Speciation diagrams

Fig. 6. Speciation diagram of plutonium as function of Eb and pH in aqueous solution at 25°C (a) carbonate-free (89) (b) 0.004 M total carbonate (90). Eb...

From a structural point of view Bruno s hypothesis is sound. In chemical terms the hydrolysis proceeds with the formation of hydroxide bridges up to the formation of the trimer. Then at high concentrations trimeric units are fused together to form the higher aggregates, that can be seen as precursors to the structure adopted by Be(OH)2 in the solid state. (See Section IV, C, precipitation equilibria). Speciation diagrams for Bruno s model are shown in Fig. 3. Two points are noteworthy in these diagrams. In the first place the concentrations of the... 

Between pH values of ca. 6 and 12 aqueous solutions hold very little dissolved beryllium because of the low solubility of Be(OH)2. When the pH is raised above 12, the hydroxide begins to dissolve with the formation of, first, Be(OH)3 and then, at even higher pH values, Be(OH) (52). The presence of these species in strongly alkaline solutions was confirmed by means of solvent extraction experiments (90) and infrared spectroscopy (31). A speciation diagram is shown in Fig. 7, which was constructed using the values of log /33 = 18.8 and log /34 = 18.6 critically selected from Table III. The diagram illustrates clearly the precipitation and dissolution of Be(OH)2. 

The fluoride ion is the only inorganic ligand to form a complete substitution series, Be(H20)4 flFJ(2 1 (n = 1-4), though there is considerable variation in the equilibrium constants that have been reported. The most reliable values are probably those of Anttila et al. (117) who used both glass and fluoride-ion selective electrodes and also took account of the competing hydrolysis reactions. They did not, however, make measurements in the conditions where BeF2 would have been formed. A speciation diagram based on reported equilibrium constants is shown in Fig. 12. It can be seen that the fluoride ion competed effectively with hydroxide at pH values up to 8, when Be(OH)2 precipitates. 

In a final example of the use of a sliding activity path, we calculate a speciation diagram, plotted versus pH, for hexavalent uranium in the presence of dissolved phosphate at 25 °C. We take a 10 mmolal NaCl solution containing 1 mmolal each ofUO +, the basis species for U(VI), and HPO4... 

Fig. 14.12. Speciation diagram at 25 °C for a 1 mmolal solution of hexavalent uranium containing 1 mmolal dissolved phosphate, calculated as a sliding activity path.

Fig. 2 Speciation diagram for 2.0 x 10" 3 M La3+ in methanol as a function of pH. Data superimposed on the figure as ( ) are second-order rate constants for La3 +-catalyzed methanolysis of p-nitrophenyl acetate (2) as a function of pH.

Figure 4.13 Distribution (speciation) diagram (----) for the system H+/[Fe(CN)e]4 /[27]aneN9 and...

The lower limit of e for which self-assembly may be realistically studied is e 4, since at this efficiency the self-assembled complex constitutes more than 90 % of the species present over a concentration range of only 102 mol dm-3. Even then, it is in equilibrium with significant amounts of oligomers. This may be readily appreciated from the speciation diagram for the tetramer 10.8 (Figure 10.14). 

Primary sources documented after each chapter minimize the need to search the literature, 80 illustrations provide structural information, reaction schemes, spectra, speciation diagrams, and biochemical schemes, and 22 tables present detailed information with references to primary sources. Packed with current and authoritative information, the book covers chemistry and bioinorganic vanadium chemistry in a broad and systematic manner that engenders comprehensive understanding. 

The knowledge of both thermodynamic constants at zero ionic strength and of the specific interaction coefficients will allow the speciation diagram of the element in the considered medium to be established. At higher electrolyte concentrations, more sophisticated theories (Pitzer, MSA,. .. [29,31]) have been developed. However, they involve a larger number of characteristic parameters, which unfortunately are unknown for the majority of chemical elements. 

In addition to speciation diagrams on a percent species basis (Section 2.1.4), solubility diagrams can be plotted on a logarithmic scale of molar concentration. For example, in the case of Al(OH)3s, a mineral which regulates aluminum in the solution of many soils, the solubility can be expressed as follows ... 

Figure 12.10. Iron Eh-pH (or pe-pH) speciation diagram with Cy = 2 x 1Q 3 and soluble metal 10 5 mol L 1 (adapted from Stumm and Morgan, 1981, with permission).

Figure 12.16. Speciation diagram of selenium (selenium = 1 pmol L ) (from Neal et a]., 1987, with permission).

Control of the particle valence/conduction band oxidation/reduction potential is not only achieved through a judicious choice of particle component material band edge redox thermodynamics of a single material are also affected by solution pH, semiconductor doping level and particle size. The relevant properties of the actinide metal are its range of available valence states and, for aqueous systems, the pH dependence of the thermodynamics of inter-valence conversion. Consequently, any study of semiconductor-particle-induced valence control has to be conducted in close consultation with the thermodynamic potential-pH speciation diagrams of both the targeted actinide metal ion system and the semiconductor material. 

Figure 3 Speciation diagrams ofberyllium hydroxo species, (a) Cbc = 0.2 M, (b) Cse = 0.002 M...

Figure 4 Speciation diagram of berylliiun fluoro complexes in solution. Cse = 0.25 M Cp = 0.5 M...

Appendix Speciation Diagrams of Metallic Ions in Aqueous Solutions 378... 

Chromium exists in aqueous solution primarily as Cr(ll) (quite unstable and easily oxidized), Cr(III) (stable over a wide range of conditions), or Cr(VI) (stable in strongly oxidizing conditions). In natural water streams the principal trivalent Cr species are CrOH and Cr(OH), [122], while the principal hexavalent species are HCrO and Cr04- . Trivalent Cr has a propensity to form the mononuclear Cr(OH)-", Cr(OH) , and Cr(OH)4" species as well as the neutral Cr(OH),(aq). At room temperature, the rate of formation of the polynuclear species Cr (OH)4 " and Cr (OH)4 " is quite low [123], The speciation diagrams of both trivalent and hexavalent chromium have been reproduced recently by Leyva-Ramos and coworkers [124,125] see also Figure A.3. 

The speciation diagram for uranium has been presented recently by Qadeer and Saleem [246] and Park et al. [247], Uranium hydrolysis begins at pH ca. 3, the... 

Arsenic is a commonly occurring toxic impurity in natural ecosystems [299]. It also pollutes the waters from the chemical industry s effluents as well as from mine tailings. The speciation diagrams of As(lll) [299,300] and As(V) [299,300,46] indicate that the predominant trivalent species is H,AsOi (except... 

APPENDIX SPECIATION DIAGRAMS OF METALLIC IONS IN AQUEOUS SOLUTIONS... 

Here we assume that the hydrolysis products are exclusively mononuclear and that the solution concentrations do not allow precipitation to occur (see Table Al). Speciation diagrams can thus be constructed from knowledge of global hydrolysis constants P which govern the formation of mononuclear hydroxides. For example, if a metallic cation M forms several hydroxide species M(OH) - M(OH) "-, M(OH), - , . . . , M(OH),/- " , the corresponding... 

Thus, for example, the speciation diagrams for Zn(ll), Hg(II), and Cr(lII) can be obtained from their respective global hydrolysis constants [J. Kragten, Atlas of Metal-Ligand Equilibria in Aqueous Solution, New York John Wiley, 1978], and this is shown in Figs. A1-A3. 

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