1310
ATANASSOVA
for the HQ–18C6 mixture, while the values of SC well. The synergistic enhancement factors are larger
obtained for HQ–B18C6 are almost the same. Reddy et when 18C6 is used in combination with HQ. Their val-
al. [18] have also reported that the synergistic constant ues increase with increasing atomic number of the lan-
increases from Nd3+ to Tm3+ in their extraction with
mixtures of 3-phenyl-4-(4-fluorobenzoyl)-5-isox-
azolone and various crown ethers. The lanthanide con-
traction and steric effects are probably significant in
establishing the different synergism.
thanides. But such a tendency was not found for the
extraction with HQ–B18C6.
The SF between the adjacent metals are high for the
system HQ–18C6. Still, their values are much larger for
the heavier lanthanides.
The metal separation can be assessed using separa-
tion factors. The separation factors of the pairs were
calculated as a ratio of the overall equilibrium constant
KQ, S values of every pair as the metals form complexes
of the same type when they are extracted with
HQ−B18C6 (18C6) mixtures. The values of the separa-
tion factors are given in Table 2. The data show that the
separation factors between adjacent metal ions for the
lanthanides extracted with the mixture HQ–18C6 are
rather high. It is seen that the separation factors
decrease in the order HQ–18C6 > HQ–B18C6, and that
the lanthanide extraction decreases in the same order. It
is interesting to note that the synergistic agent 18C6
exhibits a separation for the heavier lanthanides two
times higher than those obtained when B18C6 is used.
The separation factor between Tm/Eu obtained when
the HQ–18C6 mixture is used is rather high, 7.9 × 103,
which is somewhat higher than those obtained by
Reddy et al. [11–13, 18] for some lanthanide pairs with
various chelating extractants (4-acylpyraolones and
isoxazolones) and DB18C6. It is interesting to note that
HQ, which is a much poorer extractant for lanthanides
than β-diketones (the values of the equilibrium constants
are some orders of magnitude lower), exhibits a separa-
tion for the pair Tm/Eu, which is 1000 times higher. But
very high separation factors 1 × 103 and 2 × 103 have been
observed between U(VI) and Th(VI) by Reddy et al. [37]
when they are extracted into chloroform from nitric
acid solutions using 3-phenyl–4-benzoyl–5-isox-
azolone, which is a strong acidic extractant with pKa =
1.23, and DB18C6 and B15C5 respectively.
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RUSSIAN JOURNAL OF INORGANIC CHEMISTRY Vol. 52 No. 8 2007