Speciation redox

Although Se and S are similar chemically, their redox speciation is different enough so that decoupling of Se from S can occur. This is illustrated in the Eh-pH stability diagrams for Se and S given in Figure 1. Under moderately reducing conditions, Se is stable as Se(I V) or Se(0), whereas S(tV) is not stable at all, and S(0) is stable only under a restricted set of conditions. Thus, Se may be separated from S if it is precipitated as Se(0), for example. 

Aquatic chemists have defined their own electrochemical standard state to fecilitate calculation of redox speciation in aqueous solutions. In this standard state, all reactions are conducted at pH 7.0, 25°C, and 1 atm. The concentrations of all other solutes are 1 molal (unless otherwise specifically noted). Values so obtained are designated with the subscript w. The pe s for the most important redox couples in seawater are given in Table 7.4. 

These values can be used to predict redox speciation in seawater as illustrated by the following example. Consider the half-reactions... 

Pourbaix diagrams Graphs that show redox speciation as a function of pH and either pe or i(j,. 

It is important to note that the application of electrochemical methods to the analysis of samples of art objects and archaeological artifacts allows much more than only simple identification of certain constituents advanced methods of speciation may provide information about constituents that are only slightly differing in then-composition, or for which there are only slight differences in the matrices in which the components are embedded. Further, redox speciation—and in the case of solid samples, phase speciation—can be used to derive information on production processes or corrosion (deterioration) of the components in the time that passed since their formation. The second part of this chapter is devoted to illustrating the capabilities of advanced speciation strategies. 

Spectrophotometric techniques combined with flow injection analysis (FIA) and on-line preconcentration can meet the required detection limits for natural Fe concentrations in aquatic systems (Table 7.2) by also using very specific and sensitive ligands, such as ferrozine [3-(2-bipyridyl)-5,6-bis(4-phenylsulfonic acid)-l,2,4-triazine], that selectively bind Fe(II). Determining Fe(II) as well as the total Fe after on-line reduction of Fe(III) to Fe(II) with ascorbic acid allows a kind of speciation.37 A drawback is that the selective complexing agents can shift the iron redox speciation in the sample. For example, several researchers have reported a tendency for ferrozine to reduce Fe(III) to Fe(II) under certain conditions.76 Most ferrozine methods involve sample acidification, which may also promote reduction of Fe(III) in the sample. Fe(II) is a transient species in most seawater environments and is rapidly oxidized to Fe(III) therefore, unacidified samples are required in order to maintain redox integrity.8 An alternative is to couple FIA with a chemiluminescence reaction.77-78... 

Pohl, P. and B. Prusisz. 2006. Redox speciation of iron in waters by resin-based column chromatography. Trends Anal. Chem. 25 909-916. 

Redox Couples. The model calculates the redox potential of the couples H2O2/O2, H2O/O2, Fe +/Fe , NO2/NO3, S /SO , and As " /As, given the requisite concentrations of the couple members. Dissolved oxygen is all that is required for calculation of both the H2O/O2 and H2O2/O2 couples. The H2O/O2 couple is kinetically inhibited and is grossly out of equilibrium except at elevated temperatures (80). Therefore, the option of using pE from dissolved oxygen for redox speciation has been dropped from the model. 

When analyzing corresponding reaction networks in more detail, as will be done for Mg afterwards (Section 2.2.12), the electrochemical ligand parameter and its changes must be considered once again to understand transformations. For now, this is not yet feasible for fluxes of a multitude of essential metals just because informations on the involved chelators are missing this precludes the kind of analysis done for Mg before, notwithstanding issues like redox speciation. As, moreover there is neither... 

Solid-phase speciation. While most speciation studies have been concerned with redox speciation in solution, speciation in the solid phase is also of interest. Both reduced and oxidized arsenic and selenium species can be adsorbed on minerals, sods, and sediments albeit with differing affinities (see Sections 9.02.5.3 and 9.02.7.2). Such adsorption has been demonstrated on metal oxides and clays and also probably takes place, to some extent, on carbonates, phosphates, sulhdes, and perhaps organic matter. Structural arsenic and selenium may also be characterized. 

The strong affinity of iron oxides for Se(IV) has been well documented (Dzombak and Morel, 1990) and calculations based on the Dzombak and Morel (1990) diffuse double-layer model and default HFO database show the principal response to pH and redox speciation changes (Figure 11). The selenate species is less strongly adsorbed by iron oxides at near-neutral pH than the selenite species (Figure 11). Clay minerals (Bar-Yosef and Meek, 1987) also adsorb Se(IV). 

Fuentes, E., Pinochet, H., De Gregori, L, and Potin-Gautier, M. (2003). Redox speciation analysis of antimony in soil extracts by hydride generation atomic fluorescence spectrometry. Spectrochim. Acta B 58, 1279-1289. 

The (metabolic) pathways of dietary vanadium, such as vanadate [H2V04], can be expressed as illustrated in Scheme 5.1 after oral uptake, vanadate reaches the gastrointestinal tract, where it is partially reduced and precipitated to vanadyl (VO ) hydroxides, which are excreted with the faeces. Another portion is absorbed and circulated in the blood, where it undergoes redox speciation and complexation by the serum proteins transferrin and albumin. Vanadate and vanadyl are finally incorporated into cells, mainly those of the liver, spleen and kidney. Excretion is achieved via the urine. Part of the vanadium is taken up by bones, where the mean retention time is comparatively long. 

Like iron complexes, copper complexes have been shown to be an important sink for photochemically-generated superoxide in seawater and, based on the high reactivity of Cu(ii) complexed by cyanobacterial-derived ligands, it is likely that redox reactions with superoxide significantly influence Cu redox speciation in the ocean [221,222]. These reactions also have important effects on the steady state concentrations of superoxide in seawater, reducing the concentration by at least an order of magnitude compared to previous estimates that ignored the reactions with copper complexes [222]. 

Antonio MR, Soderholm L, Williams CW, Blaudeau J-P, Bursten BE (2001) Neptunium redox speciation. Radiochim Acta 89 17-25... 

Choppin G R, Bond AH, Hromadka PM (1997) Redox speciation of plutonium. J Radio Nucl Chem 219 203-210... 

The extent of arsenic sorption in natural waters will be influenced by many factors, relating to both the sorbent and the water composition. As(V) and As(III) have different affinities for various sorbent phases that may be present in sediment, soils, and aquifers. Thus the redox speciation of arsenic and the characteristics of available sorbents will strongly affect the extent of arsenic sorption as will the pH and concenPations of co-occurring inorganic and organic solutes in the aqueous phase. Since sorption is a surface phenomenon and is limited by the availability of surface sites on the sorbing phase(s), the extent of competition between arsenic and other sorbates will depend not only on the affinity of each sorbate for the surface but also on their concentrations relative to each other and to the surface site concentration. Elevated concenPations of phosphate have been used to desorb arsenic from clays (51) and from soils contaminated with arsenical pesticides (113). 

Another application of solvent extraction to redox speciation studies is the method developed for iodine (Malmbeck and Skamemark 1995). It was used for online speciation of iodine in the Forsmark BWR power plant in Sweden. Iodine can enter the reactor water in two ways, either by fission of tramp uranium (uranium that is adsorbed on the outer surface of the fuel pins) or via leaking fuel pins. In this method, a tiny stream of water was withdrawn fi-om the reactor system and used as feed for a 20 stage mixer-settler battery. The mixer-settlers were arranged in four batteries with 4, 4, 6, and 6 mixer-settler units. Part of such a mixer-settler battery is shown in O Fig. 52.4. 

Seawater samples should normally be analysed immediately upon sampling to prevent loss of analyte such as due to changes in the redox speciation or adsorption on the container (see also Chapter 1). Adsorption on the sample bottles can be mostly prevented in stored samples by sample acidification using nitric or hydrochloric acid. Addition of nitric acid can lead to interference with the voltanunetric determination of chromium in seawater, so in that case hydrochloric acid should be used. The amount of acid should be minimized as it will have to be neutralized prior to the voltanunetric determination of most elements. Reagent use should generally be minimized to avoid sample contamination. 

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