P] are the distribution coefficients due to the lanthanoid extraction with HTTA or HP alone under the same experimental conditions).
853] that a combination of one weak and one strong [beta]-diketone can produce the extraction of a mixed adduct accompanied by a synergistic effect as well as with the statement of Hala  that the weaker acid is involved in the mixed adduct as a neutral molecule (the pKa values of HTTA, HP, and PAR are 6.
Taking into account that the partition of HP , HTTA , and PAR  toward the aqueous phase is very low and on the basis of the slope analysis data, the overall equilibrium constants [K.
It can be seen that the curves of HTTA and HP vary practically in the same manner as those including a synergistic agent.
The separation factors and the synergistic coefficients (given also in Table 2) show that the addition of PAR to the chelating extractants HTTA and HP not only enhances the extraction efficiency but improves the selectivity among the lanthanoids as well although it has been generally believed that synergistic extraction makes the separation of metals worse, compared with the extraction with a chelating ligand only.