Mononuclear Th IV hydroxide complexes

The evaluation of equihbrium constants for mononuclear species Th(OH) is difficult because they are usually minor species under the conditions used in potentiometric studies. In addition, most of the equilibrium constants reported for mononuclear complexes cannot be accepted by this review, because they were calculated from potentiometric or solubility data under conditions where polynuclear species are known to be predominant, a fact that the authors often neglected in some cases they also proposed inadequate speciation schemes cf. Appendix A reviews of [1954GAY/LEI], [1955PAN/HSE], [1963BIL/FUR], [1964NAB/KUD], 

Method Medium logic Xi ogioA.i logic Xl Reference 

The reported conditional equilibrium constants for the complex Th(OH)2, either determined in dilute solutions or in NaC104 media (Table VII-11), are extrapolated to zero ionic strength with the SIT using s( Th(OH)2, CIO4) = (0.33+0.1) kg-moP estimated in Section VII.3.6.1 and As(2,l) = -(0.09 + 0.14) kg-mol. The 

Baes and Mesmer [1976BAE/MES] selected logm 2° -(6.93+0.2) and specific interaction coefficients from the value of [1971K1C/STE] in dilute solutions and that of [1965BAE/MEY] in 1 M NaC104. The value of logi jd° = -(6.0+ 0.6) 

In the review of Baes and Mesmer [1976BAE/MES] an upper hmit of logj -11.7 was calculated from the solubility data of [1964NAB/KUD] while Neck and Kim [2001NEC/KIM] selected an estimated value of logn, /3° = -(11 + 1) in their review. The present review does not recommend an equilibrium constant for the complex Th(OH)3 whieh is eonsidered to be a minor speeies in both potentiometrie and solubility studies. 

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Nabivanets and Kudritskaya studied the solubility of an amorphous Th(OH)4(am) precipitate in 0.1 M NaC104 at (17 + 1)°C. The solubility was measured in the pH range 3.4 to 7.0 after equilibration times of 3-6 days. The Th concentrations were determined colorimetrically after centrifugation for 1 hour at 10000 rpm with a usual laboratory centrifuge. The measured solubilities, which decrease from 1.6 x 10 M at pH 3.40 to 5 X 10 M at pH 5.35-7.00, were described by assuming only the presence of mononuclear Th(IV) hydroxide complexes. The stepwise equilibrium constants for the reactions ... 

In the present study, Hietanen and Sillen provide convincing evidence for the formation of complexes containing one and tw o coordinated hydroxides. By using additional information on the equihbrium constants for the mononuclear hydroxide complexes of Th(IV) they suggest that the complexes formed are binuclear, that is Th2(OH) and Th lOH) ". The equihbrium constants for these complexes at 25°C are logic A 2 = -(2-8 + 0.2) and logi X2 = (5 02 + 0.04) in 0.5 M ThCh log,o / , 2 = -(2.9+ 0.2) and log,o X2 = -(5.09 + 0.04) in 0.7 M ThCL, + 2.2 M NaCl. The experimental technique and data analysis are described in detail. It is not possible to extrapolate the equilibrium constants to zero ionic strength and therefore no constants have been selected by this review. 

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