Iron acid—base speciation

The boundary between all oxidized forms and all reduced forms of a substance can be drawn from Equation (18) by expanding Q (Equation (17)) to include acid/base speciation. Figure 16.1 shows this for five substances that exhibit moderately complex, but well characterized, speciation as a function of pH (uncomplexed Fe(II)/Fe(III), iron porphyrin, juglone, lawsone, and anthraquinone disulfonate). The resulting Eh-pH diagram shows, for example, that the hydroquinone of lawsone is a reductant relative to anthraquinone disulfonate, below pH 7.5, but the relationship is inverted at higher pH. A similar crossing... 

Figure 10.2. Acid-base speciation of (a) citrate, (b) aluminum, and (c) iron(III) as a function of solution pH at 25°C and in 0.01 mol L NaNOs solutions. The distributions were computed using a total soluble citrate (citys) concentration of 10 mol and total soluble aluminum (Aljs) and iron(in) [Fe(ni)Ts] concentrations controlled by gibbsite (A1(OH)3(x)) and goethite [FeOOH(x)]. (Relevant thermodynamic data from May and Murray, 2000 Baes and Mesmer, 1986 Liu and Millero, 1999.)...

The fact that the ion activities are measured rather than concentrations is ambivalent from the point of view of practical measurements. It is a great advantage for speciation purposes and for study of acid—base complexation and precipitation equilibria in solution. On the other hand, it is a drawback when the total content of an analyte in a sample is to be foimd, as great care must be taken to compensate or correct for interactions of the analyte with the sample matrix and the ambient atmosphere during calibration and measurement itself (cf. the problem of complexation of fluoride with iron(III) and alumi-num(III) ions in analyses of natural waters, or oxidation of sulfide by atmospheric oxygen). 

In operationally defined speciation the physical or chemical fractionation procedure applied to the sample defines the fraction isolated for measurement. For example, selective sequential extraction procedures are used to isolate metals associated with the water/acid soluble , exchangeable , reducible , oxidisable and residual fractions in a sediment. The reducible, oxidisable and residual fractions, for example, are often equated with the metals associated, bound or adsorbed in the iron/manganese oxyhydroxide, organic matter/sulfide and silicate phases, respectively. While this is often a convenient concept it must be emphasised that these associations are nominal and can be misleading. It is, therefore, sounder to regard the isolated fractions as defined by the operational procedure. Physical procedures such as the division of a solid sample into particle-size fractions or the isolation of a soil solution by filtration, centrifugation or dialysis are also examples of operational speciation. Indeed even the distinction between soluble and insoluble species in aquatic systems can be considered as operational speciation as it is based on the somewhat arbitrary definition of soluble as the ability to pass a 0.45/Am filter. 

Building upon a study that was performed for the Western States Petroleum Association (WSPA) [1] to optimize iron-based selenium removal processes, a scheme was developed to measure total selenium, and the individual selenium species present in the SSW and aqueous streams. Total selenium was measured by ICP-MS after a peroxide/acid digestion pretreatment of the samples. Speciation of the selenium was accomplished using an anionic ion chromatographic separation followed by quantification of the selenite, selenate, and selenocyanate using ICP-MS. 

Iron speciation of Fe(II) and Fe(III) is reported more often than any other speciation. The methods are based on cation exchange, anion exchange, and ion-pairing chromatography. Only a few of the methods are discussed here. Saitoh and Oikawa [11] simultaneously determined Fe(ll) and Fe(lll) by postbathophenanthrolinedisulfonic acid and ascorbic acid. This procedure was found to be successful in spring water samples. Moses et al. [12] reported the determination of Fe(ll) and Fe(lll) in water samples. 

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