Lanthanide fluoride stability constants

Previous assessments of lanthanide fluoride stability constants appropriate at 25 C and zero ionic strength indicate that logp/J is on the order of 4 or less for all lanthanides. The works of Walker and Choppin (1967) and Bilal and co-workers (Bilal et al. 1979, Bilal and Becker 1979, Bilal and Koss 1980, Bilal 1980, Bilal and Becker 1980, Becker... 

Table 4.2 Aqueous stability constants (log /3i) for lanthanide (3-I-) ions with fluoride, EDTA, and DTPA ...

Question 4.1 Study the data in Tables 4.1 and 4.2 (above). Plot a graph of log ft against atomic number for complex formation with EDTA ". Deduce and explain the patterns in behaviour for the lanthanides compare the values of the stability constants for EDTA and fluoride complexes. 

Solvent extraction, potentiometry, and calorimetry have been used to determine the thermodynamic parameters of the formation of the monofluoride complex of the trivalent lanthanide ions at 25°C. and an ionic strength of IM (NaClOj ). The enthalpies were all endothermic, ranging from 4.0 to 9.5 Kcal./mole consequently, the large, positive entropies, ranging from 25 to 48 cal./°C./mole, explain the high stability constants. This large entropy results from the decrease in overall water structure when the fluoride ion is complexed. The difference in the enthalpies of formation of LnF and LnAc " can possibly be explained by a difference in covalence for Ln-F and Ln-O bonds. 

These values compare favorably with those reported by Kury at /x = 0.5 and 25 °C. The stability constants of the lanthanide fluoride complexes were obtained by comparing changes in the potential in cell C with those in that of cell B upon adding sodium fluoride solution. Owing to the insolubility of LnFg, only a few (often only six) aliquots of sodium fluoride solution could be added to cell C before precipitation interferred. Precipitation could be observed easily since upon its occurrence, the potential between cells C and A would drift rapidly to lower values. 

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