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Solvent extraction equilibria of uranium with 7-dodecenyl-8-quinolinol
Analytica Chimica Acta, 146 (1983) 237-241 Elsevier Scientific Publishing Company, Amsterdam - Printed in The Netherlands
SOLVENT EXTRACTION EQUILIBRIA 7-DODECENYGS-QUINOLINOL
OF URANIUM
WITH
LIN ZHU* and HENRY FREISER* Department
of Chemistry,
University
of Arizona,
Tucson, AZ 85721
(U.S.A.)
(Received 18th June 1982)
SUMMARY Solvent extraction equilibrium constants in the system uranyl-7-dodecenyl-%quinolinol (HL) in chloroform are reported. A mixed ligand complex is formed in the presence of acetate ion, UO,(OAc)L*HL, which adversely affects the extraction. In the absence of acetate, the extracted species is the self-adduct, UO,L*HL. The extraction is enhanced by l,lO-phenanthroline (phen) which forms UO,L, - phen.
The study of the metal extraction equilibria of the higher molecular weight analogs of metal chelating extractants not only provides an interesting comparison with the parent compounds that sheds light on the role of the substituents in affecting reagent behavior, but also is essential in evaluating the efficacy of the reagents themselves in both micro and macro scale separations. This report concerns the equilibria involved in the extraction of uranyl ion by 7dodecenyl-8-quinolinol (DDQ). The study is part of a systematic evaluation of chelating extractants for the separation of individual lanthanides and actinides [l-6]. An earlier study, devoted to the behavior of DDQ with representative lanthanides [ 41, revealed that the high-molecular-weight extractant exhibited a marked decrease in self-adduct formation. EXPERIMENTAL
7-Dodecenyl-Squinolinol [7-(l-vinyl-3,3’,6,6’-tetramethylhexyl)~%quinolinol; DDQ] is commercially known as Kelex 100 (Ashland Chemicals). It was purified by washing with 1 M hydrochloric acid [4]. The purity of DDQ was determined by converting it to its copper complex and determining the copper content by atomic absorption spectrometry. To prepare uranyl stock solution, 1.004 g of U02(N03h 6H20 (analytical reagent) was dissolved in a few milliliters of 6 M hydrochloric acid and the solution was evaporated to dryness. A few drops of concentrated hydrochloric acid was added and evaporation was continued until fuming was comaOn leave from Sichuan University, Chengdu, Sichuan, People’s Republic of China. 0003-2670/83/0000-0000/$03.00
o 1983 Elsevier Scientific Publishing Company
238
plete. The residue was dissolved in 0.01 M HCl, and then diluted to 100 ml with 0.01 M HCl. The concentration of the uranyl solution was determined gravimetrically . Chloroform (analytical grade) was purified by washing with water and distilling. All other reagents used were of analytical reagent purity. An Eberbach box shaker with a shaking speed of 280 oscillations/min was employed to equilibrate the extraction mixtures which were contained in 50-ml screwcap glass vials fitted with polyethylene stoppers and plastic screwcaps. Absorbance measurements were made in a Gilford Model 2400 spectrophotometer and pH measurements were made using an Orion 701 pH meter. The experimental procedures for extraction were as previously described [4]. RESULTS
AND DISCUSSION
The distribution of uranyl ion between aqueous buffer and DDQ in chloroform was examined as a function of the concentrations of uranyl ion UOz+, pH, acetate ion, and DDQ. The distributionratio was found to be independent of metal ion concentration in the range 10-3~5-104~5 M, demonstrating the monomeric character of the extracted complex. In the range 10-‘.8-10-2.6 M DDQ, the slope of the log D vs. log [DDQ] 0 plot was constant at 2.1. The corresponding log D vs. pH plot from pH 5.5 to 6.5 had a slope of only 1.08, rather than the value of two expected from the formation of a simple 2:l DDQ-UO:’ chelate. This problem was clarified by the observation of the dependence of D upon the acetate concentration. The adverse effect of acetate ion on the extraction can be explained by the formation of a series of uranyl acetate complexes from U02(0Ac)+ to U02(0Ac);. Such complexing can be taken into account by utilizing the fraction of aquated uranyl ion concentration, au, (YU =
(1 + ?
fli [OAC-]‘)-I
(1)
i= 1
where p1 = 102.61, pZ = 104*9,and p3 = 106*23[7], to correct the distribution ratio D. A plot of log D/au vs. log [OAc-] was approximately linear with a slope close to unity, demonstrating that a single acetate was incorporated in the extracted species. The extraction data are consistent with the following formulation of the reaction: UO;+ + OAc- + 2HL(O) “gx U02(0Ac)L
- HL(0) + H’
(2)
where HL represents DDQ. The experimental results are summarized in Table 1. The good agreement of values of the extraction constant, (log K,, = 2.50 f 0.05) calculated from all the data according to Eqn. (2), is seen to validate this formulation. Extraction equilibria of uranium by DDQ in the absence of acetate exhibited different linear relationships. The slopes of log D vs. pH and log [DDQ],,
239 TABLE
1
Extraction equilibria in presence of acetate buffer Plot coordinates Log Log Log Log Log
D vs. log CL, D vs. pH D vs. log C, D/a, vs. log [OAc-] D/a, vs. log [ OAc-]
(pH 6.04) (pH 5.80)
Slope
log K,
2.1 f 0.07 1.1 f 0.02 0.0 f 0.02 0.84 f 0.07 0.74 -I 0.05
2.54 2.43 2.54 2.53 2.49
f + f * +
0.08 0.02 0.03 0.02 0.02
Mean 2.50 f 0.05
were found to be close to 2.0 and 3.0, respectively (Table 2). Thus, the extraction equation UO;+ + 3HL(O) “k
U02Lz - HL(0) + 2H+
satisfactorily explains the data, as seen by the low variability of the extraction constant (log K’ = -2.56 + 0.02). It is interesting to note that while the extraction of uranyl by DDQ occurs at a much lower pH in the absence of acetate, the value of Kb, is lower than that of K,,. In comparing uranium extraction in the presence and absence of acetate, it is useful to remember the composite nature of the extraction constant. It can be shown [6] that for KeX of Eqn. (2),
where PCs1and Knct2) represent the formation and distribution constants of the species U02 (0Ac)L HL, respectively, and K, and KDR are the acid dissociation and distribution constants of the reagent, respectively. Similarly, for Kk, of Eqn. (3), l
TABLE
2
Extraction equilibria in unbuffered solutions Plot coordinates
CHL
Log D vs. pH
CHL: 0.05 M 0.04 0.03 0.02
2.1 2.0 1.9 1.8
Log D vs. log CL,
pH:
2.9 f 0.11 3.0 f 0.10 3.0 f 0.07
Or PH
3.80 3.60 3.40
Slope f + c +
0.03 0.09 0.17 0.15
-log
K
2.56 2.54 2.60 2.55
+ 0.02 t 0.03 f 0.06 -+ 0.05
2.54 k 0.06 2.57 it 0.03 2.56 + 0.02 Mean 2.56 k 0.02
240
Kb,
= PO>
(5)
&x,3,K:IK;,
whereD(3)and K~c(3) represent the formation and distribution constants of the species U02LZ HL, respectively. If Eqn. (3) is subtracted from Eqn. (2), the reaction l
UOzLz - HL(0) + H+ + OAc- %6’ UO,(OAc)L
- HL(0) + HL(0)
(6)
is characterized by an equilibrium constant, Kt6), given by (7) The large positive value of KC6) is, nevertheless, far smaller than that of for DDQ which is 10 “*’ [ 41, signifying that the PKDc product for the acetate-containing extractive species is lower than the other by a factor of 10i”*s. A good part of this (10’“) can be explained by the difference between the formation constants of the uranyl complex of acetate (pl = 102*6) and DDQ (assumed to be close to that of &quinolinol, fil = lo”), with the rest of the difference (103”) attributable to the reasonable assumption tht the DDQ anion contributes more to the value of KDc than does acetate anion. In contrast to the behavior of DDQ with the lanthanides, the presence of the large 7-substituent does not eliminate the formation of self-adduct complexes [4] . For further comparison with the behavior of the lanthanides with DDQ, extractions were repeated in the presence of 1 ,lO-phenanthroline (phen). In order to correlate the observed increase in extent of extraction obtained with phen, the entire increase in the value of D was attributed to the extraction of a mixed ligand complex. This increase, AD, was then subjected to the same slope analysis technique, i.e., log AD vs. pH, log [HL] o, or phen to define the stoichiometry and equilibrium constant of the new species. As can be seen from a summary of the experimental data in Table 3, the following reaction expresses the results ,I UO:’ + 2HL(O) + phen(0) “sx UOZL2 - phen(0) + 2H’ (3)
K&Ka
The value of this extraction constant, 10-2*50,is very close to that of the selfadduct complex, making it appear as if replacing the DDQ as monodentate adduct by the probably bidentate phen has no advantage. Thus, the equiliof the exchange reaction, brium constant Keg,, U02LZ - HL(0) + phen(0) Kg)U02L2
l
phen(0) + HL(0)
(9)
is given by Keg,= Kik/Kk,= 10**06. Analysis of Kb: and Kk,,however, gives where the ratio of the distribution constants of phen (KD(p)) and DDQ (KDR) has a value of 10S2. Inasmuch as the distribution constant of the phen complex is either equal to or less than that of the DDQ self-adduct, the value of P(S) is, therefore, at least 10 2.5 times greater than pC3),indicating the bidentate
241 TABLE 3 Extraction equilibria in presence of phenanthroline Plot coordinates
Slope
-log
;.(I;
2.0 + 0.14
2.39 f 0.03
0:01
1.6 1.7 *f 0.18 0.20
2.40 f* 0.04 2.55
Log D vs. C& (C&a, = 0.02 M) pH: “3.;
2.0 f 0.16 1.9 i 0.16
2.52 f 0.06 2.44 * 0.06
Log D vs. pH (C&
1.6 i 0.38
2.56 f 0.05
0:01
1.8 1.6 f 0.18 0.42
0.07 2.55 2.57 f* 0.04
4.0 3.8
1.1 + 0.06 1.2 f 0.12
2.59 f 0.02 2.53 f 0.04 Mean 2.51 + 0.07
Log D vs. pH (Cshen = 0.02 M) C&:
LogDvs.
= 0.02 M) C&en: ;.(I;
C;ben(C&=
0.02M)pH:
Ic
binding of l,lO-phenanthroline to uranium. It should be further noted that, by using a 5- or 7-alkyl substituted phenanthroline which would result in a larger Km,,) without any significant change in Pt8), then, in accord with Eqn. (lo), the value of &,, i.e., the advantage of the use of the alkyl-l,lO-phenanthroline in this system, would increase substantially. This project
was supported
by a grant from the Department
of Energy.
REFERENCES 1 T. Hori, M. Kawashima and H. Freiser, Separ. Sci. Technol., 15 (1980) 861. 2 M. Kawashima and H. Freiser, Anal. Chem., 53 (1981) 902. 3 0. Tochiyama and H. Freiser, Anal. Chem., 53 (1981) 909. 4 E. Yamada and H. Freiser, Anal. Chem., 53 (1981) 2115. 5 0. Tochiyama and H. Freiser, Anal. Chim. Acta, 131(1981) 233. 6 G. H. Morrison and H. Freiser, Solvent Extraction in Analytical Chemistry, Wiley, New York, 1957. 7 R. M. Smith and A. E. Martell, Critical Stability Constants, Plenum Press, New York, 1975.
SOLVENT EXTRACTION EQUILIBRIA 7-DODECENYGS-QUINOLINOL
OF URANIUM
WITH
LIN ZHU* and HENRY FREISER* Department
of Chemistry,
University
of Arizona,
Tucson, AZ 85721
(U.S.A.)
(Received 18th June 1982)
SUMMARY Solvent extraction equilibrium constants in the system uranyl-7-dodecenyl-%quinolinol (HL) in chloroform are reported. A mixed ligand complex is formed in the presence of acetate ion, UO,(OAc)L*HL, which adversely affects the extraction. In the absence of acetate, the extracted species is the self-adduct, UO,L*HL. The extraction is enhanced by l,lO-phenanthroline (phen) which forms UO,L, - phen.
The study of the metal extraction equilibria of the higher molecular weight analogs of metal chelating extractants not only provides an interesting comparison with the parent compounds that sheds light on the role of the substituents in affecting reagent behavior, but also is essential in evaluating the efficacy of the reagents themselves in both micro and macro scale separations. This report concerns the equilibria involved in the extraction of uranyl ion by 7dodecenyl-8-quinolinol (DDQ). The study is part of a systematic evaluation of chelating extractants for the separation of individual lanthanides and actinides [l-6]. An earlier study, devoted to the behavior of DDQ with representative lanthanides [ 41, revealed that the high-molecular-weight extractant exhibited a marked decrease in self-adduct formation. EXPERIMENTAL
7-Dodecenyl-Squinolinol [7-(l-vinyl-3,3’,6,6’-tetramethylhexyl)~%quinolinol; DDQ] is commercially known as Kelex 100 (Ashland Chemicals). It was purified by washing with 1 M hydrochloric acid [4]. The purity of DDQ was determined by converting it to its copper complex and determining the copper content by atomic absorption spectrometry. To prepare uranyl stock solution, 1.004 g of U02(N03h 6H20 (analytical reagent) was dissolved in a few milliliters of 6 M hydrochloric acid and the solution was evaporated to dryness. A few drops of concentrated hydrochloric acid was added and evaporation was continued until fuming was comaOn leave from Sichuan University, Chengdu, Sichuan, People’s Republic of China. 0003-2670/83/0000-0000/$03.00
o 1983 Elsevier Scientific Publishing Company
238
plete. The residue was dissolved in 0.01 M HCl, and then diluted to 100 ml with 0.01 M HCl. The concentration of the uranyl solution was determined gravimetrically . Chloroform (analytical grade) was purified by washing with water and distilling. All other reagents used were of analytical reagent purity. An Eberbach box shaker with a shaking speed of 280 oscillations/min was employed to equilibrate the extraction mixtures which were contained in 50-ml screwcap glass vials fitted with polyethylene stoppers and plastic screwcaps. Absorbance measurements were made in a Gilford Model 2400 spectrophotometer and pH measurements were made using an Orion 701 pH meter. The experimental procedures for extraction were as previously described [4]. RESULTS
AND DISCUSSION
The distribution of uranyl ion between aqueous buffer and DDQ in chloroform was examined as a function of the concentrations of uranyl ion UOz+, pH, acetate ion, and DDQ. The distributionratio was found to be independent of metal ion concentration in the range 10-3~5-104~5 M, demonstrating the monomeric character of the extracted complex. In the range 10-‘.8-10-2.6 M DDQ, the slope of the log D vs. log [DDQ] 0 plot was constant at 2.1. The corresponding log D vs. pH plot from pH 5.5 to 6.5 had a slope of only 1.08, rather than the value of two expected from the formation of a simple 2:l DDQ-UO:’ chelate. This problem was clarified by the observation of the dependence of D upon the acetate concentration. The adverse effect of acetate ion on the extraction can be explained by the formation of a series of uranyl acetate complexes from U02(0Ac)+ to U02(0Ac);. Such complexing can be taken into account by utilizing the fraction of aquated uranyl ion concentration, au, (YU =
(1 + ?
fli [OAC-]‘)-I
(1)
i= 1
where p1 = 102.61, pZ = 104*9,and p3 = 106*23[7], to correct the distribution ratio D. A plot of log D/au vs. log [OAc-] was approximately linear with a slope close to unity, demonstrating that a single acetate was incorporated in the extracted species. The extraction data are consistent with the following formulation of the reaction: UO;+ + OAc- + 2HL(O) “gx U02(0Ac)L
- HL(0) + H’
(2)
where HL represents DDQ. The experimental results are summarized in Table 1. The good agreement of values of the extraction constant, (log K,, = 2.50 f 0.05) calculated from all the data according to Eqn. (2), is seen to validate this formulation. Extraction equilibria of uranium by DDQ in the absence of acetate exhibited different linear relationships. The slopes of log D vs. pH and log [DDQ],,
239 TABLE
1
Extraction equilibria in presence of acetate buffer Plot coordinates Log Log Log Log Log
D vs. log CL, D vs. pH D vs. log C, D/a, vs. log [OAc-] D/a, vs. log [ OAc-]
(pH 6.04) (pH 5.80)
Slope
log K,
2.1 f 0.07 1.1 f 0.02 0.0 f 0.02 0.84 f 0.07 0.74 -I 0.05
2.54 2.43 2.54 2.53 2.49
f + f * +
0.08 0.02 0.03 0.02 0.02
Mean 2.50 f 0.05
were found to be close to 2.0 and 3.0, respectively (Table 2). Thus, the extraction equation UO;+ + 3HL(O) “k
U02Lz - HL(0) + 2H+
satisfactorily explains the data, as seen by the low variability of the extraction constant (log K’ = -2.56 + 0.02). It is interesting to note that while the extraction of uranyl by DDQ occurs at a much lower pH in the absence of acetate, the value of Kb, is lower than that of K,,. In comparing uranium extraction in the presence and absence of acetate, it is useful to remember the composite nature of the extraction constant. It can be shown [6] that for KeX of Eqn. (2),
where PCs1and Knct2) represent the formation and distribution constants of the species U02 (0Ac)L HL, respectively, and K, and KDR are the acid dissociation and distribution constants of the reagent, respectively. Similarly, for Kk, of Eqn. (3), l
TABLE
2
Extraction equilibria in unbuffered solutions Plot coordinates
CHL
Log D vs. pH
CHL: 0.05 M 0.04 0.03 0.02
2.1 2.0 1.9 1.8
Log D vs. log CL,
pH:
2.9 f 0.11 3.0 f 0.10 3.0 f 0.07
Or PH
3.80 3.60 3.40
Slope f + c +
0.03 0.09 0.17 0.15
-log
K
2.56 2.54 2.60 2.55
+ 0.02 t 0.03 f 0.06 -+ 0.05
2.54 k 0.06 2.57 it 0.03 2.56 + 0.02 Mean 2.56 k 0.02
240
Kb,
= PO>
(5)
&x,3,K:IK;,
whereD(3)and K~c(3) represent the formation and distribution constants of the species U02LZ HL, respectively. If Eqn. (3) is subtracted from Eqn. (2), the reaction l
UOzLz - HL(0) + H+ + OAc- %6’ UO,(OAc)L
- HL(0) + HL(0)
(6)
is characterized by an equilibrium constant, Kt6), given by (7) The large positive value of KC6) is, nevertheless, far smaller than that of for DDQ which is 10 “*’ [ 41, signifying that the PKDc product for the acetate-containing extractive species is lower than the other by a factor of 10i”*s. A good part of this (10’“) can be explained by the difference between the formation constants of the uranyl complex of acetate (pl = 102*6) and DDQ (assumed to be close to that of &quinolinol, fil = lo”), with the rest of the difference (103”) attributable to the reasonable assumption tht the DDQ anion contributes more to the value of KDc than does acetate anion. In contrast to the behavior of DDQ with the lanthanides, the presence of the large 7-substituent does not eliminate the formation of self-adduct complexes [4] . For further comparison with the behavior of the lanthanides with DDQ, extractions were repeated in the presence of 1 ,lO-phenanthroline (phen). In order to correlate the observed increase in extent of extraction obtained with phen, the entire increase in the value of D was attributed to the extraction of a mixed ligand complex. This increase, AD, was then subjected to the same slope analysis technique, i.e., log AD vs. pH, log [HL] o, or phen to define the stoichiometry and equilibrium constant of the new species. As can be seen from a summary of the experimental data in Table 3, the following reaction expresses the results ,I UO:’ + 2HL(O) + phen(0) “sx UOZL2 - phen(0) + 2H’ (3)
K&Ka
The value of this extraction constant, 10-2*50,is very close to that of the selfadduct complex, making it appear as if replacing the DDQ as monodentate adduct by the probably bidentate phen has no advantage. Thus, the equiliof the exchange reaction, brium constant Keg,, U02LZ - HL(0) + phen(0) Kg)U02L2
l
phen(0) + HL(0)
(9)
is given by Keg,= Kik/Kk,= 10**06. Analysis of Kb: and Kk,,however, gives where the ratio of the distribution constants of phen (KD(p)) and DDQ (KDR) has a value of 10S2. Inasmuch as the distribution constant of the phen complex is either equal to or less than that of the DDQ self-adduct, the value of P(S) is, therefore, at least 10 2.5 times greater than pC3),indicating the bidentate
241 TABLE 3 Extraction equilibria in presence of phenanthroline Plot coordinates
Slope
-log
;.(I;
2.0 + 0.14
2.39 f 0.03
0:01
1.6 1.7 *f 0.18 0.20
2.40 f* 0.04 2.55
Log D vs. C& (C&a, = 0.02 M) pH: “3.;
2.0 f 0.16 1.9 i 0.16
2.52 f 0.06 2.44 * 0.06
Log D vs. pH (C&
1.6 i 0.38
2.56 f 0.05
0:01
1.8 1.6 f 0.18 0.42
0.07 2.55 2.57 f* 0.04
4.0 3.8
1.1 + 0.06 1.2 f 0.12
2.59 f 0.02 2.53 f 0.04 Mean 2.51 + 0.07
Log D vs. pH (Cshen = 0.02 M) C&:
LogDvs.
= 0.02 M) C&en: ;.(I;
C;ben(C&=
0.02M)pH:
Ic
binding of l,lO-phenanthroline to uranium. It should be further noted that, by using a 5- or 7-alkyl substituted phenanthroline which would result in a larger Km,,) without any significant change in Pt8), then, in accord with Eqn. (lo), the value of &,, i.e., the advantage of the use of the alkyl-l,lO-phenanthroline in this system, would increase substantially. This project
was supported
by a grant from the Department
of Energy.
REFERENCES 1 T. Hori, M. Kawashima and H. Freiser, Separ. Sci. Technol., 15 (1980) 861. 2 M. Kawashima and H. Freiser, Anal. Chem., 53 (1981) 902. 3 0. Tochiyama and H. Freiser, Anal. Chem., 53 (1981) 909. 4 E. Yamada and H. Freiser, Anal. Chem., 53 (1981) 2115. 5 0. Tochiyama and H. Freiser, Anal. Chim. Acta, 131(1981) 233. 6 G. H. Morrison and H. Freiser, Solvent Extraction in Analytical Chemistry, Wiley, New York, 1957. 7 R. M. Smith and A. E. Martell, Critical Stability Constants, Plenum Press, New York, 1975.