Electrochemistry of ionophore-coordinated Cs and Sr ions in the tri-1-butylmethylammonium bis((trifluoromethyl)sulfonyl)imide ionic liquid

The selective extraction of Cs+ and Sr2+ from aqueous solutions by using the ionophores calix[4]arene-bis(tert-octylbenzo-crown-6) (BOBCalixC6) and dicyclohexano-18-crown-6 (DCH18C6), respectively, was demonstrated in the hydrophobic, room-temperature ionic liquid (RTIL), tri-1-butylmethylammonium bis((trifluoromethyl)sulfonyl)imide (Tf2N-). The electrochemistry of Cs+ coordinated by BOBCalixC6 and Sr2+ coordinated by DCH18C6 was examined at a mercury film electrode (MFE) in this ionic liquid by using cyclic staircase voltammetry, sampled current voltammetry at a rotating electrode, and chronoamperometry. Both BOBCalixC6(.)2Cs(+) and DCH18C6(.)Sr(2+) exhibit well-defined reduction waves at approximately -2.4 and -2.9 V versus the ferrocene/ferrocenium (Fc/Fc(+)) couple, respectively, in which the coordinated ions are reduced to their respective amalgams, permitting the recycling of the ionophores. The diffusion coefficients of BOBCalixC6(.)2Cs(+) and DCH18C6(.)Sr(2+) are (2.7 +/- 0.1) x 10(-9) and (2.1 +/- 0.1) x 10(-9) cm(2) s(-1), respectively, at 30 degrees C. The coulometric efficiency for the reduction and stripping of Cs at mercury pool electrodes was about 90% and was independent of the deposition time, whereas the efficiency for Sr was slightly less than 90% at short times and decreased with the deposition time, probably due to the formation of a passive layer of Sr(Tf2N)(2). (c) 2004 Elsevier Ltd. All rights reserved.