Quantitative aqueous speciation

IONIC SPECIATION. Ions interact continually in aqueous solution. Ions are complexed with water molecules. Even when we say that a certain ion is uncomplexed, the fact is that the ion is still complexed, in this case with water molecules. Association constants (also known in the literature as stability or formation constants) allow one to quantitate the extent to which an ion is complexed with any particular substance in solution. They also allow comparisons of the relative affinity of different complex-ing agents for a particular chemical substance. Speciation is a chain of linked binding functions (see Fig. 3). Such diagrams show the relative concentrations of the various complexes in solution, and the reversible equilibria existing between these pools are shown by the arrows. 

The existence of an element in different chemical forms in the gaseous, solid or aqueous solution phase provides the conceptual basis for speciation in soils. More particularly, a chemical species in soil refers either to a specific molecular arrangement of the atoms of an element or, quite often, to the result of an operational process of detection and quantitation aimed at elucidating chemical forms (Bernhard et at., 1986, pp. 7-14). In principle, the former definition should be the outcome of the latter, methodological definition. In practice, this connection is difficult to achieve in natural systems (Bernhard et al., 1986) (see Chapter 1 for a definition of speciation). Understanding speciation is important in assessing the availability of plant nutrients, plant uptake of potentially toxic elements (e.g. Al, Cd), and the movement of both nutrient and toxic substances into waterways or other parts of an ecosystem (Da Silva et al., 1991). 

Methods involve extractions of analytes into organic solvents, as well as treatments with acidic or basic reagents. Solid-phase extraction can be used for removal and pre-concentrations of analytes in aqueous solutions. Applications of low-power focused microwave technology have been investigated as a means of dissolution, and good results have been reported for extractions of organometal-lic compounds of tin and mercury (Schmitt et al., 1996 Szpunar et al., 1996). Analyses of CRMs were used for verification. The time necessary for quantitative isolations of the analytes was greatly reduced, e.g. 24 h to 5 min. In addition, there were reductions in solvent volumes, and improvement in analyte recoveries. Some of the analytical procedures for speciation of particular elements such as mercury, described later in this chapter, include microwave-assisted sample preparation. 

A rapid and simple MW-assisted digestion method with alkaline solution (TMAH or methanolic KOH solution) was developed for speciation analysis of inorganic Hg and methyl-Hg in biological tissues [41]. Extracts with quantitative recoveries of Hg species after the alkaline dissolution of the sample were directly analyzed by an automated on-line hyphenated system incorporating aqueous HG, cryogenic trapping, GC, and detection by A AS. The proposed method was validated by the analysis of biological CRMs (CRM 463, DORM-1, TORT-1) and one BCR sample from an interlaboratory study (Tuna Fish 2). 

The use of GC-MIP-AES is advantageous because it avoids the predecomposition step required in the AAS detection mode. The first applications of the MIP-AES detector for Hg speciation and detection were reported in the 1970s [27-29]. Despite the overall good detection ability of the detectors, however, most of the above methods require large sample volumes, tedious solvent extraction procedures, and usually lead to the final determination of only the Me-Hg species. The description of the feasibility of quantitative in situ aqueous ethylation of Hg2-1- and Me-Hg followed by on-line preconcentration and detection by atomic fluorescences pectro-metry (AFS) or AAS certainly produces a wealth of information since it allows all Hg species to be detected in the same chromatographic run. Also on-line speciation of Hg and Me-Hg by chromatography-AFS hydride generation (HG) was used [30]. 

Methods of Speciation and Fractionation. It is apparent that in order to understand the mobility of arsenic and its availability for reactions, methods of speciation and fractionation must be applied to sediment samples in field and laboratory studies. In this paper speciation refers to the separation and quantitative determination of inorganic arsenic, methanearsonic acid, and cacodylic acid. Compartmentalization involves identifying the major compartments for arsenic in a heterogeneous system (e.g. aqueous, adsorbed, occluded,...) and determining the amounts of arsenic in each compartment. Fractionation involves the extraction of arsenic from operationally defined fractions of the solid phase of an aquatic system (e.g. sediment). 

For polynuclear complexes of aluminum, Al-NMR spectroscopy has been used extensively to characterize the structure of the complexes as well as the speciation of the aqueous fluids. The characteristics of NMR spectroscopy—nucleus specific, quantitative intensities, and sensitivity to only short-range structure—combined with the high natural abundance of K make this a powerful technique at millimolar concentrations. However, the quadrupolar nature of the K nucleus (spin number I = 5/2) introduces some complications in spectral interpretation that are worth mentioning here. 

Using liquid/liquid phase separation by thenoyltrifluoracetone, TTA, in benzene, the authors studied the speciation of Zr in the concentration range 10 to 0.1 M in 2 M perchloric acid solution as well as in 1 M HCIO4/I M LiC104 solutions. TTA is known to selectively extract free tetravalent ions such as Zr or the tetravalent actinide ions. Polymer formation by Zr in the aqueous phase is reflected quantitatively as a decrease in the distribution coefficient. The experiments were conducted very carefully spectroscopic determination of species in the benzene phase, correction for TTA loss of the benzene phase, consideration for complexation of Zr by TTA in the aqueous phase, recrystallisation of starting solids, consideration of impurities in the test, assurance that equilibrium has been reached and discussion of errors related to the variation of proton activity in the aqueous phase due to the extraction reaction. 

Attempts to quantitatively determine the extent of ionic dissociation of all relevant species including macroradicals and polymer molecules and to correlate such speciation with the variations observed for kp is difficult, if not impossible, in view of the complex acid-base properties and polyelectrolyte behavior as well as the coupled electrochemical equilibria. Studies into polyelectrolyte behavior in aqueous solution carried out so far, have been performed at conditions precisely defined with respect to solvent composition, ionic strength, concentration regime, and molecular weight. These conditions differ from the ones met in the actual free-radical polymerization experiments presented in Figure 3 and in Reference Despite this complexity, it has been reaUzed that with... 

Chau et al pointed out that as the authenticity of the compounds to be analyzed must be preserved, any of the digestion methods with acids or alkalis are not suitable, and that extraction seemed to be the method of choice for removing these compounds from samples. For this traction, they adopted benzene as recommended by Sirota and Uthe for the quantitative extraction of tetramethyllead and tetraethyllead from fish homogenates suspended in aqueous EDTA solution. Although ionic forms of lead such as Pb(II), diethyllead dichloride, and trimethyllead acetate do not extract in the benzene phase, any lead compounds that distribute into the benzene phase as tetraalkyllead will be determined. Chau et al421 found that environmental samples can contain other forms of organolead compounds that are extractable into benzene but which are not volatile enough to be analyzed by the GC-AAS technique, hence the need for a speciation specific analytical system. 

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