The extraction of lanthanides(III) (Ln(III)) from a 0.1 M sodium perchlorate solution with 2-thenoyltrifluoroacetone (Htta, HA) with pyridine (Py) into chloroform was investigated in a thermostatted room at 298 K, in order to examine in detail the variation of the reactivity of Ln(tta) 3 with an N-donor ligand across the lanthanide series. It was found that i) The extracted species was the ion-pair (HPy) + LnA 4 as well as LnA 3 Py and LnA 3 . ii) The values of the formation constants of LnA 3

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... dducts with pyridine are smaller than those with carboxylic acids across lanthanide series. iii) The variation in the formation constants of the ion-pairs represented by Ln 3+ + H + + 4A -+ Py (o) (HPy) + LnA 4 - (o) showed a maximum value in the vicinity of Nd or Sm. Then the separation factor (SF) for Pr from Sm estimated from the ion-pair extraction is a little larger than that from LnA 3 . However, the SF calculated for the adduct, LnA 3 Py, was not improved on comparison with the SF for LnA 3 .