Short communication
Synergistic Extraction of Strontium into Nitrobenzene
by Using Hydrogen Dicarbollylcobaltate and N,N,W,W-Tetracyclohexyl-oxybis(o-phenyleneoxy)
diacetamide
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 Department of Analytical Chemistry, Institute of Chemical Technology, Prague, Technicka 5,
166 28 Prague 6, Czech Republic
3 Nuclear Research Institute, 250 68 Rez, Czech Republic
* Corresponding author: E-mail: makrlik@centrum.cz
Received: 09-08-2011
Abstract
Extraction of microamounts of strontium by a nitrobenzene solution of hydrogen dicarbollylcobaltate (H+B-) in the presence of N,N,N',N'-tetracyclohexyl-oxybis(o-phenyleneoxy)diacetamide (abbrev. barium ionophore I, L) has been investigated. The equilibrium date have been explained assuming that the species HL+, SrL2+ and SrL2,+ are extracted into the organic phase. The values of extraction and stability constants of the cationic complexes in nitrobenzene saturated with water have been determined.
Keywords: Strontium, hydrogen dicarbollylcobaltate, barium ionophore I, water-nitrobenzene system, extraction and stability constants
1. Introduction
Electrically neutral, lipophilic organic complexing agents are widely used as components for ion-selective electrodes.1-3 Sensors for lithium,4 sodium,5 potassium,1'2'6-8 ammonium,910 calcium,11 strontium12 and ba-rium13-16 are based on such molecules. Because of their analytical potential, considerable effort had been directed towards the design of selective and stable electrodes for barium.13-17
The dicarbollylcobaltate anion18 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,19-33 and on the technological scale for the separation of some high-activity isotopes in the reprocessing of spent nuclear fuel and acidic
radioactive waste.34,35
In the current work, the solvent extraction of mi-croamounts of strontium by a nitrobenzene solution of hydrogen dicarbollylcobaltate (H+B-)18 in the presence of N,N,N',N'-tetracyclohexyl-oxybis(o-phenyleneoxy)diace-tamide (abbrev. barium ionophore I, L) (see Scheme 1)
Scheme 1. Structural formula of N,N,N',N'-tetracyclohexyl-oxybis (o-phenyleneoxy)diacetamide (abbrev. barium ionophore I or L, respectively).
was studied. We intended to find the composition of the species in nitrobenzene phase and to determine the corresponding equilibrium constants.
2. Experimental
N,N,N',-W-Tetracyclohexyl-oxybis(o-phenyle-neoxy)diacetamide, called also barium ionophore I (Scheme 1), was supplied by Fluka. Cesium dicarbollylcobalta-te, Cs+B-, was synthesized by the method published by Hawthorne et al.36 A nitrobenzene solution of hydrogen dicarbollylcobaltate (H+B-)18 was prepared from Cs+B- by the procedure described elsewhere.37 The other chemicals used (Lachema, Brno, Czech Republic) were of reagent grade purity. The radionuclide 85Sr2+ (DuPont, Belgium) was of standard radiochemical purity.
The extraction experiments in the two-phase wa-ter-HCl-Sr2+ (microamounts)-nitrobenzene-barium ionophore I-H+B- system were performed in 10 mL glass test-tubes with polyethylene stoppers, using 2 mL 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 mL samples were taken from each phase and their y-activities were measured by means of a well-type NaI(Tl) scintillation detector connected to a y-analyzer NK 350 (Gamma, Budapest, Hungary).
The equilibrium distribution ratios of strontium, D, were determined as the ratios of the measured radioactivities of 85Sr2+ in the nitrobenzene and aqueous samples.
3. Results and Discussion
The dependence of the logarithm of the strontium distribution ratio (log D) on the logarithm of the numerical value of the total (analytical) concentration of the barium ionophore I ligand in the initial nitrobenzene phase, log c(L), is given in Figure 1. The initial concentration of hydrogen dicarbollylcobaltate in the organic phase, cB = 0.0002 mol/L, as well as the initial concentration of HCl in the aqueous phase, c(HCl) = 0.01 mol/L, are always related to the volume of one phase.
With respect to the results of previous pa-pers,18,27,29,31,38-48 the considered water-HCl- Sr2+ (mi-croamounts)-nitrobenzene-barium ionophore I (L)-H+B-system can be described by the set of reactions
Figure 1. Log D as a function of log c(L), where L is barium ionophore I, for the water-HCl-Sr2+ (microamounts) - nitrobenzene- barium ionophore I - H+B- system; c(HCl) = 0.01 mol/L, cB = 0.0002 mol/L. The curve was calculated using the constants given in Table 2.
Sr-+ + nL„r8 + 2H:rg^SrL;;rg + 2H;
(4)
to which the following equilibrium constants correspond:
[LJ
ex H"™ I - -
ISr^HH^

[SrÇj[H;j2 [Sr^L^rtH^f
(5)
(6)
(7)
(8)
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 barium ionophore I ligand and the electroneutrality conditions in both phases of the system under consideration, was formulated39,44 and introduced into a more general least-squares minimizing program LETAGROP49 used for determination of the "best" values of the extraction constants Kex(SrLn+org) (L = barium ionophore I). The minimum of the sum of errors in log D, i.e., the minimum of the expression
+ Lorg » HL^

(1) (2) (3)
U:
:Z(logDcaIc-logDexp)2
(9)
was sought.
The values log KD =
log ß(HL+rg) = 5.85
3.6 (see Table 2, footnote a),51 ■2+ \ _ n were used
and log Kex(SrO+g) = 0.7
for the respective calculations. The results are listed in Table 1. From this table it is evident that the extraction data can be best explained assuming the species SrL2+ and SrL22+ (L = barium ionophore I) to be extracted into the nitrobenzene phase.
log/J<SrL^rs) = logKss(SrL^re)-logKw(Sr£)
(13)
The respective equilibrium constants are summarized in Table 2.
Table 1. Comparison of three different models of strontium extraction from aqueous solution of HCl by nitrobenzene solution of H+B- in the presence of barium ionophore I.
Strontium complexes	log	Kex °	Ub
in the organic phase			
SrL2+	9.18	(9.42)	0.47
SrL22+	14.95	(15.50)	3.98
SrL2+, SrL22+	9.04 ± 0.06,	13.63 ± 0.16	0.01
a The values of the extraction constants are given for each complex. The reliability interval of the constants is given as 3c(K), where c(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 + 1.5c(K)] - log [K - 1.5c(K)]}. For c(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 +3c(K)]).49 b The error-square sum U = E(log Dcalc -
-l°gDelp)2.
Knowing the values logKex(Sr+g)
ex
the extraction constants log K
0.7,39 as well as
2+ N
(SrL2o+g) = 9.04 and
log K(SrL2+org) = 13.63 determined here (see Table 1), ex 2,org 2+ 2 the stability constants of the complexes SrL and SrL2
(L = barium ionophore I) in the nitrobenzene phase defined as
Figure 2. Distribution diagram of hydrogen cation in the equilibrium nitrobenzene phase of the water-HCl-Sr2+(microamounts) -nitrobenzene- barium ionophore I - H+B- extraction system in the forms of H+ and HL+; c(HCl) = 0.01 mol/L, c
0.0002 mol/L.
+\
1 S(H+) = [H+rg]/c(H+)0rg, 2 5(HL+) = [HL+lg]/c(H+)oig, where c(H+)oig = [H+J + [HL+J.
The distribution curves were calculated using the constants given in Table 2.
(10)
(11)
can be evaluated applying the following simple relations: log fi (SrL;;) = log KJSrL^)- log KJSO (12)
Table 2. Equilibrium constants in the water-HCl-Sr2+ (microa-mounts)-nitrobenzene- barium ionophore I - H+B- system.
Equilibrium
log K
La+q « Lorg	+
H+rg + Lorg « HL+org
Sr2++ 2H+org « Sr2+g + 2H+aq
Sr2++ Lorg + 2H+rg « SrL2+g + 2H+aq
Sr2+ + 2Lorg + 2H+rg « SrL2+org + 2H+aq
Sr2+g + Lorg « SrL2+
Sr2+ + 2L„
■ « SrL2+org
3.6 a 5.8 b 0.7 c 9.04 13.63 8.34 12.93
a Determined by the method of the concentration dependent distribution.50 b Ref. 51. c Ref. 39.
1.0
«5
0.S
	l l l .
	\ Y
	
J— =■"—	----- 1 1
-5.0
-4.5
-4.0
log c[L)
-3.5
-3.0
Figure 3. Distribution diagram of strontium in the equilibrium nitrobenzene phase of the water-HCl-Sr2+ (microamounts) -nitrobenzene- barium ionophore I -H+B- extraction system in the forms of S^+, SrL2+ and SrL22+ c(HCl) = 0.01 mol/L, cB = 0.0002 mol/L. 1 5(Sr^) = [Sr2o+!]/c(Sr2+)olJ!, 2 5(SrL2+) = [SrL^MSr^,
:+)0„, where c(Sr2+)ols = [Sr^ + [SrL2+]
[SrL22+2r,]/c(Sf
3 5(SrL|+) =
+ [^L+J .
The distribution curves were calculated using the constants given in Table 2.
Moreover, Figure 2 depicts the contributions of the species H+rg and HL+rg to the total hydrogen cation concentration in the equilibrium nitrobenzene phase, whereas Figure 3 shows the contributions of the cations Sr;+g,
SrLo+g and SrL:^ to the total strontium concentration in
org 2,org
the equilibrium organic phase. From Figure 3 it follows that the cationic complex species SrL2+org is present in significant concentrations only at relatively high amounts of the barium ionophore I ligand in the system under consideration.
Finally, Table 3 presents the stability constants of the ML+ complexes, where M+ = Li+, Na+, H3O+, NH+, Ag+, K+, Rb+, Tl+, Cs+ and L is barium ionophore I, determined previously in water-saturated nitrobenzene.51 Thus, from the data given in Tables 2 and 3 it follows that in the mentioned nitrobenzene medium, the stability constants of the considered complexes ML+ and SrL2+ (L = barium ionophore I) increase in the order: Cs+ < Rb+ < NH+, K+ < H3O+ < Na+ < Ag+, Tl+ < Li+ < Sr2+.
In conclusion, Table 4 summarizes the stability constants of the complexes SrL0+g and SrL2+org with 19 ox-yethylene ligands L, denoted by the symbols diglyme, triglyme, tetraglyme, PEG 200, PEG 300, PEG 400, Slo-vafol 909, 15C5, B15C5, N15C5, 18C6, B18C6, DB18C6, DCH18C6, DB21C7, DB24C8, DCH24C8, DB30C10 and barium ionophore I, in nitrobenzene saturated with water. From the data reviewed in this table it follows that in this nitrobenzene medium, the stability constants of the complexes SrLo+g increase in the series of diglyme < triglyme < tetraglyme < DB18C6 < DB21C7 < DB24C8 < barium ionophore I < PEG 200 < DB30C10 « B18C6 < DCH24C8 < Slovafol 909 < PEG 300 < PEG 400 < DCH18C6 < 18C6, whereas the stability of the cationic complex species SrL2+org increases in the following sequence: triglyme < tetraglyme < DB18C6 < DB21C7 < N15C5 < DB24C8 < barium ionophore I < B15C5 < B18C6 < DCH24C8 < Slovafol 909 < DCH18C6 < 15C5 < 18C6.
re I) in nitrobenzene saturated with water at 25 °C.
M+	Li+	Na+	H3O+	NH+	Ag+	K+	Rb+	Tl+	Cs+
log AML+rg)"	6.6	5.9	5.8	4.8	6.0	4.8	4.2	6.0	4.0
a Ref. 51.
Table 3. Stability constants of the complexes ML+ (M+ = Li+, Na+, H3O+, NH+, Ag+, K+, Rb+, TT, Cs+; L = barium ionopho-
Table 4. Stability constants of the complexes SrL2+ and SrL|+ [L = diglyme, triglyme, tetraglyme, PEG 200, PEG 300, PEG 400, Slovafol 909, 15-crown-5 (15C5), benzo-15-crown-5 (B15C5), 2,3-naphtho-15-crown-5 (N15C5), 18-crown-6 (18C6), benzo-18-crown-6 (B18C6), dibenzo-18-crown-6 (DB18C6), dicyclohexano-18-crown-6 (DCH18C6), dibenzo-21-crown-7 (DB21C7), diben-zo-24-crown-8 (DB24C8), dicyclohexano-24-crown-8 (DCH24C8), dibenzo-30-crown-10 (DB30C10), barium ionophore I] in nitrobenzene saturated with water at 25 °C.
L	log ß (SrL2o+g)	log ß^rL^g)	Ref.
diglyme	3.06	-	38
triglyme	4.34	6.77	38
tetraglyme	4.90	7.52	38
PEG 200	9.06	-	39
PEG 300	10.41	-	39
PEG 400	11.03	-	39
Slovafol 909	10.22	14.52	40
15C5	-	14.89	41
B15C5	-	13.20	42
N15C5	-	11.28	43
18C6	11.50	16.24	44
B18C6	9.29	13.68	27
DB18C6	6.38	8.94	29
DCH18C6	11.19	14.74	31
DB21C7	6.61	10.00	45
DB24C8	8.15	12.77	46
DCH24C8	9.99	14.44	47
DB30C10	9.28	-	48
barium ionophore I 8.34		12.93	This work
4. Acknowledgements
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
Raziskovali smo ekstrakcijo mikrokoličin stroncija v benzenovo raztopino hidrogen dikarbolilkobaltata (H+B-) v prisotnosti N,_V,_V,.V-tetracikloheksil-oksibis (o-phenilenoksi)diacetamida (barijev ionopfor I, L). Eksperimentalne podatke smo obravnavali s pomočjo modela, da se kompleksi HL+, SrL2+ in SrL^+ekstrahirajo v organsko fazo. Ocenili smo koeficiente porazdelitve ter konstante stabilnosti kompleksov v nitrobenzenu, nasičenem z vodo.