Zirconium 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. 

Hydroxyl Compounds. The aqueous chemistry of zirconium is complex, and in the past its understanding was compHcated by differing interpretations. In a study of zirconium oxide chloride and zirconium oxide bromide, the polymeric cation [Zr4(OH)g (H20)jg was identified (189) the earlier postulated moiety [Zr=0] was discarded. In the tetramer, the zirconium atoms are coimected by double hydroxyl bridges (shown without the coordinating water molecules) ... 

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... 

Hock, A.L. "Zirconium Compounds The Industrial Importance of Their Aqueous Chemistry," Chemistry and Industry. November 2, 1974. 

Toney JH, Marks TJ (1985) Hydrolysis chemistry of the metallocene dichlorides M(r)5-CsHs CL, M = titanium, vanadium, or zirconium. Aqueous kinetics, equilibria, and mechanistic implications for a new class of antitumor agents. J Am Chem Soc 107 947-953... 

The aqueous chemistry of zirconium is complex and dominated by hydrolysis. One aspect is that polymerization takes place when salt solutions are diluted. The polymeric species can be cationic, anionic, or neutral. Polymers that are formed include ammonium zirconium carbonate, zirconium acetate, and zirconium oxychloride. 

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. 

Aqueous Chemistry and Complexes. Zr02 is more basic than Ti02 and is virtually insoluble in an excess of base. There is a more extensive aqueous chemistry of zirconium because of a lower tendency toward complete hydrolysis. Nevertheless hydrolysis does occur and it is very doubtful indeed if Zr4 + aquo ions exist even in strongly acid solutions. The hydrolyzed ion is often referred to as the zirconyl ion and written Zr02 +. However, there is little, if any, reliable evidence for the existence of such an oxo ion either in solution or in crystalline salts,12 although the infrared spectrum of one form of ZrOCl is reported to have a band at 877 cm-1, perhaps indicative of a Zr=0 group.13 No such group has ever been directly identified by diffraction methods. 

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. 

VEY/DUP] Veyland, A., Dupont, L., Rimbault, J., Pierrard, J.-C., Aplincourt, M., Aqueous chemistry of zirconium(IV) in carbonate media, Helv. Chim. Acta, 83, (2000), 414-427. Cited on pages 210, 351. 

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... 

Zirconium forms anhydrous compounds in which its valence may be 1, 2, 3, or 4, but the chemistry of zirconium is characterized by the difficulty of reduction to oxidation states less than four. In aqueous systems, zirconium is always quadrivalent. It has high coordination numbers, and exhibits hydrolysis which is slow to come to equiUbrium, and as a consequence zirconium compounds in aqueous systems are polymerized. 

The chemistry of aqueous solutions of zirconium salts (does the zirconyl ion exist ). A. S. Solovkin and S. V. Tsvetkova, Russ. Chem. Rev. (Engl. Transl), 1962,31, 655-669 (161). 

The solution chemistry of zirconium, both aqueous and solvent-based, is dominated by the tendency to form polymeric species. This has been reviewed in several articles [3], but the basic facts bear repeating here once again. 

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. 

Zirconium has no significant redox chemistry in solution. Formation of soluble peroxy complexes in dilute aqueous acid can be used to purify zirconium compounds by extraction in organic media. Subsequent decomposition of the complex yields ultra-pure zirconia ZrC>2, which is of interest as a high performance ceramic material.271... 

Inorganic precursors are much cheaper and easier to handle than metal alkoxides. Therefore the industrial production of oxide powders for ceramics and catalysts is mainly based on the precipitation or coprecipitation of inorganic salts from aqueous solutions. Gibbsite, Al(OH)3, (see Aluminum Inorganic Chemistry) is precipitated from aluminate solutions. Ti02 powders are made via the controlled hydrolysis of titanium salts. Stabilized zirconia is coprecipitated from aqueous solutions of zirconium oxychloride, ZrOC, and yttrium nitrate, YlKOsjs. 

There are several examples of M-Si systems. Silicides of composition M2 Si, MSi, and MSi2 are known for both metals. Zirconium also forms the Zrs Si2 phase and hafhiiun the Hfs Sis phase (see Silicon Inorganic Chemistry). All the silicides are insoluble in all mineral acids (except HF), and, unlike TiSi2, in 10% aqueous KOH. 

Zirconium is the principal FP to arise in oxidation state (IV). Where Zircaloy clad fuel is involved, nonradioactive zirconium isotopes may also be present from fuel can residues. As with ruthenium, there may be a variety of nitrato complexes present in the solution including the aquated complexes Zr(N03)s where x = 1-6, and hydroxy nitrato complexes. However, species containing ZrO " " are not expected to be present since this ion is unstable in aqueous media and is rapidly hydrated to Zr(OH)2. The extraction chemistry is further complicated by the formation of inextractable polymeric species when the Zr" concentration exceeds ca. 10 M. An example of such oligomerization is afforded by the [Zr(0H)2(H20)4]4 ion which contains four Zr ions in a square arrangement linked by two /u-OH ligands on each square edge. Four water molecules complete the Zr coordination sphere in an approximately D2d dodecahedral geometry. 

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