Thorium stability constants

U1 "1" (16), and Np1 "1" (17) (Table IV). Only in the thorium system have stability constants been determined as a function of acidity. Zebroski et al., (12) found that the 1 2 complex had the form TMSOi, at 0.5 M and 1.0 M acidity but suggested a contribution from the monoprotonated, bis-sulfato complex in 2 M acid. However, Zielen (9) found no evidence for such a species. The Kj/K2 ratio found in the latter study (n.8) compares favorably with the Am(III)-S0 data of DeCarvalho and Choppin (10)... 

Thorium(IV) complexes of some catecholate, hydroxypyridonate, and desferri-oxamine ligands have been investigated and stability constants determined in connection with searches for sequestering agents for Pu4+.16... 

Table 11.2 gives some stability constants for complexes of Th + and U02 +. Thorium forms stronger complexes with fluoride, the hardest halide ion, than with chloride and bromide this is the behaviour expected of a hard Lewis acid. Thorium also forms quite... 

The donor action of [NOs]" as an anionic ligand towards thorium(iv) and uranium(iv) in the presence of trimethylphosphine and tris(dimethylamino)phosphine oxide in aqueous media has been found to be very similar. The larger nitrate ion was observed to form more stable species with thorium than the chloride ion whereas in the uranium(iv) case both complexes formed equally readily. A study of the sulphate complexes of uranium(iv), neptunium(vi) and plutonium(vi) in HCIO4-H2SO4 solution showed the stability constants to follow the order U[Pg.453]

The available thermodynamic data are of two types stabihty constants, enthalpy and entropy of reaction for the formation of soluble complexes Th(S04) " " and solubihty data for various solid phases. The two sources are linked because the solubility of the solid phases depends on the chemical speciation, i.e., the sulphate complexes present in the aqueous phase. The analysis of the experimental stability constants has been made using the SIT model however, this method cannot be used to describe the often very high solubility of the solid sulphate phases. In order to describe these data the present review has selected a set of equilibrium constants for the formation of Th(S04) and Th(S04)2(aq) at zero ionic strength based on the SIT model and then used these as constants in a Gibbs energy minimisation code (NONLINT-SIT) for modelling experimental data to determine equilibrium constants for the formation of Th(S04)3 and the solubility products of different thorium sulphate solids phases. 

The experimental data are interpreted through the average number of fluorides per thorium atom. The concentration-dependent stability constants thus calculated were converted to constants at zero ionic strength through the modified Debye-Hiickel relationship for the experimental temperatures. These constants were then used to calculate other thermodynamic parameters through the use of van t Hoff relationship assuming that Aj/f ° (A.60) is independent of temperature. The log, (A.60) values for ... 

Danon, J., Determination of the stability constants of thorium nitrate complexes with anion-exchange resins, J. Inorg. Nucl. Chem., 13, (1960), 112-118. Cited on pages 317, 318, 462. 

Studies of the speciation of actinides in environmental waters are made difficult by the very low concentrations involved and the possibility that minor, undetected contaminants may dominate the binding of a particular metal ion. The environmental behaviour of the actinides has been reviewed. Americium and thorium exhibit simpler behaviour than other actinides since their oxidation states under such conditions are limited to Am and Th. Both are readily adsorbed by granitic rocks and tend to exhibit low solubilities, The thermodynamic solubility product of amorphous Am(OH)3 has been measured as log = 17.5 0.3 and no evidence for amphoteric behaviour or the formation of Am(OH)4 was found below pH 13. Stability constants for the binding of Am to humic acid have been found to vary with the degree of ionization, a, and were given by log = 10.58a -1-3.84 and log 2 = 5.32a -b 10.42. These were larger than the corresponding values for Eu. Humic acids also bind Th as described in Section 65.2.1. 

Thiophen Derivatives of Analytical Interest.—2-Thenoyltrifluoroacetone has maintained its position as a chelating agent in analytical chemistry. Papers describing its use in the extraction or determination of thorium, copper, europium, thallium, niobium, and molybdenum have appeared. The effect of copper(n) on the formation of monothenoyltri-fluoroacetonatoiron(iii) has been studied. The stability constants of some bivalent metal chelates of di-(2-thenoyl)methane have been determined. 3-Thianaphthenoyltrifluoroacetone has been proposed as a reagent for the spectrophotometric determination of iron(iii) and cerium(iv). The stabilities of metal chelates formed from derivatives of thiophen-2-aldehyde and of rare-earth carboxylates of thiophen-2-carboxylate have been studied. 

Hydrolysis studies of the tetravalent cations are limited to Ce and the light (Pa, Th, U, and Pu) actinides (tables 8B and 9). Only in strong acid media do the tetravalent ions exist free of hydrolysis. Similarly, only at very low concentrations are mononuclear hydroxides of significance. For Ce(I V), stability constants have been determined for bi-, ter-, tetra-, and dodeca-cerium units and for species with molecular weights as large as 40,000 (Hardwick and Robertson 1951, Danes 1967, Louwrier and Steemers 1976). X-ray measurements of thorium and uranium solutions show polynuelear complexes built from... 

Choppin GR, Ekten HA, Xia YX (1996) Variation of stability constants of thorium citrate complexes with ionic strength. Radiochim Acta 74 123-127... 

It would appear clear from observation of Figure 10.21 that the variation in the stability constant of Th2(OH)3 increases as the ionic strength increases. At an ionic strength of 0.5 mol kg the variation in the stability constant is about 0.5 log units, whereas at around 3.25 mol kg the variation has increased to about 1.0 log unit. This variation maybe due to all except one of the values coming from the work of Milic and co-workers and the fact that they only acquired data up to a bound hydroxide to thorium ratio of 0.4 (the ratio for the Th2(OH)3 species is 1.5) more data are required in chloride media where the bound hydroxide to thorium ratio is extended to the point ofthe onset of precipitation reactions which should enable more precise stability constants to be acquired for this, and other, species. 

Data determined for the stability constant of the neutral thorium(lV) monomer, Th(OH)4(aq), are listed in Table 10.15. It is clear from the data provided in the table that the proposed stability constants span many orders of magnitude and very few are consistent. The accepted data are those that are consistent with the stability constant proposed by Neck and Kim (2001) for zero ionic strength and 25 "C. 

Danesi et al, 1968 Hietanen and Sillen, 1968). The data from Milic and coworkers are within the combined uncertainty limits of the work of Hietanen and Sillen (1968), but the stability constant of Danesi et at. (1968) appears to be somewhat larger (see Table 10.16). There are also some data from perchlorate media (Baes, Meyer and Roberts, 1965 Hietanen and Sillen, 1968 Grenthe and Lagerman, 1991). Analysis of the data from the nitrate, chloride and perchlorate media leads to stability constants at zero ionic strength that are consistent and within the limits of the calculated uncertainties. Unfortunately, the vast majority of the experimental data from the studies of Milic and co-workers only reached a bound hydroxide to thorium ratio of 0.4, making it impossible to obtain stability constants with substantially higher ratios (they typically only postulated stability constants for Th2(OH)2 and Th2(OH)3 ). 

Table 10.13 Data for the stability constant of the second monomeric hydrolysis species of thorium(IV), Th(OH)2 + (reaction (2.5), M = Th , p= 1, j = 2).

Physical and Chemical Properties. Some of the physical and chemical properties (i.e., K°w K°<= and Henry s law constant) that are often used in the estimation of environmental fate of organic compounds are not useful or relevant for most inorganic compounds including thorium and its compounds. Relevant data concerning the physical and chemical properties, such as solubility, stability, and oxidation-reduction potential of thorium salts and complexes have been located in the existing literature. 

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