Short communication
Individual Extraction Constants of Some Univalent Anions in the Two-Phase Water-Phenyltrifluoromethyl Sulfone System
Emanuel Makrllk,1'* Pavel Selucky2 and Petr Vanura3
1 Faculty of Applied Sciences, University of West Bohemia, Husova 11, 306 14 Pilsen, Czech Republic
2 Nuclear Research Institute, 250 68 Rez, Czech Republic
3 Department of Analytical Chemistry, Institute of Chemical Technology, Prague, Technicka 5,
166 28 Prague 6, Czech Republic
* Corresponding author: E-mail: makrlik@centrum.cz
Received: 31-05-2011
Abstract
From extraction experiments and y-activity measurements, the extraction constants corresponding to the general equilibrium Cs+(aq) + A- (aq) ^ Cs+(org) + A- (org) taking place in the two-phase water-phenyltrifluoromethyl sulfone (FS 13) system (A-= I-, ClO- MnO,-, Br-, I-, picrate, tetraphenylborate (BPh-); aq = aqueous phase, org = FS 13 phase) were evaluated. Furthermore, the individual extraction constants of these 7 anions in the mentioned two-phase system were calculated; they were found to increase in the series of I-< ClO^ < Br- < MnO,-, picrate < I- < BPh-!.
Keywords: Univalent anions, water-phenyltrifluoromethyl sulfone system, individual extraction constants
1. Introduction
Nitrobenzene, a dipolar aprotic solvent, has frequently been used in liquid-liquid extraction, especially in the separation of cesium, strontium, barium, europium and americium from aqueous solutions.1-24 Nitrobenzene is also suitable for accurate conductometric measure-ments.25 A considerable attention has been paid to the study of charge transfer across the water/nitrobenzene interface.26-28 Moreover, a process involving chlorinated cobalt dicarbollide, polyethylene glycol (PEG 400) and dip-henyl-N,N-dibutylcarbamoylmethyl phosphine oxide (DPDBCMPO), also called UNEX, has been reported for the simultaneous recovery of cesium, strontium, lanthanides and actinides from highly acidic media into phenyl-trifluoromethyl sulfone (abbrev. FS 13).29,30 At this point it should be noted that the mentioned FS 13 diluent was developed for the UNEX process as an alternative organic diluent to the highly polar nitrobenzene. Besides this, FS 13 has the advantage of low viscosity and very good solubility of the UNEX extractants and metal solvates.30 On the other hand, nitrobenzene derivatives have been suc-
cessfully utilized as diluents for cobalt dicarbollide processes in Russia, however, they are deemed unsuitable for use in the United States due to the perceived hazards associated with nitrobenzene.
In order to estimate the individual extraction constants of some univalent ions in the water-nitrobenzene system,31 a nonthermodynamic assumption of equal ex-tractabilities of the tetraphenyllarsonium cation, Ph4As+, and the tetraphenylborate anion, BPh-, of the "reference" electrolyte32 from the aqueous into nitrobenzene phase, expressed by the identity log KPhAs+ = log K^Phi = 6.3,31 was adopted. Both these ions are sufficiently and practically equally bulky and their phenyl group "covers" are identical. Thus, it is quite justifiable to assume that their specific interactions with the molecule of the solvent are very limited and virtually equal. The above-mentioned "Ph4AsBPh4 assumption" was further employed for a number of the two-phase water-polar organic solvent sys-
33—35
tems. 35
The individual extraction constants of many univalent ions have been evaluated in the water-nitrobenzene system.31,36 38 Recently, analogous thermodynamic data
concerning some divalent and trivalent metal cations, as well as some univalent organic cations, have been reported for this two-phase system.39-41 On the other hand, in the current communication, the individual extraction constants for a series of 7 univalent anions in the two-phase water-FS 13 system are presented.
2. Experimental
Phenyltrifluoromethyl sulfone (FS 13) was supplied by Khlopin Radium Institute, St. Petersburg, Russia. Cesium iodide (Csl), cesium perchlorate (CsClO4), cesium permanganate (CsMnO4), cesium tribromide (CsBr3), cesium triiodide (CsI3) and cesium tetraphenylborate (Cs-BPh4) were purchased from Aldrich and were employed as received. The radionuclide 137Cs+ (Techsnaveksport, Russia) was of standard radiochemical purity. The other chemicals used (Lachema, Brno, Czech Republic) were of reagent grade purity. A solution of cesium picrate in water was prepared by dissolving stoichiometric amount of picric acid in an aqueous solution of CsOH.
The extraction experiments were carried out in 10 mL glass test-tubes with polyethylene stoppers: 2 mL of FS 13 and microamounts 137Cs+ were added to 2 mL of an aqueous solution of Cs+, A- (A- = I-, ClO-, MnO-, Br-,, I-,, picrate, BPh-) of the concentration in the range from 1 x 10-4 to 1 x 10-3 mol/L. The test-tubes filled with the solutions were shaken for 2 h at 25 ± 1 °C, using a laboratory shaker. Then the phases were separated by centrifugation. Afterwards, 1 mL samples were taken from each phase and their y-activities were measured using a well-type Na-I(Tl) scintillation detector connected to a y-analyzer NK 350 (Gamma, Budapest, Hungary).
The equilibrium distribution ratios of cesium, DCs, were determined as the ratios of the measured radioactivities of 137Cs+ in the FS 13 and aqueous samples.
3. Results and Discussion
With respect to the results of previous papers,1,31,36,37 the two-phase water-Cs+A- (A- = I-, ClO,-, MnO,-, Br-, I-, picrate, BPh,)-FS 13 extraction system (see Experimental) can be described by the following general equilibrium
(1)
with the corresponding extraction constant Kex(Cs+, A); aq and org denote the presence of the species in the aqueous and FS 13 phases, respectively.
The changes of the standard Gibbs energies corresponding to the transfer of the ions Cs+ and A- (A- = I-, ClO-, MnO,-, Br-, I-, picrate, BPh-,) from the aqueous in-
to FS 13 phase
Cs+(aq) » Cs+(org) A~(aq) « A"(org) are given by definitions
^QO^-MTg _ t 'J.I'V _ ,,0-aq
= M ....... - Li.....-
"rv "
tr.Cs"
^QÖ.aq-njrg _ jjO.org _ ^ o.aq
(2)
(3)
(4)
(5)
Here - s are the standard chemical potentials of the ions Cs+ and A- in the two phases concerned.
The individual extraction constants KCs+ and KA-(A = I-, ClO,-, MnO,-, Br-, I-, picrate, BPh-) for the considered univalent ions in the water-FS 13 extraction system are defined by
AG"'™=-RTlnK^
tr,Cs	Cs
AG0«""8 =-RTln K'
tr.A	A
Combination of the relation 1 - -i _ ,, . ,,
- RT In Kh (Cs+, A" ) - ^ + ^ - -H
O.org
(6)
(7)
(8)
corresponding to Equilibrium (1) with Eqs (,) - (7) yields
The standard Galvani potential difference of the considered anion A- between the FS 13 and aqueous phases (see, e. g., Refs ,2 and ,3), denoted by the symbol A"g ^A-, is defined by the following relation
A°V - AG^^/F
IUI ™A"	lr.A '
(10)
Taking into account the conditions of electroneutra-lity in the organic and aqueous phases of the system under study (see Experimental)
and the measured equilibrium distribution ratio of cesium
then combination of relations (11) - (13) with the expression for K (Cs+, A ) (see Equilibrium (1)) in the form
Ktx(Cs\A") =
[Csq^A-L [Cs+L[A"L
gives the final expression for the extraction constant K(Cs+, A-):
Kts(Cs\A")=D
(15)
In this study, from the extraction experiments and Y-activity measurements (see Experimental) by means of Eq. (15), the logarithms of the constants Kex(Cs+, A) were determined, as listed in Table 1.
lity of the A- ions from the aqueous into organic phase. This means - with regard to the experimental data presented in Tables 1 and 2 - that the extractability of the anions I-, ClO4, picrate, I- and BPh4 from the aqueous phase is somewhat higher into the nitrobenzene phase than into the FS 13 one.
In conclusion, it is necessary to emphasize that the thermodynamic constants log KA_, AG0raq^org and A"g ^ listed in Table 1 can be employed, e. g., for the studies of
Table 1. Thermodynamic data for some univalent organic anions in the two-phase water-FS 13 extraction system at 25 °C (for the meaning of the constants see text).
A-	log Kex(Cs+, A-) "	log KA->	AG^T*^ (kJ/mol) c	AO? <(V)
I-	-7.8	-3.5	20.0	0.207
ClO4	-6.0	-1.7	9.7	0.101
MnO4	-4.2	0.1	-0.6	-0.006
Br-	-5.9	-1.6	9.1	0.095
I-	-1.8	2.5	-14.3	-0.148
picrate	-4.2	0.1	-0.6	-0.006
BPh-4	0.9	5.2	-29.7	-0.308
2 Calculated from Eq. (15); b Calculated from Eq. (9); c Calculated from Eq. (7); d Calculated from Eq. (10).
Using the constants log Kex(Cs+, A ) given in Table 1, the value log K'Cs+ = -4.3,44 and applying Eq. (9), we obtain the logarithms of the individual extraction constants of the univalent anions A- (A- = I-, ClO4, MnO4, Br-, I3, picrate, BPh4), log KA_, in the two-phase water-FS 13 system. These data are summarized in Table 1. Besides this, by means of Eqs (7) and (10), one gets the thermodynamic data AG0r™ and A"g (pA-, respectively, reviewed in Table 1 as well.
Finally, from the data given in Tables 1 and 2 it follows that the individual extraction constants log KAi - in the water-FS 13 system increase in the sequence of I- < ClO4 < Br3 < MnO4, picrate < I3 < BPh4, whereas log KA_ in the water-nitrobenzene system increase in the following anion order: I- < ClO4 < picrate < I- < BPh4. In this context it should be noted that the log KAi - constants express, in principle, the affinity of the considered anions A- for the organic phase, i. e., the log KA_ constants are a quantitative measure of the hydrophobicity, as well as the extractabi-
Table 2. Individual extraction constants (log Ky of I-, ClO- picrate , I- and BPh- in the two-phase water-nitrobenzene system at 25 °C (according to Ref. 31).
A-	log KA-a
I-	-3.3
ClO4-	-1.4
picrate	0.8
I-	4.1
BPh-4	6.3
a Ref. 31.
chemical equilibria in the two-phase water-FS 13 system, analogously as in some previous papers.45-50
4. Acknowledgement
This work was supported by the European Project NTIS-New Technologies for Information Society No.: CZ.1.05/1.1.00/02.0090 and by the Czech Ministry of Education, Youth and Sports, Project MSM 6046137307.
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Povzetek
Z uporabo ekstrakcijskh eksperimentov in meritev y-aktivnosti smo v dvofaznem sistemu voda-feniltrifluorometilsulfon (FS 13) določili konstante ekstrakcije za ravnotežja, ki jih splošno lahko zapišemo kot Cs+(aq) + A- (aq) ^ Cs+(org) + A- (org); A-= I-, ClO4, MnO4, Br-, I-, pikrat, tetrafenilborat (BPh4); aq = vodna faza, org = faza FS 13. Izračunali smo tudi konstante ekstrakcije za posamezne anione in ugotovili, da naraščajo v smeri: I-< ClO4 < Br- < MnO4, pikrat < I-< BPh-4.