Hafnium complexes aqueous solutions

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. 

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 extraction of metals by liquid amines has been widely investigated and depends on the formation of anionic complexes of the metals in aqueous solution. Such applications are illustrated by the use of Amberlite LA.l for extraction of zirconium and hafnium from hydrochloric acid solutions, and the use of liquid amines for extraction of uranium from sulphuric acid solutions.42,43... 

In aqueous solution, zirconium(IV) and hafnium(IV) form complexes M(NCS)4-", where n = 1-8.104 Selective extraction of hafnium thiocyanate complexes from acidic aqueous solution by methyl isobutyl ketone is a widely used industrial method for the separation of zirconium and hafnium. Separation methods have been reviewed by Vinarov.105... 

Because K2HfF6 is —1.7 times more soluble in water than K2ZrF6,528 zirconium and hafnium can be separated by fractional crystallization of the hexafluorometallates. This approach is used on an industrial scale in the USSR.286 Conductance measurements on aqueous solutions of M2MF6 (M = K, Rb, or Cs) indicate very little hydrolysis of the [MF6]2- ions.529 Alkaline hydrolysis of potassium and ammonium fluorozirconates yields crystalline M ZrF3(0H)2-H20 complexes, which are easily dehydrated to M ZrF3(OH)2. 

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... 

Formation of zirconium and hafnium complexes with polyhydroxyaromatic compounds in aqueous solution has been studied by potentiometric, spectrophotometric and ion exchange methods. Much of this work has been reviewed by Larsen. A potentiometric and lightscattering study of the zirconium(IV)-Tiron (Tiron = disodium l,2-dihydroxybenzene-3,5-disulfonate) system provides evidence for mixed hydroxo-Tiron chelates at Tiron/Zr " ratios less than 2.5, but at higher Tiron/Zr ratios the unhydrolyzed complexes [Zr2L5] and [ZrL4] (L = tetranegative anion of l,2-dihydroxybenzene-3,5-disulfonic acid) are present. ... 

A variety of peroxo and hydroperoxo complexes of zirconium(IV) and hafnium(IV) have been isolated from aqueous or aqueous methanolic hydrogen peroxide solutions that contain additional ligands such as sulfate, oxalate or fluoride. Examples of recently reported complexes are listed in Table 8 along with characteristic vibrational frequencies and the pH employed in the aqueous preparations. Earlier work on peroxo compounds has been reviewed by Connor and Ebsworth180 and by Larsen.5... 

Equilibrium constants for the formation of nitrate complexes at hydrogen ion concentrations of 2 and 4 M and metal ion concentrations of 5 X 10 M or less, were determined using ion exchange techniques (353, 465) (Table XVIII). Activity coefficient data for aqueous zirconium and hafnium species are scarce, although there is one report (319) of activity coefficients for metal nitrate solutions as determined by the isopiestic method. 

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