Thorium solubility constant

Solubility constants of log = — 10.3 0.2 and —10.6 0.6 were also calculated for Ih02(s) prepared from thorium hydroxide and thorium oxalate, respectively. 

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. 

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... 

Simple thermodynamic calculations based on literature data (5-12) support the choice of phosphates as the optimum mineral phases for actinide immobilization. The calculations considered every relevant species reported (5-72) that contained protons, hydroxide, or the ligand in question for each metal ion. Where necessary, equilibrium constants were corrected to 0.1 M ionic strength using the Davies equation. As an example, the calculated solubility of europium, thorium, and uranium in various media at p[H] 7.0 (p[H] = - log of the hydrogen ion concentration), 0.001 M total ligand concentration, 0.1 M ionic strength, and 25 °C are shown in Table I. Within the constraints of the calculation, the solubility of thorium is limited by Th(OH)4, but the lowest europium and uranyl solubilities are observed for phosphates. 

Experimental studies on the hydrolysis of the Th" ion, its complexes with strong inorganic hgands, and the solubility of thorium oxides or hydroxides ate usually performed with low concentrations of thorium in perchlorate, chloride, and nitrate media. There is no evidence for complex formation between Th" and CIO4 however, chloride and nitrate form weak Th(lV) complexes as discussed in Sections Vlll.2.2.1 and X. 1.3.3, respectively. For the evaluation of equihbrium constants at zero ionic strength from data in chloride and nitrate media we have therefore the general problem to decide if the activity of Th", , should be calculated using a... 

Figure VII-14 shows in addition that the solubility data of thorium hydroxide or hydrous oxide can be divided into two pH regions. At pH < 6, the thorium concentration shows a steep decrease with increasing pH while at pH 6-14 the thorium concentration remains at a constant level. In the following sections the published data are discussed and evaluated as follows ...

The equilibrium constants logi A °4= -(8.5+ 1.0) and log,o A°i = - (17.4 + 0.7) can be used as "operational values" for geochemical modelling. However it is by no means ascertained that the thorium concentration measured in solubility studies at pH 6-14 is actually due to the mononuclear complex Th(OH)4(aq). This review finds it more likely that the soluble uncharged complex is one or more small (< 1.5 nm) polynuclear species Th O K(OH) 4 2i)(aq), more simply written as... 

Figure VII-16 Solubility of thorium hydroxide determined by Higashi [1959HIG] at room temperatnre from oversaturation after 1, 3, 7, and 100 days. The solnbihty and speciation lines are calcnlated with the hydrolysis constants and SIT coefficients selected in this review, logj,...

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. 

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