Short communication
Solvent Extraction of Microamounts of Calcium into Nitrobenzene Using Hydrogen Dicarbollylcobaltate
and 2,3-Naphtho-15-crown-5
Emanuel Makrlik,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, Technickà 5, 166 28 Prague, Czech Republic 3 Nuclear Research Institute, 250 68 Rež, Czech Republic * Corresponding author: E-mail: makrlik@centrum.cz Received: 07-11-2008
Abstract
Extraction of microamounts of calcium 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+, CaL2+ and CaL2+ 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, hydrogen dicarbollylcobaltate, 2,3-naphtho-15-crown-5, extraction and stability constants, water - nitrobenzene system
1. Introduction
In 1967, Pedersen1 published his first paper dealing with cyclic polyether compounds with oxyethylene groups -CH2-CH2-0- 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 cavity to the ion radius of the central cation is a decisive or at least an important factor in the stability of the complex formed. The complexing properties of the crowns are just due to the rapid development of the chemistry of these cyclic polyethers that we have witnessed in the recent decades.
Dicarbollylcobaltate anion and some of its halogen derivatives are very useful reagents for the extraction of alkali metal cations (especially Cs+), and also-in the presence of polyoxyethylene compounds - for the extraction of Sr2+ and Ba2+ from aqueous solution into an organic polar phase, both under laboratory conditions for purely theoretical or analytical purposes,2 and on the technological scale for the separation of some high-activity isotopes
in the reprocessing of spent nuclear fuel and acidic radioactive waste.3-5
Numerous naphtho-crown ethers have been synthesized and studied. The literature reports spectroscopic characteristics,6'7 X-ray structure analyses,8 complexation properties 9 and studies of chiral naphtho-crowns.10-12 However, up to now, the protonation of these compounds has not been investigated. On the other hand, in the present work, the extraction of microamounts of calcium by a nitrobenzene solution of hydrogen dicarbollylcobaltate (H+B-)2 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 nitroben-
Scheme 1. Structural formula of 2,3-naphtho-15-crown-5 (abbrev. N15C5 or L, respectively).
zene phase and to determine the corresponding equilibrium constants.
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.13 A nitrobenzene solution of hydrogen dicarbollylcobaltate (H+B-)2 was prepared from Cs+B- by the procedure described elsewhere.14 The other chemicals used (Lachema, Brno, Czech Republic) were of reagent grade purity. The radionuclide 45Ca2+ (DuPont, Belgium) was of standard radiochemical purity.
The extraction experiments in the two-phase wa-ter-HQ-Ca2+ (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 oC, 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. After evaporating aliquots (1 cm3) of the respective phases on Al plates, their ß-activities were measured by using the apparatus NRB-213 (Tesla Premysleni, Czech Republic).
The equilibrium distribution ratios of calcium, D, were determined as the ratios of the corresponding measured radioactivities of 45Ca2+ in the nitrobenzene and aqueous samples.
3. Results and Discussion
value of the total (analytical) concentration of the ligand N15C5 in the initial nitrobenzene phase, log c(L), is given in Figure 1. The initial concentration of hydrogen dicarbollylcobaltate in the organic phase, cB = 0.001 mol dm-3, as well as the initial concentration of HCl in the aqueous phase, c(HQ) = 0.01 mol dm-3, are always related to the volume of one phase.
With respect to previous results,2,15-17 the considered water-HCl-Ca2+ (microamounts)-nitrobenzene -N15C5(L)-H+B- system can be described by the set of reactions
(1) (2)
(3)
(4)
to which the following equilibrium constants correspond:
[Lo J
Kn =
[L.

[HiirL„„r '
v (re, 2+ ^ -
(5)
(6)
(7)
The dependence of the logarithm of the calcium distribution ratios (log D) on the logarithm of the numerical
Figure 1. Log D as a function of log c(L), where L is N15C5, for the water-HCl-Ca2+ (microamounts) - nitrobenzene -N15C5 -H+B- system. c(Ha) = 0.01 mol dm-3, cB = 0.001 mol dm-3. The curve was calculated using the constants given in Table 2.


(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 N15C5 ligand and the elec-troneutrality conditions in both phases of the system under study, was formulated19 and introduced into a more general least-squares minimizing program LETAGROP18 used for determination of the "best" values of the equilibrium constants ß(HL+m,org) and Kex(CaL2n+org) (L = N15C5). The minimum of the sum of errors in log D, i.e., the minimum of the expression

was sought.
Hydrogen and calcium		
complexes	log ß log Ka	U b
in the organic phase		
HL+, CaL2+	2.54 (2.81), 4.43 (4.68)	0.15
HL+, CaL22+	5.46 (6.25), 12.29 (13.36)	0.39
HL+2, CaL22+	4.58 ± 0.02, 4.02 ± 0.12	0.04
HL+2, CaL22+	5.48 ± 0.22, 7.65 ± 0.03	0.07
HL+,HL+, CaL2+	transformed to HL^, CaL2+	
HL+,HL+2, CaL22+	3.03 (3.59), 5.67 (5.95), 8.31 (8.88)	0.06
HL+, HL+, C:aL?+ C:aL2+	3.13 (3.41), 5.53 ± 0.18, 4.29 ± 0.02, 8.22 (8.56)	0.002
complexes in the organic phase
log ß log KX
U
HL+, CaL2+ HL+, CaL2+ HL+, CaL2+ HL+2, CaL2+ HL+,HL+, CaL2+ HL+,HL+2, CaL2+ HL+, HL+, C:aL?+ C:aL5+
2.54 (2.81), 4.43 (4.68)	0.15
5.46 (6.25), 12.29 (13.36)	0.39
4.58 ± 0.02, 4.02 ± 0.12	0.04
5.48 ± 0.22, 7.65 ± 0.03	0.07 transformed to HL^, CaL2+
3.03 (3.59), 5.67 (5.95), 8.31 (8.88)	0.06
3.13 (3.41), 5.53 ± 0.18, 4.29 ± 0.02, 8.22	(8.56) 0.002
a The values of the protonation and extraction constants are given for each complex. The reliability interval of the constants is given as 3g(K), where g(K) is the standard deviation of the constant K. 18 These values are given in the logarithmic scale using the approximate expression log K ± {log[K + 1.5g(K)] - log[K -1.5g(K)]}. For g(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 + 1.5g(K)].18 ^ The error-square sum U = Z(logDcaje - log Dexp)2.
The values log KD = 2.5715 andlog Kex(Cao^g) = 0.216 were used for the respective calculations. The results are listed in Table 1, from which it is evident that the extraction data can be best explained assuming the complexes HL+, HL+, CaL2+ and CaL2+(L = N15C5) to be extracted into the nitrobenzene phase.
Knowing the value log Kex(Ca2+g) = 0.2 ,16 as well as the extraction constants log Kex(CaLo^g) = 4.29 and log Kex (CaL2+org) = 8.22 determined here (Table 1), the stability constants of the complexes CaL2+ and CaL2+ in the nitrobenzene phase defined as
ß(C2i\}L) =
. [CaL^


(10)
(11)
can be evaluated applying the following simple relations:
Table 2. Equilibrium constants in the water-HCl-Ca2+ (mi-croamounts)-nitrobenzene-N15C5-H+B- system.
Equilibrium
log K
Laq ^ Lolg
H+lg + Lolg ^ HL+lg
H+ + 2L o HL+
org	org	2,olg
Ca2q + 2H+ o Ca2^ + 2H+
aq	org	org	aq
Ca2q + L + 2H+ o CaL2^ + 2H+
aq org	org	org	aq
Ca2q + 2L + 2H+ o CaL?^ + 2H+
aq	org	org	2,org	aq
Ca2^ + L o CaL^^
org org	org
Ca2^ + 2L o CaL2-+
org	org	2,org
2.57 a 3.13 5.53 0.2 b 4.29 8.22 4.09 8.02
a Determined by the method of the concentration dependent
distribution.15 bRef 16
log;ö(CaLtg) = log K,,(CaLtg)-log	, (12)
log	^ log	- log	. (13)
The respective equilibrium constants are summarized in Table 2.
Furthermore, Figure 2 depicts the contributions of the species H+lg, HL+lg and HL+ olg to the total hydrogen cation concentration in the equilibrium nitrobenzene phase, whereas Figure 3 shows the contributions of the cations Cao+g, CaL2+g and CaL2+olg to the total calcium concentration in the equilibrium organic phase. From Figures
Figure 2. Distribution diagram of hydrogen cation in the equilibrium nitrobenzene phase of the water-HCl-Ca2+ (microamounts) -nitrobenzene-N15C5-H+B- extraction system in the forms of H+, HL+ and HL+.
c(HCl) = 0.01 mol dm-3, cB = 0.001 mol dm-3.
1	5(H+) = [H+rg] / c(H+)olg,
2	5(HL+) = [HL+rg] / c(H+)olg, 5 5(HL+) = [HL+ org] / c(H+)olg,
where c(H+)olg = [H+lg] + [HL+lg] + [HLqolg].
The distribution curves were calculated using the constants given in
Table 2.
Figure 3. Distribution diagram of calcium in the equilibrium nitrobenzene phase of the water-HCl-Ca2+ (microamounts) -nitroben-zene-N15C5-H+B- extraction system in the forms of Ca2+, CaL2+ and CaL22+.
c(HCl) = 0.01 mol dm-3, cB = 0.001 mol dm-3.
1	5(Ca2+) = [Ca2;g] / c(Ca2+)„,g,
2	5(CaL2+) = [CaL2;,] / c(Ca2+)„rg,
3	5(CaL2+) = (CaL2+„,g) / c(Ca2+)„„,,
where c(Ca2+)„r, = [Ca2+,] + [CaLf;,] + [CaL2+„r,].
The distribution curves were calculated using the constants given in
Table 2.
2 and 3 it follows that the complexes HL+ org and CaL2+org are present in significant concentrations only at relatively high amounts of the N15C5 ligand in the system under consideration.
Finally, the stability constants of the complexes HL+, HL2, CaL2+ and CaL22+(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 HLorg, HL+ org, CaLo+g and CaL2+org, where L = 15C5, in water saturated nitrobenzene (see Table 3) in comparison with the stability of the respective complexes of H+ and Ca2+ with N15C5 in the mentioned medium determined here [log ß (HLorg) = 3.13, log ß (HL+ org) = 5.53, log ß (CaL2o+g) = 4.09 and log ß (CaL2+org) = 8.02] can be obviously explained on the basis of the higher flexibility of the 15C5 ligand 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
6076137307 and MSM 4977751303, and by the Specific Research of the Faculty of Applied Sciences, University of West Bohemia, Pilsen, Czech Republic.
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Table 3. Stability constants of the complexes, HL+, HL+, CaL2+ and CaL2+, where L = 15-crown-5 (15C5), 2,3-naphtho-15-crown-5 (N15C5), in nitrobenzene saturated with water at 25 °C
L	log ßCHL^rg)
log ß(HL+,„rg)
log ß(CaLo^g)
log ß(CaL2+,rg)
15C5 N15r5
4.27 a 3.13 c
6.32 a 5.53 c
7.63 b 4.09 c
11.57 b 8.02 c
a Ref. 20.
b Ref. 21.
c TTi 1 « wnrV
Povzetek
Raziskovali smo ekstrakcijo mikrokolicin kalcija z nitrobenzenovo raztopino vodikovega dikarbolilkobaltata (H+B-) ob prisotnosti crown etra 2,3-naftalen-15-crown-5 (N15C5, L). Dobljene eksperimentalne podatke smo obravnavali ob predpostavki, da so v organsko fazo ekstrahirani kompleksi HL+, HL+, CaL2+ in CaL2+ Določili smo ravnotežne koncentracije ter konstante stabilnosti kompleksov v nitrobenzen, nasičenem z vodo.