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Thermodynamic studies of TOPO adducts of europium and terbium tris-TTA chelates
Z inorg, nucl. Chem. Vol. 42, pp. 1491-1494 © PergamonPress Ltd., 19~0. Printed in Great Britain
0022-1902/80f1001-1491t$02.00/0
T H E R M O D Y N A M I C STUDIES OF TOPO A D D U C T S OF E U R O P I U M A N D TERBIUM TRIS-TTA C H E L A T E S A. T. KANDIL and K. FARAH Department of Nuclear Chemistry,Atomic Energy Establishment,Cairo, Egypt (First received 2 November 1978; receivedfor publication 26 June 1979)
Abstract--Solvent extraction of Tb3+ and Eu3+ from an aqueous phase of 0.1 M ionic strength (Na+, HCIO4)and pH = 3.4 with thenoyltrifluoroacetone,HTTA, in benzene and by mixtures of HTI'A and trioctylphosphineoxide, TOPO were made at differenttemperatures.The thermodynamicdata obtained explainthe mechanismof formation of the species Eu(TTA)3(TOPO), Eu(TTA)3(TOPO)z,Tb(TTA)3(TOPO)and Tb(TrA)3(TOPO)2.
INTRODUCTION Marcus and Kertes[l] have proposed three possible mechanisms to explain the data of synergistic solvent extraction of metal ions by the combined action of two specific solutes in the organic phase: (1) The neutral adduct replaces the residual waters of hydration in the first coordination sphere of the metal ion OH2
S
I
I
of the experimental data[7]. Systems that did not show synergism were Pa 5+ with HTTA+TBP and HTI'A+ TPPO[8] and Tb 3+, Eu a+ with mixtures of di(2-ethylhexyl)-phosphoric acid, DEHP+TBP and DEHP+ TOPOI9]. No attempt was done in the past to measure the thermodynamic parameters of both the 1:1 and 1:2 adducts. Such measurements could add informations to the mechanism by which the 1:2 adduct is formed and would show the influence of the already existing synergistic base molecule in the 1 : 1 adduct.
M(TI'A)x + 2S = M(TTA)x + 2H20
I
OH2
I
S
EXPERIMENTAL
The experimental techniques and the reagents used in this study are similar to those of Ref. [2]. Tb and Eu radioactive tracers were prepared by irradiatingtheir respective oxides at the 2 MW reactor of the Atomic Energy Establishment at Inshas. The oxides were dissolved in HCI and the chloride was transformed into perchlorate by the successive evaporations and dissolutionsin diluted HCIO4.The ionic strength was adjusted by NaCIO4 at 0.1 M and the pH by HCIO4at 3.4.
M = metal ion, S = synergistic base and X = charge on ion. (2) The neutral adduct attaches the metal chelate by means of a hydrogen bond to the coordinated waters of the first hydration sphere OH2
OH2---S
J
I
RESULTS
M(TTA)x + 2S = M(TTA)x
I
OH2
I
OH2---S
(3) The coordination number of the non-hydrated chelate expands from 6 to 8 to accomodate direct coordination of the adduct to metal S
J
Figures 1 and 2 show respectively the extraction of Eu 3+ and Tb 3+ from a 0.1 M aqueous phase at pH = 3.4 with varying concentrations of HTTA in benzene at various temperatures. Slope analysis of the straight lines obtained based on the assumptions that: hydrolysis and polymerization in the aqueous phase are absent, activity coefficients in solution are constant and that no side reactions occur, indicate that the following equation represents the extraction of Eu 3+ and Th3÷:
M(TTA)x + 2S = M(TTA)x. Ln~+)+ 3 HTTA~o,. Kt" Ln(TTA)uo)+ 3 H~a,
(1)
S The knowledge of the formation constants alone do not allow choice between the three mechanisms proposed. However, such a choice is more feasible once the enthalpies of the extraction reactions are known. Such measurements have been reported on some lanthanide elements [2], Zn 2+ [3, 4], Co2+ [5] and Th4+ [6] and showed that the third mechanism makes a better fit to the experimental data. Studies on Co2+ with HTTA and with mixtures of HTI'A + TOPO, HTrA +tributylphosphate(TBP) and HTTA+TOPO (triphenylphosphine oxide) showed that the first mechanism makes a better fit
where a = aqueous phase, o =organic phase, Ln 3+= either Th3÷ or Eu 3+, and [Ln(TTA)3]o[H÷]o3 K, = [Ln~+]co)[HTrA]~o; Table 1 contains the thermodynamic parameters of this reaction. The errors in log Kt are estimated at 5% and the error is AS and AH is 10%. Figures 3 and 4 represent the extraction of Eu 3+ and Th3÷ from a 0.1 M aqueous phase and pH = 3.4 with
1491
1492
A.T. KANDIL and K. FARAH
I0
/
/ A,45"¢ 0.1
e, 35"C 0,25"C o, lO*C
0.01
,
I
I
0.01
0.1
I
03
_
,
,
,
. . . .
i ,tl
I
fo-s
io-~
[HTTA] Fig. !. The extraction of Eu3÷by various concentrations of HTrA in benzene from an equeous phase of 0.1 M (NaCIO4)and pH = 3.4 (HCI04) at temperature of 10, 25, 35 and 45"C.
Fig. 3. The extraction of Eu 3+ by 0,05 M HTTA in benzene and varying concentrations of TOPO from an equeons phase of 0.1 M (NaCIO4) and pH of 3.4 (HC104) at temperatures of 10, 25, 35 and 45°C, the data of the species Eu (TTA)3 (TOPO)2 formed at about 5 x 10-5 M TOPO concentration is shown only at temperatures of 10 and 45°C. The data at 25 and 35°C coincident (see Table 2).
IO IO
) D I
,,,,45oc o.I
o,
e,55°C
C///
,..,,25oc
3soc
o, 25"c
O, 12"C
o. 12"C
0.01
0.0'1
I
0.1
I
I
[HTTAI Fig. 2. The extraction of Tb3+by various concentrations of HTI'A in benzene from an equous phase of 0.1 M (NaCIO4)and pH = 3.4 (HC104) at temperatures of 12, 25, 35 and 45°C.
0.1
I
i
I
I
I
i0 "s
I
I
a aJ
,
I
'
'
IO"~
~'OPO] Fig. 4. The extraction of Th3. by 0.05 M HTTA in benzene from an equeous phase of 0.1 M (NaCIO4) and pH of 3.4 (HCIO4) at temperatures of 12, 25, 35 and 45°C.
Thermodynamicstudies of TOPO adducts
1493
Table I. Thermodynamic parameters for the formation of EuO'TA)3, Tb(TI'Ah, Eu(TrAh(TOPO), Tb(TTA)3(TOPO),Eu(TTAh(TOPOhand Tb(TrAh('l'OPOh at 25"C Species
Log K~
Eu(TrA)3 Tb(TrAh Eu(TrA)3(TOPO) Tb(TTA)3(TOPO) Eu(TrA)3(TOPO): Tb(TrAh(TOPO)2
LOgK2
-7.22 + 0.05 -7.25 _+0.05 -0.63-+ 0.02 -0.93 _+0.03 4.74+-0.02 4.32-+0.02 Log B 6.59± 0.05 6.32± 0.06 11.96-+0.05 11.57± 0.05
Eu(TTA)3(TOPO) Tb(TrA)3(TOPO) Eu(TTA)3(TOPO)2 Tb('II'A)3(TOPO)2
0.05 M HTTA and various concentrations of TOPO at various temperatures. The extraction equation can be represented by: LnS+ <~)+ 3 HTT&o) + TOPO(o). =K2" Ln(TTAh(TOPO)(o) + 3 H~)
(2)
+3
K2 = [Ln(TrA)30"OPOI~oT[H ]~o~ [Ln3+]to)[I-I'I~A]~o>[TOPO]
K2
Ln(TTA)3(TOPO)2~o~+ 3 H +
(3)
with [Ln(TTA)3(TOPO)2I K2 = [Ln3+](,.[HTrA]~o>[TOPO]~o~ The data in Fig. 5 for Eu(TTA)3(TOPO)2 is shown only at 25 and 35"C falls on these data. Values of the equilibrium constant of this reaction as a function of temperature are shown in Table 2. Thermodynamic parameters for reaction 3 are also given in Table 1. The synergism reaction of interest that occurs presumably in the organic phase is: fl
Ln(Tl'Ah(o) + TOPO
Log K~
10
4.74± 0.02
25 35 45
4.74± 0.02 4.74+-0.03 4.75± 0.03
9.91 9;64 9.95 I 1.10 0.87 2.76 1.28 3.92 -6.50 0.00 -5.93 2.67 AG AH Kcal/mole Kcal/mole -9.04 -6.88 -8.67 -7.18 - 16.41 -9.64 - 15.88 -8.43
- 1.0 +3.9 6.0 8.9 21.8 28.8 AS e.u. 7.0 5.0 22.8 24.9
Ln(TTA)3(o~ + 2 TOPO
where ~_ [In(TrA),(TOVO)ko ) K2 - [In(TTA)3]
and
The thermodynamic data of this synergistic organic phase reaction is also contained in Table 1.
The thermodynamic parameters of this reaction are also contained in Table 1. At TOPO concentrations larger than about 5 x 10-6 M the species In(TTA)3(TOPO)2 is formed. The extraction equation of this species could be represented by: 3+
AS e.u.
/~, _ [Ln(TTAh(TOPO)2],o) _ K~ - [Ln(TI'A)al(o~[TOPO]2o)- Kf
with
Ln~> + 3 HTI'A~o)+ 2 TOPO(o~
AG AH Kcal/mole Kcal/mole
DISCUSSION The data in Table 1 shows an endothermic reaction for the formation of the tris-TTA chelates of Eu3÷ and Tb 3÷. This behaviour is quite familiar for chelation reactions in which the waters of the inner coordination sphere of the metal ion are displaced by the chelating reagent [I0]. On the contrary to this, the enthalpy change for the formation of the synergistic species in the organic phase is exothermic for both Eu3÷ and Tb3÷ where values of -7.18Kcal/mole and -6.88Kcal/mole are obtained for the Th(TTA)3(TOPO) and Eu(TTAh(TOPO) species, respectively. The entropy change is quite small in both cases where it amounts to 5 and 7 e.u. for the formation of the Tb(TI'A)3(TOPO) and Eu(TYA)3(TOPO) species respectively. In both cases the negative enthalpy is the dominant favour causing a favourable equilibrium. This result actually supports a mechanism for the synergism in both systems in which the coordination number of the lanthanide ion increases by one to accomodate the synergistic base molecule. The species Ln(TI'A)3(TOPOh is formed at larger. TOPO concentrations and the enhancement in its extraction is about 2.0 x l0 s over the Ln(TTA)3(TOPO) species. The enthalpy change given in Table 1 for the formation of the EuOTTA)3(TOPO)2 and the Tb(TTAh(TOPO)z is -9.64 Kcal/mole and -8.43 Kcal/mole, respectively. This result again supports an increase in the coordination number of the lanthanide ion from 7 to 8 accomodate the second synergistic base molecule. The entropy change accompanying this process is 24.9 e.u. for Tb and 22.8 e.u. for Eu. A large entropy change of this order reflects the steric hindrance caused by the first TOPO molecule to the entrance of a second molecule with its lengthy octyl groups.
1494
A. T. KANDIL and K. F A R A H
REFERENCES 1. Y. Marcus and A. S. Kertes, Ion Exchange and Solvent Extraction of Metal Complexes. Wiley, New York (1969). 2. A. T Kandil, H. F. Aly, M. Raieh and G. R. Choppin, J. lnorg. Nucl. Chem. 37, 229 (1975). 3. K. L. Nash and G. R. Choppin, J. lnorg. Nucl. Chem. 39, 131 (1977). 4. A. T. Kandil, N. Souka and F. Abdel Reheam, Radiochim. Acta In Press (1978). 5. A. AI-Atrash, A. T. Kandil, E. Souaya and W. Georgy, J. Radioanal. Chem. 43, 73 (1978).
6. A. T. Kandil and A. Ramadan, J. lnorg. NucL Chem. Submitted. 7. A. T. Kandil and A. Ramadan, J. Inorg. Nucl. Chem. Submitted. 8. A. T. Kandil, A. S. Atxlel Gawad and A. Ramadan, .L. Inorg. Nucl. Chem. In press. 9. A. T. Kandil and K. Farah, I. lnorg. Nad. Chem. 42, 277
(1980). 10. G. R. Choppin and H. G. Friedman, lnorg. Chem. 5, 1599 (1966).
0022-1902/80f1001-1491t$02.00/0
T H E R M O D Y N A M I C STUDIES OF TOPO A D D U C T S OF E U R O P I U M A N D TERBIUM TRIS-TTA C H E L A T E S A. T. KANDIL and K. FARAH Department of Nuclear Chemistry,Atomic Energy Establishment,Cairo, Egypt (First received 2 November 1978; receivedfor publication 26 June 1979)
Abstract--Solvent extraction of Tb3+ and Eu3+ from an aqueous phase of 0.1 M ionic strength (Na+, HCIO4)and pH = 3.4 with thenoyltrifluoroacetone,HTTA, in benzene and by mixtures of HTI'A and trioctylphosphineoxide, TOPO were made at differenttemperatures.The thermodynamicdata obtained explainthe mechanismof formation of the species Eu(TTA)3(TOPO), Eu(TTA)3(TOPO)z,Tb(TTA)3(TOPO)and Tb(TrA)3(TOPO)2.
INTRODUCTION Marcus and Kertes[l] have proposed three possible mechanisms to explain the data of synergistic solvent extraction of metal ions by the combined action of two specific solutes in the organic phase: (1) The neutral adduct replaces the residual waters of hydration in the first coordination sphere of the metal ion OH2
S
I
I
of the experimental data[7]. Systems that did not show synergism were Pa 5+ with HTTA+TBP and HTI'A+ TPPO[8] and Tb 3+, Eu a+ with mixtures of di(2-ethylhexyl)-phosphoric acid, DEHP+TBP and DEHP+ TOPOI9]. No attempt was done in the past to measure the thermodynamic parameters of both the 1:1 and 1:2 adducts. Such measurements could add informations to the mechanism by which the 1:2 adduct is formed and would show the influence of the already existing synergistic base molecule in the 1 : 1 adduct.
M(TI'A)x + 2S = M(TTA)x + 2H20
I
OH2
I
S
EXPERIMENTAL
The experimental techniques and the reagents used in this study are similar to those of Ref. [2]. Tb and Eu radioactive tracers were prepared by irradiatingtheir respective oxides at the 2 MW reactor of the Atomic Energy Establishment at Inshas. The oxides were dissolved in HCI and the chloride was transformed into perchlorate by the successive evaporations and dissolutionsin diluted HCIO4.The ionic strength was adjusted by NaCIO4 at 0.1 M and the pH by HCIO4at 3.4.
M = metal ion, S = synergistic base and X = charge on ion. (2) The neutral adduct attaches the metal chelate by means of a hydrogen bond to the coordinated waters of the first hydration sphere OH2
OH2---S
J
I
RESULTS
M(TTA)x + 2S = M(TTA)x
I
OH2
I
OH2---S
(3) The coordination number of the non-hydrated chelate expands from 6 to 8 to accomodate direct coordination of the adduct to metal S
J
Figures 1 and 2 show respectively the extraction of Eu 3+ and Tb 3+ from a 0.1 M aqueous phase at pH = 3.4 with varying concentrations of HTTA in benzene at various temperatures. Slope analysis of the straight lines obtained based on the assumptions that: hydrolysis and polymerization in the aqueous phase are absent, activity coefficients in solution are constant and that no side reactions occur, indicate that the following equation represents the extraction of Eu 3+ and Th3÷:
M(TTA)x + 2S = M(TTA)x. Ln~+)+ 3 HTTA~o,. Kt" Ln(TTA)uo)+ 3 H~a,
(1)
S The knowledge of the formation constants alone do not allow choice between the three mechanisms proposed. However, such a choice is more feasible once the enthalpies of the extraction reactions are known. Such measurements have been reported on some lanthanide elements [2], Zn 2+ [3, 4], Co2+ [5] and Th4+ [6] and showed that the third mechanism makes a better fit to the experimental data. Studies on Co2+ with HTTA and with mixtures of HTI'A + TOPO, HTrA +tributylphosphate(TBP) and HTTA+TOPO (triphenylphosphine oxide) showed that the first mechanism makes a better fit
where a = aqueous phase, o =organic phase, Ln 3+= either Th3÷ or Eu 3+, and [Ln(TTA)3]o[H÷]o3 K, = [Ln~+]co)[HTrA]~o; Table 1 contains the thermodynamic parameters of this reaction. The errors in log Kt are estimated at 5% and the error is AS and AH is 10%. Figures 3 and 4 represent the extraction of Eu 3+ and Th3÷ from a 0.1 M aqueous phase and pH = 3.4 with
1491
1492
A.T. KANDIL and K. FARAH
I0
/
/ A,45"¢ 0.1
e, 35"C 0,25"C o, lO*C
0.01
,
I
I
0.01
0.1
I
03
_
,
,
,
. . . .
i ,tl
I
fo-s
io-~
[HTTA] Fig. !. The extraction of Eu3÷by various concentrations of HTrA in benzene from an equeous phase of 0.1 M (NaCIO4)and pH = 3.4 (HCI04) at temperature of 10, 25, 35 and 45"C.
Fig. 3. The extraction of Eu 3+ by 0,05 M HTTA in benzene and varying concentrations of TOPO from an equeons phase of 0.1 M (NaCIO4) and pH of 3.4 (HC104) at temperatures of 10, 25, 35 and 45°C, the data of the species Eu (TTA)3 (TOPO)2 formed at about 5 x 10-5 M TOPO concentration is shown only at temperatures of 10 and 45°C. The data at 25 and 35°C coincident (see Table 2).
IO IO
) D I
,,,,45oc o.I
o,
e,55°C
C///
,..,,25oc
3soc
o, 25"c
O, 12"C
o. 12"C
0.01
0.0'1
I
0.1
I
I
[HTTAI Fig. 2. The extraction of Tb3+by various concentrations of HTI'A in benzene from an equous phase of 0.1 M (NaCIO4)and pH = 3.4 (HC104) at temperatures of 12, 25, 35 and 45°C.
0.1
I
i
I
I
I
i0 "s
I
I
a aJ
,
I
'
'
IO"~
~'OPO] Fig. 4. The extraction of Th3. by 0.05 M HTTA in benzene from an equeous phase of 0.1 M (NaCIO4) and pH of 3.4 (HCIO4) at temperatures of 12, 25, 35 and 45°C.
Thermodynamicstudies of TOPO adducts
1493
Table I. Thermodynamic parameters for the formation of EuO'TA)3, Tb(TI'Ah, Eu(TrAh(TOPO), Tb(TTA)3(TOPO),Eu(TTAh(TOPOhand Tb(TrAh('l'OPOh at 25"C Species
Log K~
Eu(TrA)3 Tb(TrAh Eu(TrA)3(TOPO) Tb(TTA)3(TOPO) Eu(TrA)3(TOPO): Tb(TrAh(TOPO)2
LOgK2
-7.22 + 0.05 -7.25 _+0.05 -0.63-+ 0.02 -0.93 _+0.03 4.74+-0.02 4.32-+0.02 Log B 6.59± 0.05 6.32± 0.06 11.96-+0.05 11.57± 0.05
Eu(TTA)3(TOPO) Tb(TrA)3(TOPO) Eu(TTA)3(TOPO)2 Tb('II'A)3(TOPO)2
0.05 M HTTA and various concentrations of TOPO at various temperatures. The extraction equation can be represented by: LnS+ <~)+ 3 HTT&o) + TOPO(o). =K2" Ln(TTAh(TOPO)(o) + 3 H~)
(2)
+3
K2 = [Ln(TrA)30"OPOI~oT[H ]~o~ [Ln3+]to)[I-I'I~A]~o>[TOPO]
K2
Ln(TTA)3(TOPO)2~o~+ 3 H +
(3)
with [Ln(TTA)3(TOPO)2I
Ln(Tl'Ah(o) + TOPO
Log K~
10
4.74± 0.02
25 35 45
4.74± 0.02 4.74+-0.03 4.75± 0.03
9.91 9;64 9.95 I 1.10 0.87 2.76 1.28 3.92 -6.50 0.00 -5.93 2.67 AG AH Kcal/mole Kcal/mole -9.04 -6.88 -8.67 -7.18 - 16.41 -9.64 - 15.88 -8.43
- 1.0 +3.9 6.0 8.9 21.8 28.8 AS e.u. 7.0 5.0 22.8 24.9
Ln(TTA)3(o~ + 2 TOPO
where ~_ [In(TrA),(TOVO)ko ) K2 - [In(TTA)3]
and
The thermodynamic data of this synergistic organic phase reaction is also contained in Table 1.
The thermodynamic parameters of this reaction are also contained in Table 1. At TOPO concentrations larger than about 5 x 10-6 M the species In(TTA)3(TOPO)2 is formed. The extraction equation of this species could be represented by: 3+
AS e.u.
/~, _ [Ln(TTAh(TOPO)2],o) _ K~ - [Ln(TI'A)al(o~[TOPO]2o)- Kf
with
Ln~> + 3 HTI'A~o)+ 2 TOPO(o~
AG AH Kcal/mole Kcal/mole
DISCUSSION The data in Table 1 shows an endothermic reaction for the formation of the tris-TTA chelates of Eu3÷ and Tb 3÷. This behaviour is quite familiar for chelation reactions in which the waters of the inner coordination sphere of the metal ion are displaced by the chelating reagent [I0]. On the contrary to this, the enthalpy change for the formation of the synergistic species in the organic phase is exothermic for both Eu3÷ and Tb3÷ where values of -7.18Kcal/mole and -6.88Kcal/mole are obtained for the Th(TTA)3(TOPO) and Eu(TTAh(TOPO) species, respectively. The entropy change is quite small in both cases where it amounts to 5 and 7 e.u. for the formation of the Tb(TI'A)3(TOPO) and Eu(TYA)3(TOPO) species respectively. In both cases the negative enthalpy is the dominant favour causing a favourable equilibrium. This result actually supports a mechanism for the synergism in both systems in which the coordination number of the lanthanide ion increases by one to accomodate the synergistic base molecule. The species Ln(TI'A)3(TOPOh is formed at larger. TOPO concentrations and the enhancement in its extraction is about 2.0 x l0 s over the Ln(TTA)3(TOPO) species. The enthalpy change given in Table 1 for the formation of the EuOTTA)3(TOPO)2 and the Tb(TTAh(TOPO)z is -9.64 Kcal/mole and -8.43 Kcal/mole, respectively. This result again supports an increase in the coordination number of the lanthanide ion from 7 to 8 accomodate the second synergistic base molecule. The entropy change accompanying this process is 24.9 e.u. for Tb and 22.8 e.u. for Eu. A large entropy change of this order reflects the steric hindrance caused by the first TOPO molecule to the entrance of a second molecule with its lengthy octyl groups.
1494
A. T. KANDIL and K. F A R A H
REFERENCES 1. Y. Marcus and A. S. Kertes, Ion Exchange and Solvent Extraction of Metal Complexes. Wiley, New York (1969). 2. A. T Kandil, H. F. Aly, M. Raieh and G. R. Choppin, J. lnorg. Nucl. Chem. 37, 229 (1975). 3. K. L. Nash and G. R. Choppin, J. lnorg. Nucl. Chem. 39, 131 (1977). 4. A. T. Kandil, N. Souka and F. Abdel Reheam, Radiochim. Acta In Press (1978). 5. A. AI-Atrash, A. T. Kandil, E. Souaya and W. Georgy, J. Radioanal. Chem. 43, 73 (1978).
6. A. T. Kandil and A. Ramadan, J. lnorg. NucL Chem. Submitted. 7. A. T. Kandil and A. Ramadan, J. Inorg. Nucl. Chem. Submitted. 8. A. T. Kandil, A. S. Atxlel Gawad and A. Ramadan, .L. Inorg. Nucl. Chem. In press. 9. A. T. Kandil and K. Farah, I. lnorg. Nad. Chem. 42, 277
(1980). 10. G. R. Choppin and H. G. Friedman, lnorg. Chem. 5, 1599 (1966).