Evaluation of distribution ratio in ion pair extraction using fundamental thermodynamic quantities Yumi Yoshida a , Masakazu Matsui a , Osamu Shirai b , Kohji Maeda c , Sorin Kihara c,* a Institute for Chemical Research, Kyoto University, Uji, Kyoto 611, Japan b Japan Atomic Energy Research Institute, Oarai, Ibaraki 311-13, Japan c Department of Chemistry, Kyoto Institute of Technology, Matsugasaki, Sakyoku, Kyoto 606, Japan Received 17 November 1997; received in revised form 20 April 1998; accepted 2 June 1998 Abstract Equations were derived for the quantitative expressions of distribution ratios, D M , in ion pair extractions of a cation, M  , with a counter ion, X  , from water, W, to an organic solvent, O, in the absence and presence of a special complexing neutral ligand, L, in O by using standard transfer free energies of M  and X  from W to O, ion pair formation constants of M  with X  in W and O, the stability constant of the complex, (ML)  , in O and the ion pair formation constant of (ML)  with X  in O. The D M values calculated by substituting the constants determined by electrochemical methods into the derived equations agreed very well with those obtained experimentally by ion pair extraction, which means that the extraction processes assumed in the derivation of the equations was reasonable. # 1998 Elsevier Science B.V. All rights reserved. 1. Introduction The quantitative understanding of processes involved in the ion pair solvent extraction is indis- pensable not only for the attainment of highly effective separation of ions [1], but also the elucidation of such biological phenomena as the selective transport of ions through biomembranes since the biomembranes are often regarded as organic solvents [2±4]. The extraction process has been assumed to be as in Fig. 1(a) in many text books [5±7]. That is, the objective ion, M  , forms an ion pair with the counter ion, X  , in an aqueous phase, W, prior to the transfer to the organic, O. Then, the neutral ion pair transfers into O and dissociates to ions partially. However, it is not natural to assume the distribution of the ion pair in W whose concentration is much less than that of dissociated ion. On the other hand, some authors in ®elds of elec- trolyte solution or electrochemistry assume the pro- cess to be as in Fig. 1(b) [8±16], i.e., M  and X  transfer from W to O individually depending on their own transfer free energies, ÁG tr , and holding the electroneutrality of both phases as well as the mass balance, and a part of M  forms the ion pair with X  , in O. The following are typical examples of works which support the process of Fig. 1(b). Friedman et al. [11] reported that the differences between distribution ratios, D, of two alkali metal ions in the ion pair extraction were unchanged even when the extraction was carried out by adopting different Analytica Chimica Acta 373 (1998) 213±225 *Corresponding author. Fax: +81-75-724-7518; e-mail: kiharas@ipc.kit.ac.jp 0003-2670/98/$19.00 # 1998 Elsevier Science B.V. All rights reserved. PII S0003-2670(98)00367-5