Short communication
Contribution to the Thermodynamics of Sc3+, Y3+, La3+ and Trivalent Lanthanide Cations in the Two-Phase Water-Nitrobenzene Extraction System
Emanuel Makrlik1* and Petr Vanura2
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
* Corresponding author: E-mail: makrlik@centrum.cz
Received: 03-08-2010
Abstract
From the exchange extraction constants corresponding to the general equilibrium M3+(aq) + 3H+(nb) ^ M3+(nb) + 3H+(aq) occurring in the two-phase water-nitrobenzene system (M3+ = Sc3+, Y3+, La3+, Ce3+, Pr3+, Nd3+, Sm3+, Eu3+, Gd3+, Tb3+, Dy3+, Ho3+, Er3+, Tm3+, Yb3+, Lu3+; aq = aqueous phase, nb = nitrobenzene phase), the individual extraction constants of 16 trivalent metal cations were calculated. It was found that these individual extraction constants in the mentioned two-phase system increase in the following cation order: Sc < Yb3+< Ho3+ < Er3+ < Tm3+, Lu3+ < Y3+, Dy3+ < Tb3+ < Gd3+ < Eu3+ < Sm3+ < Nd3+ < Pr3+ < La3+, Ce3+.
Keywords: Thermodynamics, individual extraction constants, water-nitrobenzene system, trivalent cations
1. Introduction
Nitrobenzene, a dipolar aprotic solvent, has frequently been used in liquid-liquid extraction, especially in the separation of cesium, strontium, barium, europium and americium from aqueous solutions.1-23 Nitrobenzene is also suitable for accurate conductometric measure-ments.24 A considerable attention has been paid to the study of charge transfer across the water/nitrobenzene interface.25-27
The dicarbollylcobaltate anion and some of its halogen derivatives are very useful reagents for the extraction of various metal cations (especially Cs+), and also - in the presence of polyoxyethylene compounds (e. g., crown ethers, PEG 400, Slovafol 909) - for the extraction of Sr2+ and Ba2+ from aqueous solutions into an organic polar phase, both under laboratory conditions for purely theoretical or analytical purposes,1 and on the technological scale for the separation of some high-activity isotopes in the reprocessing of spent nuclear fuel and acidic radioactive waste.28-30
In order to estimate the individual extraction constants of some univalent ions in the water-nitrobenzene
system,31 a nonthermodynamic assumption of equal ex-tractabilities of the tetraphenylarsonium cation, Ph4As+, and the tetraphenylborate anion, BPh-, of the "reference" electrolyte32 from the aqueous into nitrobenzene phase, expressed by the identity log KP^Aj+ = log KBPh- = 6.3,31 was adopted. Both these ions are4 sufficiently an4d practically equally bulky and their phenyl group "covers" are identical. Thus, it is quite justifiable to assume that their specific interactions with the molecule of the solvent are very limited and virtually equal.32 The above - mentioned "Ph4AsBPh4 assumption" was further employed for a number of the two-phase water-polar organic solvent sys-
33-35 tems.33-35
The individual extraction constants of many univalent ions were evaluated in the water-nitrobenzene system.31,36,37 Recently, analogous thermodynamic data concerning some divalent metal cations and univalent organic cations have been reported.38,39 On the other hand, in the current communication, the individual extraction constants for a series of 16 trivalent metal cations in the considered two-phase system are presented.
2. Results and Discussion
Regarding the results of previous papers,1'31'36-40 the two-phase water-M(NO3)3 (M3+ = Sc3+, Y3+, La3+, Ce3+, Pr3+, Nd3+, Sm3+, Eu3+, Gd3+, Tb3+, Dy3+, Ho3+, Er3+, Tm3+, Yb3+, Lu3+) -nitrobenzene-hydrogen dicarbollylcobalta-te1-system can be described by the following general equilibrium
M(aq) + 3H+ (nb) o M(nb) + 3H+ (aq); K s(M5+,3H + )
(1)
with the corresponding exchange extraction constant Kex(M3+, 3H+); aq and nb denote the presence of the species in the aqueous and nitrobenzene phases, respectively.
The changes of the standard Gibbs energies corresponding to the transfer of the mentioned trivalent metal cations M3+ and H+ from the aqueous into the nitrobenzene phase
M3+(aq) ^ M3+(nb), H+(aq) ^ H+(nb), are given by definitions
. / ■	>r'b ,, l.r'b	0,»u
a r0,aq->nb .. 0,nb .. O.aq Ir.H* =MH.
(2) (3)
(4)
(5)
Here p0 - s are the standard chemical potentials of the ions M3+ and H+ in the two phases concerned.
The individual extraction constants KM3+ and KH+ for the considered cations M3+ and H+' respectively' in the
water-nitrobenzene extraction system are defined
by1,31'36-39
AG^r- KTIn K'm,
AG^r*" = -RT I" K'
U.H	H
Combination of the relation
- RT In K

- + lu"'*'
(6) (7)
(8)
corresponding to Equilibrium (1) with Eqs (4) - (7) yields
Furthermore, the standard Galvani potential difference of the trivalent metal cation M3+ between the nitrobenzene and aqueous phases (see, e. g., References 41
and 42), denoted by the symbol A^^^, is defined by the following relation
A"V
= - AGf,J,tV
tr.M
/3F .
(10)
Using the constants log Kex(M3+, 3H+)40 given in Table 1, the value log KH+ = -5.7,31 and applying Eq. (9), we obtain the logarithms of the individual extraction constants of the trivalent metal cations M3+ (M3+ = Sc3+, Y3+, La3+, Ce3+, Pr3+, Nd3+, Sm3+, Eu3+, Gd3+, Tb3+, Dy3+, Ho3+, Er3+, Tm3+, Yb3+, Lu3+), log KM3+, in the two-phase water-nitrobenzene system. These thermodynamic data are also summarized in Table 1. Besides this, by means of Eqs (6) and (10), one gets the thermodynamic data AG0^"^ and respectively, reviewed in Table 1 as well.

Table 1. Thermodynamic data for some trivalent metal cations M^ in the two-phase water-nitrobenzene extraction system at 25 °C (for the meaning of the constants see text).
m3+	log Kex	log KM*	AG0^	Anb a*L+ aq » M3+
	(M3+, 3H+)"		(kJ mol-1)c	(V)rf
Sc3+	0.76	- 16.34	93.29	- 0.322 3
y3+	1.13	-15.97	91.17	- 0.315 0
La3+	1.42	-15.68	89.52	- 0.309 3
Ce3+	1.42	-15.68	89.52	- 0.309 3
Pr3+	1.40	-15.70	89.63	- 0.309 7
Nd3+	1.38	-15.72	89.75	- 0.310 1
Sm3+	1.27	-15.83	90.37	- 0.312 2
Eu3+	1.22	-15.88	90.66	- 0.313 2
Gd3+	1.21	-15.89	90.72	- 0.313 4
Tb3+	1.15	-15.95	91.06	- 0.314 6
Dy3+	1.13	-15.97	91.17	- 0.315 0
Ho3+	1.09	-16.01	91.40	- 0.315 8
Er3+	1.10	-16.00	91.35	- 0.315 6
Tm3+	1.12	-15.98	91.23	- 0.315 2
Yb3+	1.07	-16.03	91.52	- 0.316 2
Lu3+	1.12	-15.98	91.23	- 0.315 2
a Ref. 40; b Calculated from Eq. (9); d Calculated from Eq. (10).
Calculated from Eq. (6);
Finally, from the data given in Table 1 it follows that the individual extraction constants KiM3+ in the water-nitrobenzene system increase in the series of Sc3+ < Yb3+ < Ho3+ < Er3+ < Tm3+, Lu3+ < Y3+, Dy3+ < Tb3+ < Gd3+ < Eu3+ < Sm3+ < Nd3+ < Pr3+ < La3+, Ce3+. At this point it should be noted that the mentioned constants express, in principle, the affinity of the considered cations M3+ for the nitrobenzene phase. It means that the KM3+ constants are a quantitative measure of the hydrophobicity, as well as the extractability of the ions M3+ from the aqueous into the nitrobenzene phase.
In conclusion, it is necessary to emphasize that the thermodynamic constants log KM3+, AG0^'"^ and A!lb^M3+listed in Table 1 can be employed, e. g., for the
studies of chemical equilibria in the two-phase water-nitrobenzene extraction system, analogously as in some previous papers.43-48
3. 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
V dvofaznem sistemu voda-nitrobenzen (nb) smo za različne ione raziskovali ravnotežja porazdelitve, ki jih lahko ponazorimo z enačbo M3+(aq) + 3H+(nb) M3+(nb) + 3H+(aq), M3+ = Sc3+, Y3+, La3+, Ce3+, Pr3+, Nd3+, Sm3+, Eu3+, Gd3+, Tb3+, Dy3+, Ho3+, Er3+, Tm3+, Yb3+, Lu3+. Iz ravnotežnih konstant porazdelitve (konstant ekstrakcije) smo določili konstante ekstrakcije za posamezne ione ter ugotovili, da v proučevanem dvofazne sistemu le te sledijo v zaporedju Sc3+ < Yb3+< Ho3+ < Er3+ < Tm3+, Lu3+ < Y3+, Dy3+ < Tb3+ < Gd3+ < Eu3+ < Sm3+ < Nd3+ < Pr3+ < La3+, Ce3+.