Solubility studies at high pH and carbonate concentrations

The available solubility data for Th02(am, hyd) in carbonate solution (19 sets of experimental data from different groups of authors) under widely varying conditions pH 4.5-13, carbonate concentrations up to 2 M and ionic strength varying from 1 = 0.1 M to 4.0 M (even up more than 6 M) are well described with only a few ternary complexes, the most important ones being Th(0H)(C03)4 , Th(0H)2(C03)2 and Th(0H),(C03) -. 

the following formation constants and SIT coefficients are selected  

The solubility data at high carbonate concentrations ( 0.5 M) and pH 12 [1995RAI/FEL], [1997FEL/RAI], [1999FEL/RAI], in particular with additions of NaCl, can be better described by assuming the formation of an additional (124) complex = Th(0H)2(C03)4 , with  

The solubility data determined with a dried Th(IV) oxyhydroxide phase at / = 0.5 M, both at Ctot = 0.1 M and under CO2 partial pressures of 1.0 and 0.1 bar [19940ST/BRU], [2005ALT/NEC], set an upper limit for the formation constant of Th(C03)5. Altmaier et al. [2005ALT/NEC] indicate that logi is more negative than -16.6, which is consistent, within the uncertainties, with the value of-(16.4 + 0.3) from [19940ST/BRU]. If this value is combined with logn, = - (47.6 + 0.5) or 

EXAFS measurements in various carbonate and bicarbonate solutions [1997FEL/RAI], [1997HES/FEL], [2006ALT/NEC] give only quahtative information on the speciation because the spectra could not be deconvoluted into single component contributions. However, in combination with the known formation constant of Th(0H)(C03) (log,D ° = (35.6 + 0.5), cf. Section Xl.1.3.2.1), the most important competing species in these solutions, the information from the EXAFS studies allows the estimation of the equilibrium constant log, within a certain range. 

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