Aqueous thorium hydroxide complexes

VIL3 Aqueous thorium hydroxide complexes VII.3.1 Discussion of experimental methods... 

It has to be emphasised that these equilibrium constants must not be combined with 8(ThAN03) = (0.31 +0.12) kg-mol used in the Sections VII.3 and VIII. 1.2 on aqueous thorium hydroxide and fluoride complexes this value refers to a strict ion interaction approach where the effect of nitrate complexation is included in the interaction coefficient. The equilibrium constants for the formation of nitrate complexes must be combined with s(ThA NOj) = e(ThA CIO ,) = (0.70 + 0.10) kg-mol, ... 

Most of the recent experimental work on the detailed thermodynamic properties of thorium compounds has been on solubilities and aqueous solutions. The large number of hydroxide complexes of Th(IV) and the propensity to form colloidal solutions, makes the interpretation of these studies a challenging task. As noted in previous reviews, the development of laser-based spectroscopic techniques has been a useful tool in our increased understanding of these phenomena, and it is gratifying that... 

Thorium generally exists as a neutral hydroxide species in the oceans and is highly insoluble. Its behavior is dominated by a tendency to become incorporated in colloids and/or adhere to the surfaces of existing particles (Cochran 1992). Because ocean particles settle from the water column on the timescale of years, Th isotopes are removed rapidly and have an average residence time of = 20 years (Fig. 1). This insoluble behavior has led to the common assertion that Th is always immobile in aqueous conditions. While this is generally true in seawater, there are examples of Th being complexed as a carbonate (e.g.. Mono Lake waters, Anderson et al. 1982 Simpson et al. 1982) in which form it is soluble. 

Principle of Separation. Uranium forms a nitrate complex that is extractable into ethyl acetate (as well as other organic extractants). Thorium does not readily form an extractable nitrate complex. When ethyl acetate is contacted with an aqueous solution, the uranium-nitrate complex is partitioned favorably into the ethyl acetate whereas thorium nitrate is not. The distribution of the metal ion between the two phases is expressed as D = Corganic/Caqueous where C is the concentration in moles or dps per unit volume in the respective phases. The thorium remains in the aqueous phase and is precipitated as the hydroxide for counting. 

Thorium (Th, at. mass 232.04) occurs in solutions exclusively in the IV oxidation state. In its chemical properties it resembles Zr and Ti, as well as the rare-earth elements. In aqueous solutions at pH < 1, it exists as colourless Th ions. It is less readily hydrolysed than Ti or Zr. The hydroxide Th(OH)4 (precipitating at pH 3.5-4) has no amphoteric properties. Thorium forms stable complexes with tartrate, citrate, and EDTA and less stable complexes with sulphate, nitrate and carbonate. 

A more convincing experiment performed by Crookes [34] involved Fe(OH)3 as a carrier of uranium X (having the more convenient ti/2 = 24 days) precipitated from the soluble uranyl carbonato complex in excess (NH4)2C03. These observations show an insoluble hydroxide, but they did not establish the thorium(IV) chemistry of this Th isotope, the first descendant of Sir Ernest Rutherford (1871-1937) and Frederick Soddy (1877-1956) then showed [35] that ti 2 = 3.6 days of thorium X ( Ra), soluble in aqueous ammonia, is replaced by radio-elements precipitating as hydroxides. In 1907 Rutherford demonstrated [36] that ionium ( °Th with tj j = 80000 years, the immediate ancestor of Ra with t j = 1600 years in uranium minerals), Th (tiy2 = 14100 million years), and radiothorium Th (ti/2 = 1-9 years) cannot be separated by chemical means. 

The thorium ion, Th4+, is more resistant to hydrolysis than other 4+ ions but undergoes extensive hydrolysis in aqueous solution at pH higher than 3 the species formed are complex and dependent on the conditions of pH, nature of anions, concentration, etc. In perchlorate solutions the main ions appear to be Th(OH)3 +, Th(OH)2+, Th2(OH) +, Th4(OH) +, while the final product is the hexamer Th6(OH)95 of course, all these species carry additional water.19 Hexameric ions exist also for Nbv and for Ce1 v and Ulv [M604(0H)J12 + ions are found in crystals of the sulfates. The metal atoms are linked by hydroxo or oxo bridges. In crystals of the hydroxide, Th(OH)4, or the compound Th(0H)2Cr04 HzO, chain-like structures have been identified, the repeating unit being Th(OH)2+ in solution, the polymers may have similar form (28-1) or may additionally be cross-linked. 

I. In this method, Th is quantitatively precipitated as the phosphate, together with a small amoimt of rare earth phosphates this is accomplished by neutralization and dilution of the solution. The phosphates are dissolved in cone, hydrochloric acid and precipitated with oxalic acid, and the thoroughly washed precipitate is extracted with warm aqueous NagCOa. Most of the rare earths stay in the residue, while the thorium dissolves in the form of a carbonate complex, NaeTh(C03)s. The material is freed of the remaining traces of rare earths by repeated crystallization in the form of the sulfate Th(S04,)a 8H3O. The procedure consists of precipitation of the hydroxide with ammonia and solution of the latter in sulfuric acid to re-form the sulfate. The precipitate from the last purification stage is dissolved in nitric acid to yield the nitrate. 

Complicating the development of ISEs for higher actinide ions is their inherent radioactivity. They also have chemistry tiiat often differs from that of the uranyl cation. Actinides from americium to lawrencium display solution-phase chemical features that resemble those of the trivalent lanthanides. Conversely, in certain oxidation states, the early actinides (thorium through neptunium) often mimic transition metals. Also, as mentioned above, many of the actinides can exist in a large number of oxidation states. For instance, in the case of plutonium, four oxidation states can exist simultaneously in aqueous solution. Finally, as true for the lanthanides, complex salts with hydroxide, halogens, perchlorates, sulfates, carbonates, and phosphates are well known for most of the actinides. 

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