Short communication
Solvent Extraction of Calcium and Strontium into Nitrobenzene by Using a Synergistic Mixture of Hydrogen Dicarbollylcobaltate and 2,6-(Diphenylphosphino)Pyridine Dioxide
Emanuel Makrlik,1'* Petr Vanura,2 Pavel Selucky3 and Zdenek Spichal4
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, Technickâ 5,
166 28 Prague 6, Czech Republic
3 Nuclear Research Institute, 250 68 Rez, Czech Republic
4 Department of Inorganic Chemistry, Faculty of Science, Masaryk University, Kotlârskâ 2, 611 37 Brno, Czech Republic
* Corresponding author: E-mail: makrlik@centrum.cz
Received: 04-07-2011
Abstract
Solvent extraction of microamounts of calcium and strontium by a nitrobenzene solution of hydrogen dicarbollylcobaltate (H+B-) in the presence of 2,6-(diphenylphosphino)pyridine dioxide (DPPPDO, L) has been investigated. The equilibrium data have been explained assuming that the species HL+, HL+, CaL2+, CaL2+, SrL2+, SrL2+ and SrL2+ 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: Calcium, strontium, hydrogen dicarbollylcobaltate, 2,6-(diphenylphosphino)pyridine dioxide, water-nitrobenzene system, extraction and stability constants
1. Introduction
The dicarbollylcobaltate anion1 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,2-29 and on the technological scale for the separation of some high-activity isotopes in the reprocessing of spent nuclear fuel and acidic radioactive waste.30,31
Bidentate phosphonates, phosphine oxides and ma-lonamides have been intensively studied for the extraction
of trivalent lanthanides and actinides from acidic media.32-34 A process using octyl-phenyl-V,V-diisobutylcar-bamoylmethyl phosphine oxide (i.e. "classical" CMPO) and called TRUEX was apparently used in the United States,32 whereas malonic diamides (RR'NCO)2CHR" (DI-AMEX) were proposed in France.33
Recently, extractive properties of a synergistic mixture of hydrogen dicarbollylcobaltate (H+B-)1 and 2,6-(diphenylphosphino)pyridine dioxide (DPPPDO, L; see Scheme 1) toward Eu3+ have been investigated in the water-nitrobenzene system.35 On the other hand, in the current work, the solvent extraction of microamounts of calcium and strontium by a nitrobenzene solution of the mentioned synergistic mixture was studied. We intended
to find the composition of the species in the organic phase and to determine the corresponding equilibrium constants.
2. Experimental
Preparation of 2,6-(diphenylphosphino)pyridine dioxide (DPPPDO, L; see Scheme 1) was presented in Ref. 36. Cesium dicarbollylcobaltate, Cs+B-, was synthesized by means of the method published by Hawthorne et al.37 A nitrobenzene solution of hydrogen dicarbollylcobaltate (H+B)1 was prepared from Cs+B- by the procedure described elsewhere.38 The other chemicals used (Lac-hema, Brno, Czech Republic) were of reagent grade purity. The radionuclides 45Ca2+ and 85Sr2+ (DuPont, Belgium) were of standard radiochemical purity.
Scheme 1. Structural formula of 2,6-(diphenylphosphino)pyridine dioxide (abbrev. DPPPDO or L, respectively).
The extraction experiments in the two-phase wa-ter-HCl-M2+(microamounts; M2+ = Ca2+, Sr2+)-nitroben-zene-DPPPDO-H+B- systems 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 centrifuga-tion. In the case of the systems involving 45Ca2+, after evaporating aliquots (1 mL) of the respective phases on Al plates, their /¡-activities were measured by using the apparatus NRB-213 (Tesla Premysleni, Czech Republic). On the other hand, in the case of the systems with 85Sr2+, 1 mL samples were taken from each phase and their y-ac-tivities 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 calcium and strontium, D, were determined as the ratios of the corresponding measured radioactivities of 45Ca2+ and 85Sr2+ in the nitrobenzene and aqueous samples.
3. Results and Discussion
The dependences of the logarithm of the calcium and strontium distribution ratios (log D) on the logarithm
of the numerical value of the total (analytical) concentration of the DPPPDO ligand in the initial nitrobenzene phase, log c(L), are given in Figures 1 and 2, respectively. The initial concentration of hydrogen dicarbollylcobaltate in the organic phase, cg = 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.
t-1-1-1-r
-4.0 -3.5 -3.0 -2.5 -2.0 log c(L)
Figure 1. Log D as a function of log c(L), where L is DPPPDO, for the water- HCl-Ca2+ (microamounts)- nitrobenzene - DPPPDO -H+B- system; c(HCl) = 0.01 mol/L, cB = 0.001 mol/L. The curve was calculated using the constants given in Table 3.
-4.0 -3.5 -3.0 -2.5 -2.0 log c(L)
Figure 2. Log D as a function of log c(L), where L is DPPPDO, for the water- HCl-Sr2+ (microamounts)- nitrobenzene- DPPPDO-H+B- system; c(HCl) = 0.01 mol/L, cB = 0.001 mol/L. The curve was calculated using the constants given in Table 4.
With respect to the results of previous papers,3,9,14,28,29 the considered water-HCl-M2+ (microa-mounts; M2+ = Ca2+, Sr2+)-nitrobenzene- DPPPDO(L)-H+B- systems can be described by the set of reactions:


M=;+nLnri+2H;reoML;;rB + 2H;
(2)
(3)
(4)
(5)
to which the following equilibrium constants correspond:
(6)
(7)
(8) (9)
(10)

'WSJL org J ^ org ]
[ML^JpI]2

[M;;:][Lorj"[H;
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 DPPPDO ligand and the electroneutrality conditions in both phases of the system under consideration, was formulated39,40 and introduced into a more general least-squares minimizing program LETAGROP 41 used for determination of the "best" values of the extraction constants Kex(ML2+org) (M2+ = Ca2+, Sr2+; L = DPPPDO). The minimum of the sum of errors in log D, i.e., the minimum of the expression
Table 1. Comparison of various models of calcium extraction from aqueous solution of HCl by nitrobenzene solution of H+B- in the presence of DPPPDO.
Calcium complexes in the organic phase
log K
U b
CaL22+	22.58 (22.92)	2.15
CaL23+	26.21 (27.05)	29.13
CaL24+	29.55 (30.51)	57.08
CaL 2, CaL 3	22.19 ± 0.09, 25.56 ± 0.12	0.02
CaL 3, CaL 4	Transformed to CaL23+	
CaL22+, CaL23+, CaL24+	Transformed to CaL22+, CaL23+	
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.41 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) > 0.2 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 + 3c(K)]).41 b The error-square sum U = X(log Dcalc -
c " log Dexp)2,
Table 2. Comparison of various models of strontium extraction from aqueous solution of HCl by nitrobenzene solution of H+B- in the presence of DPPPDO.
Strontium complexes in the organic phase
log K
» ex
U"
SrL22+	23.06 (23.60)	7.16
SrL23+	26.93 (27.70)	19.90
SrL234+	30.26 (31.15)	45.08
SrL22+, SrL23+	22.21 ± 0.24, 26.43 (26.64)	0.42
SrL223+, SrL234+	Transformed to SrL23+	
SrL223+, SrL243+, SrL24+	22.21 ± 0.22, 26.23 (26.57),	0.03
	28.48 (28.87)	
a See Table 1, footnote a. b See Table 1, footnote b.
U:
X(log Dcalc log Dexp)2
(11)
was sought.
The values log KD = 2.38,35 log ^(HL^rg) = 9.45,35 log £(HL+,OTg) = 11.99,35 log Kex(Ca20+g) = 0.242 and log Kex(Sr2o+rg) = 0.739 were used for the respective calculations. The results are listed in Tables 1 and 2. From these tables it is evident that the extraction data can be best explained assuming the complexes CaL22+ CaL23+ SrL22+ SrL23+ and SrL24+(L = DPPPDO) to be extracted into the nitrobenzene phase.
Knowing the values log Kex(Ca2o+rg) = 0.242 and log K (Sr2+ ) = 07,39 as well as the extraction constants log Kex(CaL22+org) = 22.19, log KJCaL^) = 26.56, log Kex(SrL22+org) = 22.21, log ^(SrL^) = 26.23 and 28.48 determined here (see Tables 1
log Kex(SrL4+0rg)
and 2), the stability constants of the complexes ML22+, ML23+and ML24+(M2+ = Ca2+, Sr2+; L = DPPPDO) in the organic phase defined as
/(MLtn,)- [MLl"]

ß{ ^ [M-ltL^i
[M^][L0 l4
(12)
(13)
(14)
OI^ J L — Mg J
can be evaluated applying the following simple relations: log 0 (M L^) = log K„(MLLe) - log K„(M£) (15)
log¿(ML^) - log K„(ML^) - log Keï(M £ ) (16)
log ß (ML^ ) = log Kk (ML2^ ) - log	) (i7)
a
a
The respective equilibrium constants are summarized in Tables 3 and 4.
Table 3. Equilibrium constants in the water - HCl- Ca2+ (microa-mounts) - nitrobenzene - DPPPDO - H+B- system.
Equilibrium
log K
Laq « Laq
H+org + Lorg « HL+org
H+0 + 2Lorg « HL+,org
Ca2+ + 2H+rg « Cao+g + 2H+
Ca24 + 2Lorg + 2H+rg « CaL^+org + 2H+aq
Ca24 + 3Lorg + 2H+rg « CaL3+org + 2H+aq
Ca?o+g + 2Lorg « CaL2+org
CaQg + 3Lorg « CaL3,Qrg_
2.38 a 9.45 a 11.99 a 0.2 b 22.19 26.56 21.99 26.36
a Ref. 35. b Ref. 42.
Table 4. Equilibrium constants in the water - HCl - Sr2+ (microa-mounts) - nitrobenzene - DPPPDO - H+B- system.
Equilibrium
log K
La+q « Lorg	+
H+org + Lorg « HL+org H+rg + 2Lorg « HI2+2,org
S^ + 2H+rg « Sr2+g + 2H+aq
S£ + 2Lorg + 2Horg « SrLtrg + 2H+q
S£ + 3Lorg + 2Horg « SrL33+org + 2H+aq
Sr!++ 4Lorg + 2Horg « SrL4+org + 2H+aq
Sr2+ + 2L___« SrL:
2+
org	org	2,org
Sr2o+g+ 3Lorg « SrL2+org SC + 4Lorg « SrL2+org
2.38 a 9.45 a 11.99 a 0.7 b 22.21 26.23 28.48 21.51 25.53 27.78
a Ref. 35. b Ref. 39.
Moreover, Figure 3 depicts the contributions of the species H+rg, HL+rg, and HL+org, to the total hydrogen cation concentration in the equilibrium nitrobenzene phase, whereas Figures 4 and 5 show the contributions of the cations Ca2+ , CaL22+ , CaL23+ and Sr2+, SrL22+ , SrL23+ ,
org	2,org	3,org	org	2,org'	3,org'
SrL24+org, respectively, to the total divalent metal cation concentration in the corresponding equilibrium organic phase. From Figures 3, 4 and 5 it follows that the cationic complex species HL+ org, CaL23+org and SrL24forg are present in significant concentrations only at relatively high amounts of the DPPPDO ligand in the systems under consideration.
At this point it should be noted that the stability constants of the calcium and strontium complex species involving the DPPPDO ligand in nitrobenzene saturated with water are log p (CaL2^) = 21.99, log p (SrL2^) = 21.51,
2,org'
log p (CaL23+org) = 26.36 and log P (SrL^) = 25.53, as given in Tables 3 and 4. Thus, in the considered nitrobenzene medium, the stability constants of the CaL2n+complexes, where n = 2, 3 and L is DPPPDO, are somewhat higher than those of the corresponding complexes SrL2n+
Figure 3. Distribution diagram of hydrogen cation in the equilibrium nitrobenzene phase of the water-HCl-Ca2+(micro-amounts)-nitrobenzene-DPPPDO-H+B- extraction system in the forms of H+, HL+ and HL+; c(HCl) = 0.01 mol/L, cB = 0.001 mol/L.
1	S(H+) = [H+rg]/c(H+)
2	S(HL+) = [HL+rg]/c(H+)org,
3	S(HL+) = [HL+,0rg]/c(H+)0l where c(H+)or
The distribution curves were calculated using the constants given in Table 3.
: [HL1 + [HL+J + [HL+org].
Figure 4. Distribution diagram of calcium in the equilibrium nitrobenzene phase of the water- HCl-Ca2+ (microamounts) - nitrobenzene - DPPPDO - H+B- extraction system in the forms of Ca2+, CaL2+ and CaL23+. c(HCl) = 0.01 mol/L, cB = 0.001 mol/L.
1	S(Ca2+) = [Ca2o++,]/c(Ca2+)or&,
2	S(CaL22+) = [CaL^H0rg]/c(Ca )0rg,
3	S(CaL2+) = [CaL23+ where c(Ca2+)o
The distribution curves were calculated using the constants given in Table 3.
-J3,org]/c(Ca2+)org,
= [Ca2o+„] + [CaLÎ+rg] + [CaL2^].
Finally, it is necessary to emphasize that the stability constants of the complexes CaL23+ SrL23+ and EuL33+ with the DPPPDO ligand in water-saturated nitrobenzene are
1.0
0.5
^___ A	1 1 2 3
_	y
\	
-4,0
-3.5
-3,0 log C[L)
-2,5
-2.0
Table 5. Stability constants of the complex species HL+, HL+, CaL22+ CaL23+ SrL22+ SrLfand SrL24+ [L = octyl-phenyl-^N-diiso-butylcarbamoylmethyl phosphine oxide ("classical" CMPO), 2,6-(diphenylphosphino)pyridine dioxide (DPPPDO)] in nitrobenzene saturated with water at 25 °C.
Quantity
log ß(HL+org) log ß(HL+ ) log ß(CaL22+org) log ß (CaL23+org) log ß (SrL22+org) log ß(SrL23+org) log ß(SrL2+org)
L
"classical" CMPO a
DPPPDO '
6.16 9.29 14.46 19.52 13.09 17.31 18.96
9.45 c 11.99 c 21.99 26.36 21.51 25.53 27.78
a Ref. 14. b This work. c Ref. 35.
Figure 5. Distribution diagram of strontium in the equilibrium nitrobenzene phase of the water- HCl-Sr2+ (microamounts) -nitrobenzene- DPPPDO - H+B- extraction system in the forms of Sr2+, SrL|+, SrL23+ and SrL24+; c(HCl) = 0.01 mol/L, cB = 0.001 mol/L.
1	5(Sr2+) = [Sr20++!]/c(Sr2+)0rg,
2	5(SrL22+) = [SrL^+0rg]/c(Sr+)0rg,
3	5(SrL|+) = [SrL^rgMSr^rg,
4	5(SrL24+) = [SrL^H0rg]/c(Sr2+)0lg,
where c(Sr2+)ols = [Sr^] + [S^] + [SrLf+J + [SrL^]. The distribution curves were calculated using the constants given in Table 4.
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.
5. References
log p (CaL23+org) = 26.36 (Table 3), log p (SrL23+org) = 25.53 (Table 4) and log p (EuL33+) = 35.80.35 This means that the stability of these three complexes in the mentioned medium increases in the series of Sr2+ < Ca2+ << Eu3+.
In conclusion, Table 5 summarizes the stability constants of the species HL+, HL+, CaL22+, CaL23+, SrL22+, SrL23+and SrL24+with two electroneutral ligands L, denoted by the symbols "classical" CMPO and DPPPDO (see Schemes 1 and 2), in nitrobenzene saturated with water. From the data reviewed in this table it follows that in the considered nitrobenzene medium, the stability constants of the cationic complexes HL+, HL+, CaL22+, CaL23+, SrL22+, SrL23+and SrL24+, where L = DPPPDO, are substantially higher than those of the corresponding complex species involving the "classical" CMPO li-gand.

Scheme 2. Structural formula of octyl-phenyl-NN-diisobutylcar-bamoylmethyl phosphine oxide (abbrev. "classical" CMPO).
1.	E. Makrlik, P. Vanura, Talanta 1985, 32, 423-429.
2.	E. Makrlik, P. Vanura, P. Selucky, J. Solution Chem. 2009,
38,	1129-1138.
3.	E. Makrlik, P. Vanura, P. Selucky, J. Solution Chem. 2010,
39,	692-700.
4.	E. Makrlik, J. Budka, P. Vanura, Acta Chim. Slov. 2009, 56, 278-281.
5.	E. Makrlik, P. Vanura, P. Selucky, V. A. Babain, I. V. Smir-nov, Acta Chim. Slov. 2009, 56, 718-722.
6.	E. Makrlik, P. Vanura, P. Selucky, Acta Chim. Slov. 2009, 56, 475-479.
7.	E. Makrlik, P. Vanura, P. Selucky, Acta Chim. Slov. 2009, 56, 973-976.
8.	E. Makrlik, P. Vanura, P. Selucky, Acta Chim. Slov. 2010, 57, 470-474.
9.	E. Makrlik, P. Vanura, P. Selucky, Acta Chim. Slov. 2010, 57, 485-490.
10.	E. Makrlik, P. Vanura, P. Selucky, Acta Chim. Slov. 2010, 57, 922-926.
11.	E. Makrlik, P. Toman, P. Vanura, R. Rathore, Acta Chim. Slov. 2010, 57, 948-952.
12.	E. Makrlik, P. Vanura, P. Selucky, Acta Chim. Slov. 2011, 58, 176-180.
13.	E. Makrlik, P. Vanura, Z. Phys. Chem. 2009, 223, 247-252.
14.	E. Makrlik, P. Vanura, P. Selucky, Z. Phys. Chem. 2009, 223, 253-261.
15.	E. Makrlik, J. Dybal, P. Vanura, Z. Phys. Chem. 2009, 223, 713-718.
16.	E. Makrlik, P. Vanura, P. Selucky, J. Radioanal. Nucl. Chem. 2009, 279, 137-142.
17.	E. Makrlik, P. Vanura, P. Selucky, J. Radioanal. Nucl. Chem. 2009, 279, 287-291.
18.	E. Makrlik, P. Vanura, P. Selucky, V. A. Babain, I. V. Smir-nov, J. Radioanal. Nucl. Chem. 2009, 279, 743-747.
19.	E. Makrlik, P. Vanura, Z. Sedläkovä, J. Radioanal. Nucl. Chem. 2009, 280, 607-611.
20.	E. Makrlik, P. Vanura, P. Selucky, J. Radioanal. Nucl. Chem. 2009, 281, 547-551.
21.	E. Makrlik, P. Vanura, P. Selucky, J. Radioanal. Nucl. Chem.
2009,	281, 633-638.
22.	E. Makrlik, P. Vanura, P. Selucky, J. Radioanal. Nucl. Chem.
2010,	283, 45-50.
23.	E. Makrlik, P. Vanura, Z. Sedläkovä, J. Radioanal. Nucl. Chem. 2010, 283, 157-161.
24.	E. Makrlik, P. Vanura, J. Radioanal. Nucl. Chem. 2010, 283, 497-501.
25.	E. Makrlik, P. Vanura, P. Selucky, J. Radioanal. Nucl. Chem. 2010, 283, 571-575.
26.	E. Makrlik, P. Vanura, P. Selucky, J. Radioanal. Nucl. Chem. 2010, 283, 727-733.
27.	E. Makrlik, P. Vanura, P. Selucky, V. A. Babain, I. V. Smir-nov, J. Radioanal. Nucl. Chem. 2010, 283, 839-844.
28.	E. Makrlik, P. Vanura, P. Selucky, J. Radioanal. Nucl. Chem. 2010, 284, 87-92.
29.	E. Makrlik, P. Vanura, P. Selucky, J. Radioanal. Nucl. Chem. 2010, 285, 383-387.
30.	V. N. Romanovskiy, I. V. Smirnov, V. A. Babain, T. A. Todd, R. S. Herbst, J. D. Law, K. N. Brewer, Solvent Extr. Ion Exch. 2001, 19, 1-21.
31.	J. D. Law, R. S. Herbst, T. A. Todd, V. N. Romanovskiy, V. A. Babain, V. M. Esimantovskiy, I. V. Smirnov, B. N. Zaitsev, Solvent Extr. Ion Exch. 2001, 19, 23-36.
32.	W. W. Schulz, E. P. Horwitz, Separ. Sci. Technol, 1988, 23, 1191-1210.
33.	C. Cuillerdier, C. Musikas, P. Hoel, L. Nigond, X. Vitart, Separ. Sci. Technol. 1991, 26, 1229-1244.
34.	G. R. Mahajan, D. R. Prabhu, V. K. Manchanda, L. P. Badhe-ka, Waste Management 1998, 18, 125-133.
35.	E. Makrlik, P. Vanura, P. Selucky, Z. Spíchal, Acta Chim. Slov. 2011, 58, 600-604.
36.	R. Sevcik, M. Necas, J. Novosad, Polyhedron 2003, 22, 1585-1593.
37.	M. F. Hawthorne, D. C. Young, T. D. Andrews, D. V. Howe, R. L. Pilling, A. D. Pitts, M. Reintjes, L. F. Warren, P. A. Wegner, J. Am. Chem. Soc. 1968, 90, 879-896.
38.	E. Makrlik, Collect. Czech. Chem. Commun. 1992, 57, 289295.
39.	P. Vanura, E. Makrlik, J. Rais, M. Kyrs, Collect. Czech. Chem. Commun. 1982, 47, 1444-1464.
40.	P. Vanura, E. Makrlik, Collect. Czech. Chem. Commun. 1993, 58, 1324-1336.
41.	L. G. Sillén, B. Warnqvist, Arkiv Kemi 1996, 31, 315-339.
42.	P. Vanura, Czech. J. Phys. 1999, 49 (Suppl. S1), 761-767.
Povzetek
Proučevali smo ekstrakcijo mikrokoličin kalcija in stroncija z raztopino hidrogen dikarbolilkobaltata (H+B-) v prisotnosti 2,6-(difenilfosfino)piridin dioksida (DPPPDO, L). Eksperimentalne podatke smo analizirali ob predpostavki, da se kompleksi HL+, HL+, CaL22+, CaL23+, SrL22+, SrL23+ in SrL2+ ekstrahirajo v organsko fazo. Določili smo koeficiente porazdelitve in konstante stabilnosti kompleksov v nitrobenzenu, nasičenem z vodo.