was not inhibited by hydroxyl radical scavengers and, therefore the formation of hydroxyl radicals was unimportant. Lipid peroxidation depended on the Fe3+/Fe2+ ratio, with 50 50 as the optimal value [19]. Superoxide supposedly stimulated peroxidation both by reducing ferric ions and oxidizing ferrous ions. As superoxide is able to release iron from ferritin, superoxide-promoted lipid peroxidation can probably proceed under in vivo conditions [16,20]. 

The ratio of ferrous to ferric species represents a redox state considerably less oxidizing than suggested by the dissolved oxygen content. The measured Eh falls between these values. Because the values vary over a range of more than 500 mV, this water clearly is not in redox equilibrium assuming that it is gives an incorrect distribution of iron species. 

One of the steps in the leaching and purification of zinc electrolyte is the oxidation of ferrous to ferric iron, with the subsequent precipitation of Fe(0H)3. Calculations (5 ) may be performed to determine the influence of pH and oxygen pressure on the final equilibrium ratio obtained for these two ions. 

Figure 11. Determination of ferrous-ferric isotope exchange kinetics in dilute aqueous solutions using Fe-enriched tracer solutions. Measured 5 Fe values for ferrous (squares) and ferric (circles) Fe in solution versus time. Initial 5 Fe values for Fe(II), 0%o and Fe(III),q 331%o. The rapid convergence in Fe/ Fe ratios for the ferric and ferrous species indicates that isotopic equilibrium is attained within minutes. Adapted from Welch et al. (2003).

Another type of non-spectral matrix effect, associated with the oxidation state of the analyte, was proposed by Zhu et al. (2002). Figure 14 plots the relative Fe(II) to total Fe ratio of ultra pure Fe standard solutions versus the difference between the 8 Fe value of the mixed valence state standard and the 5 Fe value of the Fe(III) only standard. The oxidation state of these standards was not quantified by Zhu et al. but based on colorimetric methods using 2,2 -bipyridine the relative Fe(ll) to total Fe ratios of these standards are well known. This matrix effect appears to exert a signihcant control on isotope accuracy, where for example if a reduced ferrous solution was compared to an oxidized ferric standard, the accuracy of the 5 Fe value could be affected by up to l%o. This matrix effect associated with oxidation state is unlikely to be a result of space charge effects because the mass of an electron is unlikely to produce a large change in the mass of the ion beam. Perhaps this matrix effect may be associated with ionization properties in the plasma. 

Figure 4. Isotopic mass-balance for measured 6 Fe values of ferric ( ) and ferrous ( ) Fe in solution as a function of Fe(II)/FeT ratios. Although the initial Fe isotope compositions of the fluids have 6 Fe values of 0%o (shown hy dotted mixing line), the lighter Fe isotopes are portioned into the ferrous species following attaimnent of isotopic equilihrium. The triangles (A) represent the calculated isotopic mass balance of the different solutions. Fractionations noted are measured values based on data for equimolar and higher Fe(II)/total Fe ratio experiments. (A) and (D) are for experiments with zero Cl at 22 and 0°C, respectively. (B) and (E) are experiments done with at 11 mM Cl at 22 and 0°C, respectively. (C) and (F) are experiments done with 111 mM Cl at 22 and 0°C, respectively. Modifled from Welch et al. (2003).

As the volume of ceric ion solution increases, so the ratio of ferrous ion to ferric ion will decrease, and hence the electrode potential pe3+,Fe2+ changes during the course of the titration. This is why a potentiometric titration can be followed by using the emf as the reaction variable. 

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