Short communication
Solvent Extraction of Strontium into Nitrobenzene by Using a Synergistic Mixture of Hydrogen Dicarbollylcobaltate and Dibenzo-30-crown-10
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: 31-08-2010
Abstract
Extraction of microamounts of strontium by a nitrobenzene solution of hydrogen dicarbollylcobaltate (H+B-) in the presence of dibenzo-30-crown-10 (DB30C10, L) has been investigated. The equilibrium data have been explained assuming that the species HL+, SrL2+, SrHL3+ and SrHL32+ 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, dibenzo-30-crown-10, water-nitrobenzene system, extraction and stability constants, complexation
1. Introduction
In 1967, Pedersen published his first papers1'2 dealing with cyclic polyether compounds with oxyethylene groups -CH2-CH2-O-, that are called crowns owing to their structure. These electroneutral crown compounds form relatively stable complexes in nonaqueous solvents, especially with alkali and alkaline-earth metal cations, the cations being placed in the ligand cavities. The ratio of the size of the crown ligand cavity to the ion radius of the central cation is a decisive or at least an important factor in the stability of the complex compounds formed.3 It is the complexing properties of the crowns that are due to the rapid development of the chemistry of these cyclic polyet-hers that we have witnessed in the recent decades. At this point it should be noted that several reviews have covered many aspects of their chemistry.3-6
The dicarbollylcobaltate anion7 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,8-29 and on the technolo-
gical scale for the separation of some high-activity isotopes in the reprocessing of spent nuclear fuel and acidic radioactive waste.30'31
In the current work, the solvent extraction of microamounts of strontium by a nitrobenzene solution of hydrogen dicarbollylcobaltate (H+B-)7 in the presence di-benzo-30-crown-10 (DB30C10, L) (see Scheme 1) was studied. We intended to find the composition of the species in the organic phase and to determine the corresponding equilibrium constants.
Scheme 1. Structural formula of dibenzo-30-crown-10 (abbrev. DB30C10 or L, respectively).
2. Experimental
Dibenzo-30-crown-10 (DB30C10, L) (see Scheme 1) was supplied by Aldrich. Cesium dicarbollylcobaltate, Cs+B-, was synthesized by means of the method published by Hawthorne et al.32 A nitrobenzene solution of hydrogen dicarbollylcobaltate (H+B-)7 was prepared from Cs+B- by the procedure described elsewhere.33 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-DB30C10 -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 hours 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 /-activities were measured using a well-type NaI(Tl) scintillation detector connected to a /-analyzer NK 350 (Gamma, Budapest, Hungary).
The equilibrium distribution ratios of strontium, DSr, 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 DB30C10 ligand in the initial nitrobenzene phase, log
Fig. 1. Log D as a function of log c(L), where L is DB30C10, for the water-HCl-Sr2+ (microamounts) - nitrobenzene - DB30C10 -H+B- system; c(HCl) = 0.01 mol/L, cB = 0.001 mol/L. The curve was calculated using the constants given in Table 2.
c(L), is given in Fig 1. The initial concentration of hydrogen dicarbollylcobaltate in the organic phase, ^ = 0.001 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 papers,7,10,29,34-44 the considered water- HCl-Sr2+ (microa-mounts)-nitrobenzene- DB30C10 (L)- H+B- system can be described by the set of reactions:
(1) (2) (3)

Srü+ + 2HL O Sri: + 2H1
SC + (r + 2)H;rE + sLore o SrHrL(:,;f + 2H; (4) to which the following equilibrium constants correspond:
[L0J
Kd =
IK
(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 DB30C10 ligand and the electroneutrality conditions in both phases of the system under consideration, was formulated 35,39 and introduced into a more general least-squares minimizing program LETAGROP 45 used for determination of the "best" values of the extraction constants Kex(SrHrL(rs+2g" ) (L = DB30C10). The minimum of the sum of errors in log D, i.e., the minimum of the expression
U = x(logDaIc-logDexp)2
(9)
was sought.
The values log KD = 3.8 (see Table 2, footnote a), log £(HL+rg) = 6.047 and log Kex(Sr2g) = 0.735 were used 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+, SrHL3+ and SrHL32+(L = DB30C10) to be extracted into the nitrobenzene phase.
Knowing the value log Kex(Sro2r+g) = 0.7,35 as well as the extraction constant log Kex(SrL2r+g) = 9.9835 determined
Table 1. Comparison of various models of strontium extraction from aqueous solution of HCl by nitrobenzene solution of H+B- in the presence of DB30C10.
Strontium complexes in the organic phase
log K
U
b
2+
SrL2 SrL2 SrHL3 SrHL
3+
SrL2+, SrL2+
SrL2+, SrHL3+
SrL2+, SrHL3+
SrL2+, SrHL3+
SrL2+, SrHL3+
SrHL3+, SrHL3+
SrL2+, SrL2+, SrHL3+
SrL2+, SrL^+, SrHL3+
SrL2+, SrHL3+, SrHL3+
SrL2+, SrL2+, SrHL3+, SrHL?+
10.22 (10.47)	0.92
14.47 (15.36)	39.40
14.99 (15.62)	16.32
19.50	(19.88)	2.97 Transformed to SrL2+
10.06 (10.35), 13.32 (13.81)	0.95 Transformed to SrL2+
12.44 (12.97), 13.90 (14.39)	3.04 Transformed to SrHL3+
13.51	(13.86), 19.06 (19.45)	0.65 Transformed to SrL2+, SrHL3+
Transformed to SrL2+
9.98 (10.21), 13.36 (13.64), 18.27 (18.56)	0.03
Transformed to SrL2+, SrHL3+, SrHL23+
a
a The values of the extraction constants are given for each complex. The reliability interval of the constants is given as 3o(K), where o(K) is the standard deviation of the constant K.45 These values are given in the logarithmic scale using the approximate expression log K ± {log [K + 1.5 o(K)] - log [K - 1.5 o(K)]}. For O(K) > 0.2 K, 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 + 3o(K)]).45 b The error-square sum U = X(log Dcalc - log D )2.
here (see Table 1), the stability constant of the complex SrL2+ (L = DB30C10) in the nitrobenzene phase defined as
(10)
can be evaluated applying the following simple relation: log 0 (SrL-;) = log (SrL=;) - log K„(Sr^) (11)
Similarly, the protonation constant of the cation SrL2+g (L = DB30C10) in nitrobenzene saturated with water, i. e., the equilibrium constant of the following reaction:
Table 2. Equilibrium constants in the water -HCl- Sr2+ (microa-mounts) - nitrobenzene - DB30C10 - H+B- system.
Equilibrium
La+q « Lorg	+
H+rg + Lorg « HL+rg Sr24 + 2H+rg « Sr2o^ + 2H+q Sr?aq + Lorg + 2H+rg S£ + Lorg + 3H+rg	3
Sr24 + 2Lorg + 3H+rg « ^L+org + 2H+aq < + Lorg « SrL^o+g3
SrL!+s + H+rs « SrHL3+_
« SrL2! + 2H+q
+ 2H+aq
or
« SrHL
log K
3.8 a 6.0 b 0.7 c
9.98 13.36 18.27 9.28 3.38
a Determined by the method of the concentration dependent distri-bution.46 b Ref. 47. c Ref. 35.
SrL;;+	SrHL3;
defined as
lt(SrHL30+ ) = ■
[SrHL'; j
[SrL^JtH^]
was calculated on the basis of Relation (14):
log K(SrHLJo;) = log Kex(SrHL3;j-
org J
-logK„(Srt4)
(12)
(13)
(14)
The respective equilibrium constants are summarized in Table 2.
Moreover, Fig. 2 depicts the contributions of the species Sr2+ (curve 1), SrL2+ (curve 2), SrHL^+g (curve 3) and SrHL3+org (curve 4) to the total strontium cation con-
centration in the equilibrium organic phase. From this figure it follows that the distribution curve 3 is decreasing, whereas the distribution curves 2 and 4 are increasing.
Finally, Table 3 summarizes stability constants of the complexes SrL2+g and SrL2+,rg with 18 oxyethylene li-gands L, denoted by the symbols diglyme, triglyme, te-traglyme, PEG 200, PEG 300, PEG 400, Slovafol 909, 15C5, B15C5, N15C5, 18C6, B18C6, DB18C6, DCH18C6, DB21C7, DB24C8, DCH24C8 and DB30C10, in nitrobenzene saturated with water. From the data reviewed in this table it follows that in the mentioned nitrobenzene medium, the stability constants of the complexes SrL2+ increase in the series of diglyme < triglyme < tetraglyme < DB18C6 < DB21C7 < DB24C8 < PEG 200 < DB30C10 « B18C6 < DCH24C8 < Slovafol 909 < PEG 300 < PEG 400 < DCH18C6 < 18C6, whereas the
log c(L)
Fig. 2. Distribution diagram of strontium in the equilibrium organic phase of the water- HCl-Sr2+ (microamounts) -nitrobenzene-DB30C10 - H+B- extraction system in the forms of Sr2+, SrL2+, SrHL3+, and SrHLf; c(HCl) = 0.01 mol/L, cB = 0.001 mol/L.
1	5(Sr2+) = [Sr^MSr2^
2	8(SrL2+) = [SrL20+,]/c(Sr2+)0!g,
3	8(SrHL3+) = [SrHL3o+,]/c(Srf+)olg,
4	6(SrHL2+) = [SrHL^MSr^,
where c^r^ = [S^] + [SrL^] + [Sr^ + [SrHL^ The distribution curves were calculated using the constants given in Table 2.
Table 3. Stability constants of the complexes SrL2+ and SrL^+T = diglyme, triglyme, tetraglyme, PEG 200, PEG 300, PEG 400, Slo-vafol 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 (DCH24-C8), dibenzo-30-crown-10 (DB30C10)] in nitrobenzene saturated with water at 25 °C.
L	log 0(SrL2o+g)	log ß(SrL22+org)	Ref.
diglyme	3.06	-	34
triglyme	4.34	6.77	34
tetraglyme	4.90	7.52	34
PEG 200	9.06	-	35
PEG 300	10.41	-	35
PEG 400	11.03	-	35
Slovafol 909	10.22	14.52	10
15C5	-	14.89	36
B15C5	-	13.20	37
N15C5	-	11.28	38
18C6	11.50	16.24	39
B18C6	9.29	13.68	40
DB18C6	6.38	8.94	41
DCH18C6	11.19	14.74	42
DB21C7	6.61	10.00	43
DB24C8	8.15	12.77	29
DCH24C8	9.99	14.44	44
DB30C10	9.28	-	This work
stability of the cationic complex species SrL2+rg increases in the following sequence: triglyme < tetraglyme < DB18C6 < DB21C7 < N15C5 < DB24C8 < B15C5 < B18C6 < DCH24C8 < Slovafol 909 < DCH18C6 < 15C5 <18C6.
4. Acknowledgements
This work was supported by the Czech Ministry of Education, Youth and Sports, Projects MSM 4977751303 and MSM 6046137307.
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Povzetek
Proučevali smo ekstrakcijo mikrokoličin stroncija z raztopino hidrogen dikarbolilkobaltata (H+B-) v nitrobenzenu v prisotnosti dibenzo-30-crown-10 (DB30C10, L). Ob predpostavki, da so v ravnotežju v organski fazi prisotni kompleksi HL+, SrL2+, SrHL3+ in SrHL32+ smo določili konstante ekstrakcije in konstante stabilnosti navedenih komplekosv.