Speciation in Solution

The terms components and species in chemistry have specific meanings different from those in common parlance. There are a limited number of components, which define the composition of the solution. Species are chemical entities that actually occur in the solution. Each species can be obtained as a product of chemical reaction from components, but a component cannot be obtained as a product of chemical reaction from other components. For example, in aqueous NaCl, there are two components, namely water and NaCl, and numerous species, namely water and the ions H iaq), OH (aq), Na taq), and Cr(aq). The above list of species is usually sufficient to describe the behavior of dilute solutions, but it is not complete. Specific problems may require consideration of other species, such as ion pairs in aqueous NaCl. The concept of components and species was originally introduced in solution chemistry, but it can also be used in surface 

Components are present in solution in the form of different species. The concentrations of these species depend on the concentrations of all components in the system. Metal cations form aquo complexes and other complexes in which one or more water molecules are replaced by ligands other than water. This problem is discussed in basic handbooks of inorganic and analytical chemistry. Speciation in simple systems can be easily calculated when the stability constants of particular species are available. Specialized software that facilitates calculation of speciation in more complex systems is available. Many errors and misinterpretations related to speciation in solution can be found in the literature. 

SijOjfOipj and as monomeric SifOi i )4. The higher polymeric species are less abundant. The fraction of polymeric species increases with pH and with silica concentration. 

The literature data on stability constants of various species are not consistent, for the reasons explained above. Ligand-exchange reactions in solution have various rates for example, in Cr(iii) complexes, they are very slow. Thus, the system of interest is not necessarily in equilibrium with respect to these reactions, even with vigorous stirring and long equilibration times. 

Results reported in secondary sources (publications reporting previously published results) were avoided in the present compilation, and attempts were made to access the primary source. This process often consisted of many steps, because what appeared to be a primary source often happened to be another secondary source. Secondary sources were chiefly used to acquire information about primary sources. The results from PhD theses, conference proceedings, and publications using non-Latin alphabets are often cited after secondary sources when the original source was not accessible or difficult to understand. It was not always easy to distinguish between primary and secondary sources. In a few papers, it was not clear if they reported an original pi I,), with the reference being cited only for a method, or if the reference was also cited for the value of pH,. Such problems could not be solved without inspection of the reference. 

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The As(OH)4 and Cr+++ components follow a pattern distinct from the other metals (Fig. 14.11), sorbing at only near-neutral pH. This pattern results from the manner in which the metals speciate in solution. As(III) appears as As(OH)3 when pH is less than 9, and as As(OH)4, or As020H at higher pH. The sorption reactions for these species are,... 

AN EMPIRICAL AND MODELING STUDY OF BORON SPECIATION IN SOLUTION WITH A REACTIVE DENDRIMERIC POLYMER... 

We present here the initial modeling efforts and experimental determinations of boron speciation in solution with a reactive polymer. Also, the speciation model is applied to the ultrafiltration process and compared with data obtained from PAUF of a synthetic boron-contaminated feed. 

Birnessite (8-Mn02) Condensation of glucose and glycine under soil ambient conditions (measurement of optical density) yields of humic substances XANES study of change in speciation of Mn ESR study of Mn speciation in solution 13C CPMAS NMR spectra of FA fraction resembling spectra of natural FAs Jokic et al. (2001b)... 

The extent of speciation in solution depends on the stoichiometric coefficients of the components of a species the polyvalent nature and protonation behaviour of anionic complexing ligands the type and relative ability of different cations and anions to form complexes pH ionic strength, and the ratio of the total concentrations of the reactants in solution (the total cation anion ratio). 

The pK of formation of a species can have a significant effect on the variation of the extent of speciation with the total cation anion ratio in solution (Fig. 9.3). For weak complexes (p K 2), there is no effect of total cation anion ratio on speciation in solution at constant pH (Fig. 9.3(a)). If the anion complexes the cation moderately (pK 4), however, the fraction of total cation complexed by the ligand increases almost linearly as the cation anion ratio varies from 1 1 to 1 1000 (Fig. 9.3(b)). In the case of strong complexation (pK 6), species... 

Table 9.3 The effect of ionic strength on speciation in solution...

Speciation in solution is considered a major factor in the mobilisation and leaching of metal cations (DeKoninck, 1980 Bloomfield, 1981 Stevenson and Fitch, 1986). Complexation increases the total soluble concentration of a metal and hence increases its potential to be leached. Organic ligands (e.g. humate, ful-vate, citrate, polyphenols) are the major complexers involved in this mechanism, but they are effective only if the soluble organic complex does not become saturated and precipitate (DeKoninck, 1980). 

Inorganic speciation in solution can also affect the mobility of metal ions (Doner, 1978). The formation of an ion-pair with Cl can more than double the mobility of Cd in the presence of 200molm 3NaCl. At the same chloride concentration, however, the mobilities of Cu2+ and Ni2+ are only increased slightly (5-10%), presumably because of very weak complexation with Cl. This mechanism could increase the leaching of Cd from saline soils but it may not be effective in non-saline soils because the ratio of the total concentrations of Cd Cl must be >1 106 before >50% of total Cd is complexed by Cl (estimated using the computer model TITRATOR (Cabaniss, 1987), which considered the chloro and hydroxy complexes of Cd at pH 5.0 and a total Cd concentration of 0.1 mmolm-3 equilibrium constants were taken from Lindsay (1979)). 

The free ion, Cu2+, appears to be the major species of Cu taken up by plants (Graham, 1981 Jones and Jarvis, 1981). Hence Cu complexation will decrease uptake (DeKock and Mitchell, 1957) unless the complex can dissociate and/or diffuse quickly enough to maintain a constant supply of Cu2+ at the root surface. In the case of Zn, the presence of humic acid (Chen and Aviad, 1990) and carboxylic acids (EDTA, diethylenetriamine pentaacetic acid (DTPA), nitrilotriacetic acid (NTA) DeKock and Mitchell, 1957) has been found to decrease absorption but it is not known whether Zn uptake is correlated with Zn2+ in solution because Zn speciation was not estimated. Other work has shown that Zn initially complexed with citrate is taken up by barley from nutrient solutions (Chairidchai and Ritchie, 1993) and the presence of chelates (EDTA, citrate) can speed up the diffusion of Zn to a root surface in soils (Hodgson et al., 1967 Elgawhary et al., 1970). Speciation in solution is particularly important in the uptake of iron because of its extremely low solubility in the absence of complexing anions... 

Therefore, thermodynamics plays a fundamental role in supramolecular chemistry. However, thermodynamics is rigorous and as such, a great deal of ancillary information is required prior to the formulation of an equation representative of the process taking place in solution, such as, the composition of the complex and the nature of the speciation in solution. For the latter and when electrolytes are involved, knowledge of the ion-pair formation of the free and complex salts in the appropriate solvent is required particularly in non-aqueous solvents. This information would allow the establishment of the concentrations at which particular ions are the predominant species in solution. Similar considerations must be taken into account when neutral receptors are involved, given that in dipolar aprotic or inert solvents, monomeric species are not always predominant in solution. In addition, awareness of the scope and limitations of the methodology used for the derivation of thermodynamic data for the complexation process is needed and this aspect has been addressed elsewhere [18]. 

Although, in some cases the composition of the anion complex could be obtained from 1H NMR data, this is not universally found. Therefore, the composition of the anion complex and the nature of speciation in solution are often established through conductance measurements and some representative examples on the systems investigated are discussed in the next section. 

According to International Union of Pure and Applied Chemistry (IUPAC), the terms speciation and chemical species should be reserved for the forms of an element defined as to isotopic composition, electronic or oxidation state and/or complex or molecular structure (Templeton el al, 2000). This classical definition, appropriate to speciation in solution samples, would exclude most speciation studies on solid materials, such as soils and sediments, more properly defined as fractionation studies. The terminology used in this chapter is based on the broader definition of speciation given by Ure and Davidson (2002), which encompass the IUPAC s narrow definition and includes the selective extraction and fractionation techniques of solid samples. 

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. 

Uranium solubility is increased even more in the nitrate microcosms, and one possible explanation is the conversion of acetate to CO2, coupled with nitrate reduction, which would give higher dissolved carbonate concentrations in the nitrate microcosms. However, PHREEQE modelling showed that the higher CO concentration would not greatly affect uranium speciation in solution and is therefore unlikely to account for the enhanced solubility. Alternatively, as nitrate is reduced to ammonium (NH/), which promotes cation exchange, this could lead to displacement of U02 from surface complexes, which are the predominant uranyl species on mineral surfaces. " ... 

Once fluorescence intensity changes are related to metal ion speciation in solution, a conditional formation constant and mass balance equations may be employed to define a final nonlinear relationship between K, Cl and fluorescence intensity signals. 

Possible mechanisms of binding to the settling particles should be examined, taking into account the speciation in solution of Cu and Zn. An intriguing relationship exists between total dissolved copper, free copper ion, and copper bound in particles. The ratio of [Cu2+] to total dissolved Cu is about 10 610 As determined from the concentration in settling particles and in the water column, the distribution coefficients for Cu are Kd = 1-5 X 104 L/kg, with variations over the year. 

Algae may thus influence both the total dissolved and the free aquo ion concentrations of Cu and Zn in the water column of the lake by binding in the settling material or by releasing complexing ligands into solution. These two effects are linked to each other because binding in the particulate phase, either by uptake or by adsorption, also depends on the speciation in solution. 

Leckie [14] emphasized the advantage of chemical speciation over overall distribution coefficients in adsorption modeling. On the other hand, in many theoreticar studies of adsorption even the speciation in solution is neglected and only the total concentration of dissolved species is taken into account. One probable reason of paying no attention to well-known experimental facts is that some authors use adsorption equations borrowed from gas adsorption, and obviously these equations are not suitable to deal with multiple solution species involving the adsorbate. 

Functionally defined speciation. Functionally defined species are exemplified by the plant-available species or chemical pools in which the function is plant availability. Available forms of trace metal cations are not necessarily associated with one particular chemical species or a specific soil component. Hence, to predict the availability of trace metals, we either have to establish the species involved and develop methods that specifically determine those forms only, or we have to establish an empirical relationship between an accepted diagnostic measure of the metal and plant growth. Both speciation in solution and fractionation of the solid phase to identify the chemical pools can affect plant uptake (phytoavailability) of trace metals and water pollution. 

CCM) (Stumm et al., 1970, 1976, 1980 Schindler et al., 1976), the triple-layer model (TLM) (Davis and Leckie, 1978, 1980 Davis et al., 1978 Hayes and Leckie, 1987 Hayes et al., 1988), and the 1 pK basic Stem model (Bolt and van Riemsdijk, 1982 Van Riemsdijk et al., 1986, 1987). The application of many of the commonly used computer models in the determination of the speciation in solution phase has been dealt with exhaustively by Lumsdon and Evans (1995). 

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