Hafnium aqueous chemistry

Zirconium [7440-67-7] is classified ia subgroup IVB of the periodic table with its sister metallic elements titanium and hafnium. Zirconium forms a very stable oxide. The principal valence state of zirconium is +4, its only stable valence in aqueous solutions. The naturally occurring isotopes are given in Table 1. Zirconium compounds commonly exhibit coordinations of 6, 7, and 8. The aqueous chemistry of zirconium is characterized by the high degree of hydrolysis, the formation of polymeric species, and the multitude of complex ions that can be formed. 

The chemistry of hafnium has not received the same attention as that of titanium or zirconium, but it is clear that its behaviour follows that of zirconium very closely indeed with only minor differences in such properties as solubility and volatility being apparent in most of their compounds. The most important oxidation state in the chemistry of these elements is the group oxidation state of +4. This is too high to be ionic, but zirconium and hafnium, being larger, have oxides which are more basic than that of titanium and give rise to a more extensive and less-hydrolysed aqueous chemistry. In this oxidation state, particularly in the case of the dioxide and tetrachloride, titanium shows many similarities with tin which is of much the same size. A large... 

The oxidation states and stereochemistries of zirconium and hafnium are summarized in Table 18-A-l. These elements, because of the larger atoms and ions, differ from Ti in having more basic oxides, having somewhat more extensive aqueous chemistry, and more commonly attaining higher coordination numbers, 7 and 8. They have a more limited chemistry of the III oxidation state. 

There are several complete compilations of the literature concerning zirconium and hafnium that take the reader up to about 1960 62, 344, 420, 558). Since then several reviews of more limited scope have been published, one on the structural aspects of zirconium chemistry 116), and others on the separation of zirconium and hafnium 578), aqueous chemistry 234, 533), and ion-exchange properties of zirconium compounds 29). In general, the data in the present review are drawn from publications since 1960, although references to earlier work are included where necessary to complete the picture. 

Low oxidation states are not characteristic of zirconium and hafnium. There is no aqueous chemistry of the metals in oxidation state < III, except for the unstable orange aqua ion [Zr(H20)e] +, which is analogous to the stable violet [Ti(H20)6] + ion. Inorganic compounds of and... 

A simplified series of reactions between a hafnium salt and sulfuric acid is given in Fig. 4.3. The reactions showcase important facets of thin-film synthesis (but do not address the precise identities of intermediates or complexities of aqueous hafnium chemistry.) In the first step, a hafnium oxide chloride crystal hydrate is dissolved in water to disperse small hafnium-hydroxo molecular clusters. Sulfato ligands are subsequently added in the form of sulfuric acid. Since sulfato binds more strongly than chloro, hafnium-hydroxo-sulfato aqueous species are created. Under mild heating, these species readily poly-... 

At least some of these difficulties have been overcome in the work to be reported in this study, which deals with the adsorption of hafnium hydrolyzed species on powdered glass as a function of the acidity of the medium. The adsorption of hafnium species from aqueous solution has apparently never been investigated, yet this ion lends itself conveniently to studies of the problems discussed above. The chemistry of the hafnium ion in water is fairly well understood (23) and a suitable isotope, 181Hf, is available for adsorption studies. What makes hafnium a particularly interesting system is the fact that it forms the entire series of hydrolyzed species Hf(OH)n(4 r )+ where n 4. At intermediate acidities (pH > 4) the solutions of low concentrations contain only the neutral, soluble species Hf(OH)4. It should be emphasized that there is a pH and a concentration range over which this species is present without simultaneous formation of hafnium hydroxide. Thus, it is possible to elucidate the effect of the ionic charge upon the adsorption of hydrolyzed species in systems void of colloidal hydroxides. The glass powder was used in... 

The chemistry of the niobium-tantalum pair is quite similar to that of the zirconium-hafnium pair. In aqueous media, all solvated species containing these elements have either metal-to-oxygen bonds, metal-to-fluorine bonds, or both. 

Titanium has a more extensive redox chemistry than either zirconium or hafnium. In addition to the +4 oxidation state, the most stable for all three elements of this group, titanium(lll) and titanium(II) compounds are known. Titanium(III) is a good reducing agent and exists in aqueous solution as under acidic conditions, Ti-... 

This chapter reviews the literature up to approximately January 1984. Previous comprehensive reviews of the chemistry of zirconium and hafnium are available, and annual surveys of the inorganic and coordination chemistry of these elements have been published since 1981 The present treatment emphasizes the chemistry of discrete, isolable complexes, although some attention has been given to the extremely complex aqueous solution chemistry of Zr and Hf. In general, zirconium forms slightly more stable complexes in solution than does hafnium stability constants may be found in standard compilations.In cases where the structural chemistry of discrete complexes is closely related to that of chain, layer or extended three-dimensional structures, the discussion of the discrete complexes has been set in the broader context the section on fluorometallate complexes is a case in point. Organometallic compounds of zirconium and hafnium have been excluded almost entirely. 

The extremely complicated aqueous solution chemistry of and HP has been reviewed by Larsen, Solovkin and Tsvetkova,and Clearfield. Zirconium(IV) and hafnium(IV) ions undergo extensive hydrolysis, and the predominant solution species are polynuclear, even in dilute (>10 -10 M) solutions of high acidity (1-2 M). Spectrophotometric, " ultracentrifugation, " and light scattering studies point to trinuclear and tetranuclear hydrolysis products complexes such as [M3(OH)4] and [M4(OH)8] have been suggested. ITie mononuclear ions are predominant solution species only at trace metal ion... 

The chemistry of rutherfordium has been shown to be similar to the chemistry of hafnium rather than the chemistry of the heavier actinides, a clear demonstration of the expected end of the actinide series at lawrencium. This demonstration involved both aqueous and gas-phase chemistry. In the gas-phase experiments by Zvara etal. (1972a,b) the 3-s isotope produced in the " Pu ( Ne, 5n) reaction was used. Zvara and co-workers attempted to use thermochromatography to show a difference in volatility of RfC which seemed to condense at 220°C as compared to the chlorides of the heavier actinides which have much higher condensation temperatures. 

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