Polynuclear hydrolysis species

The observed kinetic behaviour [62NAB] poses a general problem for identifying thermodynamically stable polymer solution species in an overall hydrolysis 

---

Actinide Colloids Actinide cations undergo hydrolysis in water. Hydrolysis is a step to polynucleation and thus to the generation of actinide colloids the polynuclear hydrolysis species become readily adsorbed to the surface of natural colloids. This also applies to Th, daughter of the primordial radionuclide... 

If either a mono- or a polynuclear hydrolysis species is dominant, the solubility... 

These two mechanisms, the one emphasizing the adsorption of specific often polynuclear hydrolysis products, the other emphasizing the role of polymeric species, are clearly not mutually exclusive an earlier study on thorium (IV) adsorption suggested a combined mechanism (J). 

It is well known that hydrolyzed polyvalent metal ions are more efficient than unhydrolyzed ions in the destabilization of colloidal dispersions. Monomeric hydrolysis species undergo condensation reactions under certain conditions, which lead to the formation of multi- or polynuclear hydroxo complexes. These reactions take place especially in solutions that are oversaturated with respect to the solubility limit of the metal hydroxide. The observed multimeric hydroxo complexes or isopolycations are assumed to be soluble kinetic intermediates in the transition that oversaturated solutions undergo in the course of precipitation of hydrous metal oxides. Previous work by Matijevic, Janauer, and Kerker (7) Fuerstenau, Somasundaran, and Fuerstenau (I) and O Melia and Stumm (12) has shown that isopolycations adsorb at interfaces. Furthermore, it has been observed that species, adsorbed at the surface, destabilize colloidal suspensions at much lower concentrations than ions that are not specifically adsorbed. Ottewill and Watanabe (13) and Somasundaran, Healy, and Fuerstenau (16) have shown that the theory of the diffuse double layer explains the destabilization of dispersions by small concentrations of surfactant ions that have a charge opposite to... 

See, for example, J. Steinhardt and J. A. Reynolds, Multiple Equilibria in Proteins, Academic Press, 1969 K. S. Murray, Binuclear oxo-bridged iron(III) complexes, Coord. Chem. Rev. 12 1 (1974) W. Schneider and B. Schwyn, The hydrolysis of iron in synthetic, biological, and aquatic media, pp. 167-196, in Aquatic Surface Chemistry, ed. by W. Stumm, Wiley, New York, 1987 and P. M. Bertsch, Aqueous polynuclear aluminum species, pp. 87 115 in The Environmental Chemistry of Aluminum, ed. by G. Spnsitn, CRC Press, Boca Raton, FL, 1989. 

A plot of p([Cu2 ]/[Cu7ot]) 3S a function of pH for three separate titrations fall on a single curve despite up to fivefold differences in measured dissolved copper concentration at a given pH (Figure 2). This behavior of the ratio [Cu2+]/[Cujot] is indicative of the formation of mononuclear hydrolysis species and excludes the possibility that the observed reduction in free cupric ion may have been caused by precipitation of Cu(0H)2 (solid) or the formation of polynuclear complexes. Analysis of data for p[Cu2+], pECujoj] and pH in the pH range 7.7 to 10.8 indicated the presence of two hydrolysis species (CuOH and Cu(0H)2) whose stability constants are given in Table I. Our value of the stability constant for the monohydroxo complex (106.48) falls... 

The establishment of hydrolysis equilibria is usually very fast, as long as the hydrolysis species are simple. Polynuclear complexes are often formed rather slowly. Many of these polynuclear hydroxo complexes are kinetic intermediates in the slow transition from free metal ions to solid precipitates and are tlius thermodynamically unstable. Some metal-ion solutions age, that is, tliey change their composition over periods of weeks because of slow structural transformations of the isopoly ions. Such nonequilibrium conditions can frequently be recognized if the properties of metal-ion solutions (electrode potentials, spectra, conductivity, light scattering, coagulation effects, sedimentation rates, etc.) depend on the history of the solution preparation. 

Multinuclear hydrolysis species usually are not observed during dissolution of the most stable modification of the solid hydroxide or oxide they are formed, however, by oversaturating a solution with respect to the solid phase. Such polynuclear species, even if thermodynamically unstable, may be of significance in natural water systems. Many multinuclear hydroxo complexes may persist as metastable species for years. 

Figure 6.5. Hydrolysis of Fe(III). (a) Distribution of Fe(lII) species in a 10" M Fe(III) solution as a function of pH. The solution is considered homogeneous [it is, in the neutral pH, range slightly oversaturated with respect to amorphous Fe(OH>3] (cf. Figure 6.8b). The concentrations of the multimeric species Fe2(OH)2 and FesfOH) are below 10" M. (b, c) Distribution diagrams for the various hydrolysis species in a hypothetically homogeneous 10" M and 10 M Fe(III) solution, respectively. Formation of the dimer and trimer increases with increasing Fe(III)io(. The shaded area indicates the approximate pH range of oversaturation with regard to Fe(OH)3(s). The polynuclear species occur in appreciable concentrations only when the solutions become oversaturated. It is thus plausible that these polynuclear species are intermediates in the formation of the solid phase.

An understanding of the mechanisms of aluminum hydrolysis and the formation of crystalline species of aluminum hydroxide has been viewed as important in various fields of pure and applied chemistry, biochemistry, and geochemistry. In part, this interest results from the unique properties of certain hydrolysis species of aluminum that appear to be present as polymeric or polynuclear macro-ions. These ions have a strong positive charge and may interact with specific charge sites on surfaces they encounter. The polymeric species also may grow by accretion, and they may persist melastably for months or years under some conditions (1). 

Popov [84POP] studied the complexation of zirconium(lV) by the sulphite ion and postulated the formation of the species Zr(0H)3803 in the pH range 0.54 to 1.44. The formation of this species was premised on the belief that Zr(OH)3 was the dominant species in the pH range studied. It is likely that in this pH range, and for the Zr concentration used in the experiments (0.085 - 1.801 x 10 M), that Zr(OH)( is not the only major Zr hydrolysis species. Further, it is also likely that a mixture of polynuclear and mononuclear species form (see discussion in Appendix D). As such, this review... 

Variations of the metal ion concentration (at a fixed hydrochloric acid concentration (0.2 M)) were used to indicate the formation of polynuclear zirconium hydrolysis species. There was insufficient data to allow an estimation of stability constants. 

Equilibrium constants of the hydrolysis reaction [eq, (32)] have been reported lor the trivalent lanthanides and tetravalent cerium but not for the divalent lanthanide cations. The latter would be even weaker acids due to a lower charge density than the Ln cations. A variety of mononuclear and polynuclear hydrolytic species have been reported and the values of the hydrolysis equilibrium constants in the literature are listed in tables 12 (mononuclear) and 13 (polynuclear). Baes and Mesmer (1976) have reviewed the hydrolysis reactions of the lanthanides so the scope of this review is limited to the studies which have appeared since the publication of their book. The species are discussed in terms of mononuclear species Ln(OH) , = 1 to 6) and polynuclear species... 

The smaller ionic radius also leads to extensive hydrolysis, which helps to explain why scandium compounds are more difficult to crystallize than those of the lanthanides and why the number of basic compounds is large. In aqueous solutions, depending on the pH, the Sc ion can be presented as [Sc(H20)g] +, [Sc(H20)50H] + and [Sc(H20)5(0H)]2 (Komissarova, 1980) there is also evidence for the formation of other polymeric species (Biedermann et al., 1956). The main product of polynuclear hydrolysis for Sc +, + as well as for In appears to... 

Partial hydrolysis leads to polynuclear hydrolysis products and condensed species in water [30], which precipitate for most metal ions. The formation of these species occurs through four major reactions ... 

There is a single study that has postulatedthe formation of a polynuclear hydrolysis product for trivalent europium. Bentouhami et /. (2004) indicated the formation of Eu2(OH)3 with the logarithm ofthe stability constant being -15.4 0.2. As there is no confirmatory evidence for the formation of this species, the datum is not retained in this review. 

The species resulting from the hydrolysis of hydrated cations such as those mentioned here are often highly complex, containing more than one metal atom (i.e. they may be polynuclear). The description here is simplified to show the essentials of the processes. 

Hydroxides. Thorium (TV) is generally less resistant to hydrolysis than similarly sized lanthanides, and more resistant to hydrolysis than tetravalent ions of other early actinides, eg, U, Np, and Pu. Many of the thorium(IV) hydrolysis studies indicate stepwise hydrolysis to yield monomeric products of formula Th(OH) , where n is integral between 1 and 4, in addition to a number of polymeric species (40—43). More recent potentiometric titration studies indicate that only two of the monomeric species, Th(OH) " and thorium hydroxide [13825-36-0], Th(OH)4, are important in dilute (<10 M Th) solutions (43). However, in a Th02 [1314-20-1] solubiUty study, the best fit to the experimental data required inclusion of the species. Th(OH) 2 (44). In more concentrated (>10 Af) solutions, polynuclear species have been shown to exist. Eor example, a more recent model includes the dimers Th2(OH) " 2 the tetramers Th4(OH) " g and Th4(OH) 2 two hexamers, Th2(OH) " 4 and Th2(OH) " 2 (43). 

The pH effect in Fenton reactions is due to the Fe speciation. In highly acidic solutions containing noncoordinating species, Fe exists as Fe(H20)f. The composition as function of the pH is represented in Figure 6.3a. At increasing pH, Fe undergoes hydrolysis forming FeOH +, Fe(OH)2+, and finally FeO via binuclear, Fe2(OH)4, and polynuclear species. The aim of optimization is to avoid Fe precipitation, either bi-, poly-, or precipitated Fe oxides that are aU inactive in... 

Hydrolysis of Iron(lll) in Acid Media - Formation of Polynuclear Species... 

A review of recent advances in chromium chemistry (82) supplements earlier comprehensive reviews of kinetics and mechanisms of substitution in chromium(III) complexes (83). This recent review tabulates kinetic parameters for base hydrolysis of some Cr(III) complexes, mentions mechanisms of formation of polynuclear Cr(III) species, and discusses current views on the question of the mechanism(s) of such reactions. It seems that both CB (conjugate base) and SVj2 mechanisms operate, depending on the situation. The important role played by ionpairing in base hydrolysis of macrocyclic complexes of chromium(III) has been stressed. This is evidenced by the observed order, greater... 

Last but not least of the liquid calorimetric media are aqueous solutions used in the hydrolysis of simple and complex fluorides. Stepwise replacement of F by OH occurs, and mixed products are not unusual. Thus the BFj ion hydrolyzes to species BF (OH)l and one has to ensure that the same product composition is formed in the auxiliary heat experiments (99). The problem is accentuated when polynuclear species form, as the equilibration can be slow. The inconsistencies in the heats of alkaline hydrolysis of MoF6 and WFe found by various authors and of the enthalpy of SbF5—derived by assuming SbF5 and Sb205 dissolved in 10 M HF produced the same species in solution—illustrate the difficulties. It is as well to confirm enthalpies of higher valent fluorides obtained by hydrolysis by alternative nonaqueous methods, especially since uncertainty in the Afl (Fderived enthalpy. The advantage of hydrolysis methods, apart from the simplicity of technique, is that the heats are small and one can tolerate... 

The vanadate (1, 2), molybdate (1-5), and tungstate (1-3) systems have been described in previous reviews. Although the focus in this chapter is on more recent developments, earlier well-established knowledge is included where needed for perspective and also to present a coherent picture of the hydrolysis behavior of these oxyanions. Equilibria of mono- and polynuclear species are described and information about known structures are given. Some recent work about mixed polyoxoanions is briefly reviewed. 

---