Speciation defined

Speciation is the occurence of an element in separate, identifiable forms and so the term chemical speciation defines the oxidation state, concentration, and composition of each of the species present in a chemical sample. Improved computing and analytical methods of late have led to an increased understanding of the exact speciation underlining many... 

A contaminant can occur in many different forms in an environmental compartment, each of them having different mobility, transfer coefficients to and between living matter, and even toxicity. This variety of existing forms for a given element is termed chemical speciation, defined as the distribution of one or more elements between all its possible species (distinct chemical entities) in a given system. It should be noted that the term speciation is also used in another context to describe all the experimental methods applied to investigate the above discussed species distribution. 

Normality is the number of equivalent weights (EW) per unit volume and, like formality, is independent of speciation. An equivalent weight is defined as the ratio of a chemical species formula weight (FW) to the number of its equivalents... 

The high concentration of oxygen in the atmosphere plays a central role in the photochemistry and chemical reactivity of the atmosphere. Atmospheric oxygen also defines the oxidation reduction potential of surface waters saturated with oxygen. The presence of oxygen defines the speciation of many other aquatic species in surface waters. 

The chemical speciation of the metal is defined as its distribution among different phases and different dissolved forms. When heavy metals enter aquatic... 

Principles and Characteristics The fastest growing area in elemental analysis is in the use of hyphenated techniques for speciation measurement. Elemental spe-ciation analysis, defined as the qualitative identification and quantitative determination of the individual chemical forms that comprise the total concentration of an element in a sample, has become an important field of research in analytical chemistry. Speciation or the process yielding evidence of the molecular form of an analyte, has relevance in the fields of food, the environment, and occupational health analysis, and involves analytical chemists as well as legislators. The environmental and toxicological effects of a metal often depend on its forms. The determination of the total metal content... 

A method for estimating the TSCF for equation 14.24 is given in Table 14.10. The root concentration factor is also defined in Table 14.10 as the ratio of the contaminant in the roots to the concentration dissolved in the soil water (pg/kg root per pg/L). This is important in estimating the mass of contaminant sorbed to roots in phytoremediation systems. The values of TSCF and RCF for metals depend on the metals redox states and chemical speciation in soil and groundwater. 

As discussed in Chapter 5 of this volume [104], chemical speciation can be defined as the physicochemical distribution of a chemical among all of its possible forms. In environmental systems, ligands range from simple ligands... 

The factors which control the distribution of trace elements [defined arbitrarily in geochemistry as those elements present at less than 0.1 weight percent (wt %)] can be discussed under a number of headings - structural, thermodynamic, kinetic and, in the sedimentary environment, solubility and speciation. 

Delhi soils by studying its speciation in the soil profile and to assess if there was any spatial variability. Soils representing the Aravali Ridge and the alluvial floodplains of river Yamuna were collected as a single, undisturbed core up to a depth of lm and the profile differentiated into four layers- 0-17 cm, 17-37 cm, 37-57 cm, and 57-86 cm. Pseudo total Aluminum and Iron in the soils were speciated into the operationally defined species (weakly exchangeable, organic matter complexes, amorphous oxides and hydroxides, and crystalline or free oxides) by widely recommended selective extraction procedures. Both A1 and Fe in these soils are bound predominantly as Fe oxides and silicates and have only very low percentages in the easily mobilizable pools. 

Iron in the soil samples was also speciated to yield the following operationally defined species of Fe ... 

The concentrations of the four A1 species occurring in each layer of the soil profile are expressed as a percentage of the A1 content of the specific soil layer. While Alpstot increased with depth of the profile, the contribution of the operationally defined A1 species towards speciation however decreased with depth (Figure 3a-d). The maximum differentiation of Alpstotas A1, A1, A1, and A1 was 8.51% which was observed in the surface layer while the mean contribution was 5.43 1.60%. On the contrary, in 37-57 cm depth which had the most abundant Alpstot, the mean share of the A1 species (2.72 0.75%) was the lowest contribution to A1 in all the layers of the soil profile. This implied that pseudo total A1 in these depths are predominantly bound as silicates and hence are not available for speciation under the experimental conditions. 

On the basis of the preceding discussion, it should be obvious that ultratrace elemental analysis can be performed without any major problems by atomic spectroscopy. A major disadvantage with elemental analysis is that it does not provide information on element speciation. Speciation has major significance since it can define whether the element can become bioavailable. For example, complexed iron will be metabolized more readily than unbound iron and the measure of total iron in the sample will not discriminate between the available and nonavailable forms. There are many other similar examples and analytical procedures that must be developed which will enable elemental speciation to be performed. Liquid chromatographic procedures (either ion-exchange, ion-pair, liquid-solid, or liquid-liquid chromatography) are the best methods to speciate samples since they can separate solutes on the basis of a number of parameters. Chromatographic separation can be used as part of the sample preparation step and the column effluent can be monitored with atomic spectroscopy. This mode of operation combines the excellent separation characteristics with the element selectivity of atomic spectroscopy. AAS with a flame as the atom reservoir or AES with an inductively coupled plasma have been used successfully to speciate various ultratrace elements. 

Model Studies. In model studies of adsorption, one deals with simple, well-defined systems, where usually a single well-characterized solid phase is used and the composition of the ionic medium is known, so that reactions competing with the adsorption may be predicted. It is not a trivial problem to compare the results from such model studies with those from field studies, or to use model results for the interpretation of field data. In field studies, a complex mixture of solid phases and dissolved components, whose composition is only poorly known, has to be considered competitive reactions of major ions and trace metal ions for adsorption may take place, and the speciation of the trace metal ions is often poorly understood. In order to relate field studies to model studies, distribution coefficients of elements between the dissolved and solid phases are useful. These distribution coefficients are of the following form ... 

At equilibrium, the reactant concentrations and products can be used to define a mass ratio called an equilibrium constant (A). This constant can then be used to predict the equilibrium concentrations of the reactants and products from the total amount of C or from either the equilibrium concentration of the products or the reactants. Although K is referred to as an equilibrium constant, it is a function of salinity, temperature, and pressure. With the appropriate value of K, calculations can be made to predict the equilibrium speciation of elements in seawater. The procedure for doing this is provided in the next section along with an expansion of K to multicomponent chemical systems. 

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. 

The actual form in which a contaminant molecule or ion is present in natural water, as result of a change in the coordinative relationship, emphasizes a specific chemical speciation. A chemical species is defined by lUPAC as the isotopic composition, electronic or oxidation state, and/or complex or molecular stracture, and the speciation of an element as the distribution of an element amongst defined chemical species in a system (Templeton et al. 2000). 

Power et al. (2005) show the effeet of pH and initial As(III) coneentration on the kineties of arsenite oxidation at bimessite-water interfaees, when a competitive metal (e.g., Zn) is present in an adsorbed or nonadsorbed state (Fig. 16.5). Two well-defined trends in the As(III) oxidation reactions can be distinguished (1) the extent of As(III) oxidation decreases with increasing pH from 4.5 to 6.0 and (2) oxidation on a percent basis is suppressed with increasing initial As(III) concentration from 100 to 300 dM. The pH effects on As(III) oxidation may have been influenced by competitive adsorption reactions between As(III) and reaction products (e.g., Mn(II)) and were not influenced by arsenic solution speciation. The suppressed As(III) oxidation rate constant may be a result of differences in the amount of Mn(II) release, which compete with dissolved As(III) species for unreacted Mn(IV) surface sites, and of Mn(II) adsorption, which inhibit the reaction between As(III) and Mn(IV) surface sites. 

From the point of view of chemical modeling, aqueous solutions are treated as electrolytic solutions —i.e., solutions in which solutes are present partially or totally in ionic form. Speciation is the name for the characteristic distribution of ion species in a given aqueous solution in the form of simple ions, ionic couplings, and neutral molecules. Solutes in aqueous solutions are defined as electrolytes and may be subdivided into nonassociated and associated. Nonassociated electrolytes are also defined as strong and mainly occur in the form of simple or simply hydrated ions. An example of a strong electrolyte is the salt NaCl, which, in aqueous solution of low ionic strength, occurs in the form of completely dissociated Na and CN ions. 

Among the above-defined thermodynamic entities, the individual ionic activity coefficients are particularly useful, because they allow practical calculation of the speciation state of an aqueous phase, linking individual ionic molalities to the energy balance. We will see in the following section how these coefficients may be derived. 

Ionic liquids in molecular solvents "Second-generation" or "modern" ILs are well defined cation-anion combinations that are liquid at room (or reaction) temperature. They normally consist of exactly one cation and one anion, and they are often air- and moisture-stable. Therefore, investigations on structure and speciation of this class of ILs have a different focus. In addition, the higher stability of these... 

It is not possible to prescribe specific pretreatment procedures here because these can only be decided upon when the system and the purpose of the experiments has been properly defined. However, a wealth of information exist in various biochemical reference books on how to isolate various biological compounds. The recommended techniques and methods could be used as part of the trace element speciation protocol often after slight modification, taking into consideration the following points First, the trace element blank levels have to be low, less than 10% of the total concentration in the sample. Second, the regents used should not interfere with subsequent analytical determinations. Third, the experimental conditions should not deviate markedly from those found in vivo, especially the pH and ionic strength of the medium. 

In spite of these limitations it is possible to obtain valuable data provided specific questions are posed. For example, it has been suggested that the level of methyl-mercury in a sample is a good indicator of the level of mercury toxicity Therefore it will be most useful to detect and quantify this species rather than to use the available resources to detect a wide range of mercury species, whose biological behaviour are yet to be established. It is essential to clearly define the purpose and use to which the information will be put before any speciation experiments are performed. This helps not only in the choice of the appropriate techniques and methods, but also helps to focus attention on the parameters that are useful for the interpretation of the results. 

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