Thorium Solution Chemistry

Thorium forms relatively stable tetravalent salts of many of the oxyacids. These can be prepared by reacting thorium hydroxide or basic carbonate with the appropriate acid. 

Thorium nitrate is very soluble in water, to the extent of 65.6 g Th(NO3)4/100 g solution at 20°C. It can be crystallized from solution as the nominal tetrahydrate. Thorium nitrate solutions are used for purifying thorium by solvent extraction. Sec. 9.  

Although anhydrous thorium sulfate dissolves in water at 0°C to the extent of 20 w/o, the solution is metastable and deposits hydrates on standing. Stable solutions at higher temperature require the presence of free sulfuric acid, as in solutions used to leach thorium minerals. Sec. 

A solution of ThOCl2 is produced when ThCU reacts with water. Evaporation to dryness produces a succession of ill-defined hydrates that can be converted to anhydrous ThOCU by heating to 250 C. 

Hydrated thorium fluoride is precipitated when a soluble fluoride is added to a solution of thorium nitrate. Precipitation can be prevented by addition of aluminum nitrate to complex the fluoride ion, an expedient used in the Thorex process (Chap. 10, Sec. 5). 

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Although the exact extentis not known accurately, hydrolysis of various salts is known to occur. Since the hydroxide is not precipitated it is assumed that the hydrolysis product is some ion on the form Th(OH)2++ orThOHJ+. The solution chemistry of thonum is made more complicated because of the hydrolytic phenomena observed and the polynuclear complex ions that are formed at low acidities and higher thorium concentrations. 

The chemical properties span a range similar to the representative elements in the first few rows of the periodic table. Francium and radium are certainly characteristic of alkah and alkaline earth elements. Both Fr and Ra have only one oxidation state in chemical comhina-tions and have little tendency to form complexes. Thallium in the 1+ oxidation state has alkah-like properties, but it does form complexes and has extensive chemistry in its 3+ state. Similarly, lead can have alkaline earth characteristics, hut differs from Ra in forming complexes and having a second, 4+, oxidation state. Bismuth and actinium form 3+ ions in solution and are similar to the lanthanides and heavy (Z > 94) actinides. Thorium also has a relatively simple chemistry, with similarities to zirconium and hafiuum. Protactinium is famous for difficult solution chemistry it tends to hydrolyze and deposit on surfaces unless stabilized (e.g., by > 6 M sulfuric acid). The chemistry of uranium as the uranyl ion is fairly simple, hut... 

The separation of the lanthanides from thorium, uranium, plutonium, and neptunium can fairly readily be achieved by exploiting the greater extractability of the higher oxidation states of the light-actinide elements. However, the transplutonium actinides do not have stable higher oxidation states. In this case, separation of the lanthanide fission products from the transplutonium actinides must exploit the small differences in the solution chemistry of the lanthanides and actinides in the trivalent oxidation state. It is the separation of the lanthanides from the trivalent actinide cations that is the focus of this chapter. 

In the case of thorium, on the other hand, the tetravalent state is the only one existing in solution. The hydrated Th ion is, moreover, by far the least acidic of the actinide ions. In contrast to this, the neighbor Pa is by far the most acidic, extensively hydrolyzed even in strongly acid solutions. The acidities of Np, and Pu are intermediate. These ions all play an important part in the solution chemistries of the respective elements, though especially is fairly easily oxidized. 

The chemistry of neptunium (jjNp) is somewhat similar to that of uranium (gjU) and plutonium (g4Pu), which immediately precede and follow it in the actinide series on the periodic table. The discovery of neptunium provided a solution to a puzzle as to the missing decay products of the thorium decay series, in which all the elements have mass numbers evenly divisible by four the elements in the uranium series have mass numbers divisible by four with a remainder of two. The actinium series elements have mass numbers divisible by four with a remainder of three. It was not until the neptunium series was discovered that a decay series with a mass number divisible by four and a remainder of one was found. The neptunium decay series proceeds as follows, starting with the isotope plutonium-241 Pu-24l—> Am-24l Np-237 Pa-233 U-233 Th-229 Ra-225 Ac-225 Fr-221 At-217 Bi-213 Ti-209 Pb-209 Bi-209. 

Since transport by water is virtually the only available mechanism for escape, we will be predominantly concerned with the chemistry of aqueous solutions at the interface with inorganic solids - mainly oxides. These will be at ordinary to somewhat elevated temperatures, 20-200 C, because of the heating effects of radioactive decay during the first millennium. The elements primarily of interest (Table I) are the more persistent fission products which occur in various parts of the periodic table, and the actinides, particularly uranium and thorium and, most important of all, plutonium. 

BERZELIUS, J. J. (1779-1848). A native of Sweden, Berzelius was one of the foremost chemists of the 19th century. He made many contributions to both fundamental and applied chemistry coined the words isomer and catalyst classified minerals by chemical compound. He recognized organic radicals which maintain their identity in a series of reactions discovered selenium and thorium, and isolated silicon, titanium, and zirconium did pioneer work with solutions of proteinaceous materials which he recognized as being different from true solutions. 

S. Ahrland and co-workers, eds., Gmelin Handbook of Inorganic Chemistry, Thorium, Suppl Vol D1, Properties of Thorium Ions in Solutions, 8th ed., Springer-Vedag, Berlin, 1988. 

For detailed explanations of the chemistry of specific prreipitation titrations the reader is referred to other sources. These include methods such as the Mohr method for halides using the silver chromate end point, the Fajans adsorption indicator method, the Volhard method for FeSCN end point, the titration of halide mixtures with the attendant solid-solution and adsorption effects, the titration of fluoride with thorium(IV), and the titration of sulfate with barium ion. 

With the exception of thorium and protactinium, all of the early actinides possess a stable +3 ion in aqueous solution, although higher oxidation states are more stable under aerobic conditions. Trivalent compounds of the early actinides are structurally similar to those of their trivalent lanthanide counterparts, but their reaction chemistry can differ significantly, due to the enhanced ability of the actinides to act as reductants. Examples of trivalent coordination compounds of thorium and protactinium are rare. The early actinides possess large ionic radii (effective ionic radii = 1.00-1.06 A in six-coordinate metal complexes),and can therefore support large coordination numbers in chemical compounds 12-coordinate metal centers are common, and coordination numbers as high as 14 have been observed. 

All early actinides from thorium to plutonium possess a stable +4 ion in aqueous solution this is the most stable oxidation state for thorium and generally for plutonium. The high charge on tetravalent actinide ions renders them susceptible to solvation, hydrolysis, and polymerization reactions. The ions are readily hydrolyzed, and therefore act as Bronsted acids in aqueous media, and as potent Lewis acids in much of their coordination chemistry (both aqueous and nonaqu-eous). Ionic radii are in general smaller than that for comparable trivalent metal cations (effective ionic radii = 0.96-1.06 A in eight-coordinate metal complexes), but are still sufficiently large to routinely support high coordination numbers. 

The release of uranium and thorium from minerals into natural waters will depend upon the formation of stable soluble complexes. In aqueous media only Th is known but uranium may exist in one of several oxidation states. The standard potential for the oxidation of U in water according to equation (2) has been re-evaluated as E° - 0.273 0.005 V and a potential diagram for uranium in water at pH 8 is given in Scheme 3. This indicates that will reduce water, while U is unstable with respect to disproportionation to U and U Since the Earth s atmosphere prior to about 2 x 10 y ago was anoxic, and mildly reducing, U " would remain the preferred oxidation state in natural waters at this time. A consequence of this was that uranium and thorium would have exhibited similar chemistry in natural waters, and have been subject to broadly similar redistribution processes early in the Earth s history. Both U " and Th are readily hydrolyzed in aqueous solutions of low acidity. A semiquantitative summary of the equilibrium constants for the hydrolysis of actinide ions in dilute solutions of zero ionic strength has been... 

The chemistry of actinide ions is generally determined by their oxidation states. The trivalent, tetravalent and hexavalent oxidation states are strongly complexed by numerous naturally occurring ligands (carbonates, humates, hydroxide) and man-made complexants (like EDTA), moderately complexed by sulfate and fluoride, and weakly complexed by chloride (7). Under environmental conditions, most uncomplexed metal ions are sorbed on surfaces (2), but the formation of soluble complexes can impede this process. With the exception of thorium, which exists exclusively in the tetravalent oxidation state under relevant conditions, the dominant solution phase species for the early actinides are the pentavalent and hexavalent oxidation states. The transplutonium actinides exist only in the trivalent state under environmentally relevant conditions. 

The solution photochemistry of the actinides begins with uranium none has been reported for actinium, thorium, and protactinium. Spectra have been obtained for most of the actinide ions through curium in solution (5). Most studies in actinide photochemistry have been done on uranyl compounds, largely to elucidate the nature of the excited electronic states of the uranyl ion and the details of the mechanisms of its photochemical reactions (5a). Some studies have also been done on the photochemistry of neptunium (6) and plutonium (7). Although not all of these studies are directed specifically toward separations, the chemistry they describe may be applicable. 

The chemistry of thorium largely concerns Th(IV) and, in aqueous solution, there is no evidence for any other oxidation state. The E° value for the Th /Th couple is -1.9V. 

Nobelium is a member of the actinide series of elements. The ground state electron configuration is assumed to be (Rn)5fl47s2, by analogy with the equivalent lanthanide element ytterbium ([Kr]4fl46s2) there has never been enough nobelium made to experimentally verify the electronic configuration. Unlike the other actinide elements and the lanthanide elements, nobelium is most stable in solution as the dipositive cation No ". Consequently its chemistry resembles that of the much less chemically stable dipositive lanthanide cations or the common chemistry of the alkaline earth elements. When oxidized to No, nobelium follows the well-estabhshed chemistry of the stable, tripositive rare earth elements and of the other tripositive actinide elements (e.g., americium and curium), see also Actinium Berkelium Einsteinium Fermium Lawrencium Mendele-vium Neptunium Plutonium Protactinium Ruthereordium Thorium Uranium. 

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