Short communication
Distribution of Microamounts of Cesium in the Two-Phase Water-HCl-Nitrobenzene-2,3-Naphtho-15-crown-5-Hydrogen Dicarbollylcobaltate Extraction System
Emanuel Makrllk,1'* Petr Vanura2 and Pavel Selucky3
1 Faculty of Applied Sciences, University of West Bohemia, Husova 11, 306 14 Pilsen, Czech Republic 2 Institute of Chemical Technology, Prague, Technickâ 5, 166 28 Prague, Czech Republic 3 Nuclear Research Institute, 250 68 Rez, Czech Republic * Corresponding author: E-mail: makrlik@centrum.cz Received: 19-08-2009
Abstract
Extraction of microamounts of cesium by a nitrobenzene solution of hydrogen dicarbollylcobaltate (H+B-) in the presence of 2,3-naphtho-15-crown-5 (N15C5, L) has been investigated. The equilibrium data have been explained assuming that the complexes HL+, HL+, CsL+ and CsL2 are extracted into the organic phase. The values of extraction and stability constants of the species in nitrobenzene saturated with water have been determined.
Keywords: Cesium, 2,3-naphtho-15-crown-5, hydrogen dicarbollylcobaltate, extraction and stability constants, water -nitrobenzene system
1. Introduction
The observation that macrocyclic polyethers form stable complexes with alkali and alkaline earth metal cations has stimulated a great deal of interest in these compounds for their possible applications in various branches of chemistry and biology.1,2 Extensive thermodynamic da-ta3,4 suggest that the stability of macrocyclic complexes depends on the relative cation and ligand cavity size, the number and arrangements of the ligand bonding sites, the substitution on the macrocyclic ring and the solvent effects.
Dicarbollylcobaltate anion5 and some of its halogen derivatives are very useful reagents for the extraction of various metal cations (especially Cs+, Sr2+, Ba2+, Eu3+ and Am3+) from aqueous solutions into a polar organic phase, both under laboratory conditions for purely theoretical or analytical purposes,6-31 and on the technological scale for the separation of some high-activity isotopes in the reprocessing of spent nuclear fuel and acidic radioactive wa-ste.32-34
Numerous naphtho-crown ethers have been synthesized and studied. The literature reports spectroscopic characteristics,3536 X-ray structure analyses,37 complexa-
tion properties38 and studies of chiral naphtho-crowns.39-41 On the other hand, in the present work, the extraction of microamounts of cesium by a nitrobenzene solution of hydrogen dicarbollylcobaltate (H+B-)5 in the presence of somewhat rigid 2,3-naphtho-15-crown-5 ligand (see Scheme 1) was studied. We intended to find the composition of the species in the nitrobenzene phase and to determine the corresponding equilibrium constants.
OX0
Scheme 1. Structural formula of 2,3-naphtho-15-crown-5 (abbrev. N15C5 or L, respectively).
2. Experimental
2,3-Naphtho-15-crown-5 (N15C5, L) was supplied by Fluka, Buchs, Switzerland. Cesium dicarbollylcobalta-
te, Cs+B-, was synthesized in the Institute of Inorganic Chemistry, Rež, Czech Republic, using the method published by Hawthorne et al.42 A nitrobenzene solution of hydrogen dicarbollylcobaltate (H+B-)5 was prepared from Cs+B- by the procedure described elsewhere.43 The other chemicals used (Lachema, Brno, Czech Republic) were of reagent grade purity. The radionuclide 137Cs+ (Techsna-veksport, Russia) was of standard radiochemical purity.
The extraction experiments in the two-phase wa-ter-HCl-Cs+ (microamounts)-nitrobenzene-N15C5 -H+B- system were performed in 10 cm3 glass test-tubes covered with polyethylene stoppers, using 2 cm3 of each phase. The test-tubes filled with the solutions were shaken for 2 h at 25 ± 1 °C, using a laboratory shaker. Under these conditions, the equilibria in the systems under study were established after approximately 20 min of shaking. Then the phases were separated by centrifugation. Afterwards, 1 cm3 samples were taken from each phase and their y-activities were measured using a well-type NaI(T1) scintillation detector connected to a single-channel y-analyzer NK 350 (Gamma, Budapest, Hungary).
The equilibrium distribution ratios of cesium, D, were determined as the ratios of the corresponding measured radioactivities of 137Cs+ in the nitrobenzene and aqueous samples.
3. Results and Discussion
The dependences of the logarithm of the cesium distribution ratios (log D) on the logarithm of the numerical value of the total (analytical) concentration of the ligand N15C5 in the initial nitrobenzene phase, log c(L), are given in Figure 1. The initial concentrations of hydrogen dicarbollylcobaltate in the organic phase, cB = 0.001 and
Figure 1. Log D as a function of log c(L), where L is N15C5, for the water-HCl-Cs+ (microamounts) - nitrobenzene - N15C5-H+B- system. O c(HCl) = 0.10 mol dm-3, cB = 0.001 mol dm-3. □ c(HCl) = 0.40 mol dm-3, cB = 0.0005 mol dm-3. The curves were calculated using the constants given in Table 2.
0.0005 mol dm-3, as well as the initial concentrations of HCl in the aqueous phase, c(HCl) = 0.10 and 0.40 mol dm-3, are always related to the volume of one phase.
With regard to previous results,5,44-48 the considered water-HCl-Cs+(microamounts)-nitrobenzene-N15C5 (L)-H+B- systems can be described by the set of reactions
(1) (2)
(3)
(4)
(5)
H0* + L<*a HLorg
H* + 2L HL*
orça,	org	¿,org
CS^+h^OCS^+H;
C^+^+H^OCsl^+H;
to which the following equilibrium constants correspond:

[LJ;
/î(hl;j =
/J(HLW =
Kex(CS^)-
[HL^l
Pdmj
[HL+,WJ
[H^ltL^]2 [C>* IPC]
K„(CsL. ) =
[CsL;org][H;]

(6)
(7)
(8) (9)
(10)
The subscripts "aq" and "org" denote the aqueous and organic phases, respectively.
A subroutine UBBE, based on the relations given above, the mass balance of the N15C5 ligand and the elec-troneutrality conditions in both phases of the system under study, was formulated47,48 and introduced into a more general least-squares minimizing program LETAGROP49 used for determination of the "best" values of the extraction constants Kex(CsLi+org) (L = N15C5). The minimum of the sum of errors in log D, i.e., the minimum of the expression
(11)
was sought.
The values log KD = 2.57 (see Table 2, footnote a), log AHL+rg) = 3.13, 45 log j6(HL2+,OTg)
5.53 and log Kex(Cso+rg) = 3.0946 were used for the respective calculations. The results are summarized in Table 1, from which it is evident that the extraction data can be best explained
+ and CsL+ (L = N15C5) to
assuming the complexes CsL be extracted into the nitrobenzene phase.
Knowing the value log Kex(Cs+rg) = 3.09; the extraction constants log Kex (CsL+rg) = 6.55 log Kex (CsL+org) = 8.56 determined here (Table 1), the
46 as well as and
stability constants of the complexes CsL+ and CsL+ in the nitrobenzene phase defined as
/i(CsLM =

[Csl;J [cs;rg][Lorj ■
[Csl;0j
[Cs;j[L„r 1! '
(12)
(13)
can be evaluated applying the following simple relations: log yS(CsL+ors ) = log (CsL^ ) - log K„ (Cs^ ), (i4)
log»!,* ) = log K^CsL* ) - log IUCsL,) .(15)
Table 1. Comparison of three different models of cesium extraction from aqueous solutions of HCl by nitrobenzene solution of H+B- in the presence of N15C5.
Cesium complexes		
in the organic phase	log Kex 0	U b
CsL+	6.67 (7.03)	0.04
CsL+	9.17 (10.11)	0.33
CsL+, CsL+	6.55 (6.69), 8.56 (9.08)	0.01
Figure 2. Distribution diagram of hydrogen cation in the equilibrium nitrobenzene phase of the water-HCl-Cs+ (microamounts) -nitrobenzene-N15C5-H+B- extraction system in the forms of H+, HL+ and HL+. c(HCl) = 0.10 mol dm-3, cB = 0.001 mol dm-3. 1 8(H+) =-r" + ]/~(H+)
: [H+rg]/c(H+)„
2	S(HL+) = [HL+rg]/c(H+)
3	S(HL+) = [HL+org]/c(H+)org,
org'
where c(H+)ols = [^------^------^
The distribution curves were calculated using the constants given in Table 2.
Moreover, Figure 2 depicts the contributions of the species H+rg, HL+rg and HL+org to the total hydrogen cation concentration in the equilibrium nitrobenzene phase, whe-
a The values of the extraction constants are given for each complex. The reliability interval of the constants is given as 3a(K), where a(K) is the standard deviation of the constant K. 49 These values are given in the logarithmic scale using the approximate expression log K ± {log[K + 1.5a(K)] - log [K - 1.5a(K)]}. For a(K)>0.2K, the previous expression is not valid and then only the upper limit is given in the parentheses in the form of log K(log[K + 3a(K)]) .49 b The error-square sum U = S(log Dcalc -log Dexp)2.
The respective equilibrium constants are listed in Table 2.
Table 2. Equilibrium constants in the water-HCl-Cs+ (mi-croamounts)-nitrobenzene-N15C5-H+B- system.
Equilibrium
log K
Laq « Lorg H+org + Lorg « HL+rg H+org + 2Lorg « HL2,org Cs+q+ H+rg « Cs+rg + H+q
Cs+ + T + H+ « CsT + + H+
aq org + norg w ^^org + naq
Cs+ + 2L + H+ « CsL + + H+
aq	org org	2,org aq
Cs+rg + Lorg « CsL+rg
C^org + 2Lorg « CsL+,org_
2.57 a 3.13 b 5.53 b 3.09 c
6.55
8.56
3.46
5.47
a Determined by the concentration dependent distribution method. 44 b Ref. 45. c Ref. 46.
Figure 3. Distribution diagram of cesium in the equilibrium nitrobenzene phase of the water-HCl-Cs+ (microamounts) -nitrobenze-ne-N15C5-H+B- extraction system in the forms of Cs+, CsL+ and CsL+. c(HCl) = 0.10 mol dm-3, cB = 0.001 mol dm-3.
1	S(Cs+) = [Cs+g]/c(Cs+) ,
2	8(CsL+) = [CsL+lg]/c(Cs+)0rg,
3	8(CsL+) = [CsL+0lg]/c(Cs+)0rg,
where c(Cs+)0rg = [Cs+J + [CsL+rg] + [Cs^].
The distribution curves were calculated using the constants given in
Table 2.
Table 3. Stability constants of the complexes HL+, HL+, CsL+ and CsL+, where L = 15-crown-5 (15C5), 2,3-naphtho-15-crown-5 (N15C5), in nitrobenzene saturated with water at 25 °C.
L	log j3(HLQrg)	log j3(HL2„rg) log fffCsl/ g log KCsL+QJ
15C5	4.27 a	6.32 a	4.24 b	6.72 b
N15C5	3.13 c	5.53 c	3.46 d	5.47 d
a Ref. 50. b Ref. 51. c Ref. 45. d This work.
reas Figure 3 shows the contributions of the cations Cs+rg, CsL+ and CsL+ to the total cesium concentration in the
org	2,org
equilibrium organic phase. From Figures 2 and 3 it follows that the complexes HL2+,org and CsL+2,org are present in significant concentrations only at relatively high amounts of the N15C5 ligand in the system under consideration.
In conclusion, the stability constants of the complexes HL+, HL+, CsL+ and CsL+ (L = 15C5, N15C5) in nitrobenzene saturated with water at 25 °C are reviewed in Table 3. In this context it should be noted that somewhat higher stability of the cationic complex species HL^rg, HL+ CsL+ , and CsL+ , where L = 15C5, in water sa-
2,org	org	2,org
turated nitrobenzene in comparison with the stability of the respective complexes of H+ and Cs+ with N15C5 li-gand in the mentioned medium (see Table 3) can be obviously explained on the basis of the higher flexibility of the ligand 15C5 compared with the relatively rigid structure of N15C5.
4. Acknowledgements
The present work was supported by the Czech Ministry of Education, Youth and Sports, Projects MSM 4977751303 and MSM 6046137307, and by the Specific Research of the Faculty of Applied Sciences, University of West Bohemia, Pilsen, Czech Republic.
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Povzetek
Proučevali smo ekstrakcijo mikrokoličin cezija z raztopino vodikovega dikarbolilkobaltata (H+B-) v nitrobenzenu ob prisotnosti 2,3-nafto-15-crown-5 (N15C5, L). Dobljena ravnotežja smo pojasnili s predpostavko, da se kompleksi HL+, HL+, CsL+ in CsL+ ekstrahirajo v organsko fazo. Določili smo konstante ekstrakcije in stabilnosti kompleksov v nitrobenzenu nasičenem z vodo.