Aqueous thorium complexes with carbonate

Both quahtative and qnantitative data ate available. Only the qnantitative data can be nsed to obtain thermodynamic parameters, but the quahtative information can in some cases be nsed to corroborate the quantitative conclusions, e.g. concerning the stoichiometiy of complexes and the mode of coordination of the carbonate ligand. (Qnantitative determinations of the stoichiometiy and equilibrium constants are described in the following section. These studies are complicated by the formation of sparingly solnble sohd phases and the formation of ternary Th(lV)-hydroxide-carbonate species. This review will therefore begin with a short summary of the advantages and disadvantages of the varions methods nsed to deduce the stoichiometiy and eqnilibrinm constants in the temaiy Th(lV)-carbonate-water system. 

The analysis of experimental data is facilitated if the temaiy system is rednced to a formaf three-component system by keeping the concentration of one component, A, constant and varying the other two, B and C. These data provide information on the stoichiometry of the complexes with respect to the two components that are varied and their conditional equilibrinm constants. By repeating the investigation in new series of experiments where the concentration of A is still constant bnt with the valnes (Ai, A2, A3 etc.) in the different series, one can determine the stoichiometiy with respect to A and the equilibrium constants for the temaiy complexes. The choice of components in the thorium carbonate system depends on the pH region investigated at low pH where the partial pressure of CO2 can be measured it is practical to use C02(g) as one of the components, the other two being the concentrations of Th and H. The chemical equilibria are then formulated as  

The conditional equilibrium constants (for a given partial pressure of carbon dioxide) for these reactions are  

1 Solubility of Th02(am, hyd) in carbonate solution and formation constants of ternary Tb(IV)-bydroxide-carbonate complexes 

The equilibrium constants logj = (logj + logj ) derived by the different authors from their solubility studies with Th02(am, hyd) are given in Table XI-3. The notation yz refers to the stoichiometric coefficients in Eqs (XL 9) and (XL 10)  

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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. 

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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