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Liquid-solid extraction system based on between 40-salt-H2O without organic solvents
0039-914o/Pos3.00 + 0.00
?‘&nta, VoI. 37, No. 9, pp. 885-888, 1990
PergamonPresspk
Printed in Great Brimin
EXTRACTION SYSTEM BASED ON TWEEN 4&SALT-H,O WITHOUT ORGANIC SOLVENTS IA BUHAI
and &&~aRIGAN
Department of Chemistry, South-central College for Nationalities, Wuhan, Hubei, 430074, People’s Republic of China (Received 15 September 1989. Revised I4 February 1990. Accepted 21 February 1990)
&nmary--An aqueous solution of Tween 40 forms liquid and solid phases in the presence of various salts, depending on the Tween 40 concentration, the identity and con~tration of the salt, and the solution acidity. The distribution of Zr(IV), U(VI), Fe(III), Pb(I1) and some organic photometric reagents between the Tween 40 phase and aqueous phase containing salt was examined. The quantitative extraction, separation and determination of Zr(IV) or U(M) in the presence of Pb(II) was achieved by controlling the solution acidity in the system. The extraction mechanism has been tentatively studied.
extraction with common organic solvents, only neutral species are normally transferred from the hydrophilic to the hydrophobic phase. Therefore the method cannot be applied to the extraction of strongly hydrophilic species, especially those with multiple charges. Also, the solvents are often volatile, flammable (or even explosive) and toxic. It is known that aqueous solutions of poly(ethylene glycol) (PEG) can separate into two aqueous phases in the presence of certain inorganic salts.‘**This two-phase aqueous system has been applied to the extractive separation of metal ions.3T4 Recently, we found that aqueous solutions of poly(~nylpyrro~done~ (PVP), Tween 80 @oIyoxyethyleue sorb&an mono-oleate) and other water-soluble polymers form liquid and solid phases on addition of various salts. Under suitable conditions some water-soluble photometric reagents and their complexes with metal ions can be transferred into the solid phases formed by the polymers. Some metal ions can be separated with this system. We have called the system a liquid-solid extraction system.$ It has an advantage over the two-phase aqueous system, because it is more convenient (not requiring a separatory funnel) and is more rapid. In this paper the liquid-solid extraction system based on Tween 40--salt-H,U is reported. 1n liquid-liquid
EXPmIWTAL
Appamm
A Shanghai model 721 spectrophotometer was used for photometric meas~ements. A
Shanghai model pHs-2 pH-meter and a shaking machine were used. Reagents
An aqueous solution of Tween 40 (polyoxyethylene sorbitan palmitate) was prepared by dissolving 50 ml of Tween 40 in 200 ml of distilled water. Standard 1.O-mg/ml solutions of U(VI), Pb@) and Fe(EII) in 0.151cInitric acid, and Zr(IV) in 10% v/v hydrochloric acid were prepared frum UO,(No,), - 6H@, pure lead and iron, and ZrOCl, - 8H,O respectively. More dilute solutions were prepared by appropriate dilution as required. All other reagents were of analytical grade. Procedure
Given volumes of Tween 40 aqueous solution, photometric reagent solution and metal ion solution were put into a 25-ml tube fitted with a stopper. The acidity of strongfy acidic solutions was adjusted with hydrochloric acid, and in the pH range 1.5-6.5 by use of monochloroacetic acid or acetic acid buffers. The solution was then diluted to 10.0 ml with distilled water, a given weight of solid salt was added and the mixture was shaken for 3 min. After standing for several minutes, the solution separated into liquid and solid phases. The liquid phase was carefully decanted. The Tween 40 solid phase was washed two or three times with l-2 ml of a saturated solution of the same salt, and the washings were combined with the separated liquid phase. The washed Tween 40 phase was dissolved with distilled water. The photometric
885
886
LI BUM and
MENG RIGAN
reagent or complex in the dissolved solid or liquid phase was determined spectrophotometritally. The degree of extraction (E%) was calculated as the ratio of the amount of species extracted by the solid phase (Tween 40 phase) to the amount initially added to the system. The conditions used for determination of the metal ions with arsenazo III as photometric reagent are given in Table 1. RESULTS AND DISCUSSION
Phase separation solution
conditions for
0
Tween
40
Figure 1 shows the phase diagrams for the system based on Tween 40 and various salts, namely sodium citrate, ammonium sulphate and sodium dihydrogen phosphate. With constant Tween 40 concentration, the salt concentration necessary for phase separation depends on the identity of the salt. Because most metal sulphates are water-soluble, (NH&SO, is preferred for use in the phase separation. The necessary salt concentration increases as the Tween 40 concentration decreases. The salt concentration necessary also depends on the pH. In the pH range 0.0-2.0 it decreases with increase in pH and remains unchanged when the pH value is further increased. The reason is probably that at low pH the sulphate ion is protonated to HSOi and the salting-out ability is reduced. Distribution of organic reagents
The distribution of some water-soluble reagents between the Tween 40 and aqueous phases was examined at different pH values from 0.0 to 6.5. The organic reagents chosen, containing different chelating groups, were arsenazo III, Bromopyrogallol Red, Chromazurol S and Xylenol Orange. The E% values of these reagents were found to be independent of the pH. E% for the first three reagents was from 95 to lOO%, and for the last about 70%. It is possible to use them as extractants and extraction-photometric reagents in this system. Extraction of metals in the presence of arsenazo III E&et
v
4
5
c mu t M
Fig. I. Phase diagrams of the Tween 40-aqueous salt system: a, Na,C,H,O, (pH 6.6-7.2); b, (NH&SO, (pH 3.8-3.9); c, NaH,PO, (pH 2.9-3.1).
Pb(I1) in the Tween 40 phase on the pH value in the presence of arsenazo III. The stability of the complexes of Zr(IV), U(V1) and Fe(II1) can be increased by raising the pH from 0.0 to 3.0, so the E% is correspondingly increased. The E% for these metals is maximum at pH 3.0, and then decreases with further increase in pH, probably owing to hydrolysis and other side-reactions of the metals at high pH. In contrast to Zr(IV), U(VI) and Fe(III), for Pb(I1) E% dereases as the pH increases from 0.0 to 3.0, and then increases at higher pH. Effect of arsenazo III concentration. Figure 3 shows that at fixed pH the degree of formation of the complexes of the metals can be increased by increasing the arsenazo III concentration up to I .29 x 10v4M; the E% values for Fe(II1) and Pb(I1) continue to increase, but those of Zr(IV) and U(V1) decrease slightly with further increase in arsenazo III concentration. Without arsenazo III present, the E% values for the four metals are all zero. This means that
s 60 6 40
of PH.
dence of E%
Figure 2 presents the depenfor Zr(IV), U(VI), Fe(II1) and
Table 1. Spectrophotometric Zr(IV) Wavelength, nm 624 Acidity O.lM HCI
determination of metals Uol’I) 648 pH 2.0
Fe(II1)
Pb(II)
610 pH 3.5
646 pH 5.0
Fig. 2. Relationship between E% of metals and pH: CT_, 40 = 12%; Cuew, ,,1= 1.29 x lo-‘M; CcNHlbSOI = 2.27M; amount of metals added: Zr, 40 fig; Pb, 50 pg; U, 40 pg; Fe, 20 pg.
Liquid-solid extraction system
887
Table 2. Recoveries in extractive separation of U(W) from Pb(II)
$
WI) added, fig
60
ru40
0
0.5
1.0
1.5
2.0
2.5
3.0
3.5
Fig. 3. Effect of arsenazo III concentration on E% for the metals: extraction acidity, Zr, pH 1.0; Pb, pH 5.0; U and Fe, pH 3.0; other conditions as for Fig. 2.
the uncomplexed metal ions cannot be extracted into the Tween 40 phase. Eflect of (NH&SO4 concentration. When the (NH&SO, concentration is increased, the E% of Pb(I1) is almost constant, but the values for Zr(IV), U(W) and Fe(II1) increase to a maximum and then decrease, as shown in Fig. 4. As the salt concentration increases, the Tween 40 is salted out and E % is therefore increased. When the Tween 40 is completely salted out the salteffect of the excess of ammonium sulphate decreases the stability of the complexes and hence decreases E %. Recovery tests for quantitative separation
Figure 2 shows that at pH 3.0 the E% values for Zr(IV) and U(V1) are in the range 96100%. At the same pH the E% for Pb(I1) is only about 1l%, so after the first extraction with arsenazo
70
G 60
Fe ^
40
Pb
t
I
I
1.5
,
2.0
I
I
2.5 30
Csott,
I
3.5
1
4.0
M
Fig. 4. Relationship between E% and CNHlhso,: extraction conditions as for Fig. 3. 37,%-C
40.0
100.0 100.0 500 1000 5000
Apparent WI) recovery, %
No. of extraction operations
39.0 101.0 60.8 4.6 5.3 5.4
97.5 101.0 101.3 92 106 108
2 2 2 2 2 3
III and the Tween 4O-(NH&SO,,-H20 system, Zr(IV) or U(W) can be quantitatively transferred into the Tween 40 phase, and a large fraction of the Pb(I1) remains unextracted. To separate the residual Pb(II), the salted-out Tween 40 phase is dissolved in 10 ml of water, and then salted out with 3.0 g of ammonium sulphate; two or three such operations should result in satisfactory separation of Zr(IV) or U(V1) from Pb(II), as shown in Tables 2 and 3. Extraction mechanism for Tween 40phase Composition of complex of U(VI) with arsenazo ZIZ extracted into Tween 40 phase. The
continuous-variations and molar-ratio methods were used to determine the composition of the complex, which was found to be 1: 1 U(V1): arsenazo III (Figs. 5 and 6). Effect of cationic and anionic surfactants on
E% for U(vI). To study further the extraction mechanism for the Tween 40 phase, various concentrations of cationic or anionic surfactant were added to the solutions of the extraction system containing the complex of U(V1) with arsenazo III, and the E% of U(W) was determined. Figure 7 shows that at pH 3.0 E% is little affected by the anionic surfactant added (dodecylbenzene sodium sulphonate, DBS) (a in Fig. 7), but is decreased by increasing concentration of cationic surfactant (cetylpyridinium chloride, CPC) (b in Fig. 7). At lower pH (pH 1.O), the situation is quite different. The E% of U(V1) is decreased by increasing DBS Table 3. Recoveries for extractive separation of Zr(IV) from Pb(I1)
50
TAL
40.0
100.0 60.0 5.0 5.0 5.0
WI) found, Mr
4.0
Corr*nozo m t 10-4M
c
Pb(I1) added, Mr
Zr(IV) added, lrg 40.0 100.0 40.0 5.0 5.0 5.0
Pb(I1) added, PkY 40.0 100.0
200.0 500 1000 2500
Zr(IV) found, fig 39.0 94.0 40.0 4.7 5.0 5.0
Zr(IV) recovery, %
No. of extraction operations
97.5 94.0 100 94 100 100
2 2 2 2 2 2
LI BUHAIand MENGRIGAN
888
b’
Fig. 5. Job plot. Extraction acidity, pH 3.0.
0’
L 0
I
I
I
I
1
2
3
4
Cs”rtactont . 10 -2nf
Fig. 7. Effect of cationic and anionic surfactants on E% of UfVI): a, b, pH 3.0; a’, b’, pH 1.O.
P 0
I
0.5
I
I
I
I
1.0
1.5
2.0
2.5
I 3.0
CF?l/CMl Fig. 6. Molar-ratio plot. Acidity for Fig. 5.
concentration (a’ in Fig. 7), but is not affected by CPC (b’ in Fig. 7). As discussed above, U(V1) and arsenazo III form a 1: 1 complex, which is extracted into the Tween 40 phase. Arsenazo III is a weak polyacid, H, L, and there is considerable uncertainty about the values of the dissociation constants. At pH 1.0, however, the dominant species is probably H,L-, which would give a positive charge to the 1: 1 complex with UO:+, for which dodecylbenzene sulphonate might act as counter-ion to give a neutral species. At pH 3.0, however, the dominant species is probably’ H,L3-, which would give a negatively charged uranyl complex, which could give a neutral species with a cetylpyridinium counter-ion.
Under both conditions, E% for U(W) is decreased, indicating that only the charged complex can be extracted into the Tween 40 solid phase, and not the uncharged species. This mechanism is at present only speculative and requires experimental examination. It is in contrast to that of liquid-liquid extraction systems, in which only uncharged and hydrophobic substances can be extracted from the aqueous phase.
REFERENCES T. I. Zvarova, V. M. Shkinev, B. Ya. Spivakov and Yu. A. Zolotov, Dokl. Akad. Nauk SSSR, 1983, 273, 107. I&m, Mikrochim. Acta, 1984 III, 449. Li Buhai, Sun Xiaomei and Yang Shengcai, Gaodeng Xuexiao Huaxue Xuebao, 1989, 3, 293. Li Buhai, Sun Xiaomei and Wang Shoucheng, Fenxi Shiyanshi, 1989, 6, 10. 5. Li Buhai and Wang Kefu, Gaodeng Xuexiao Huaxue Xuebao, 1990, 4, 1207.
6. B. Budiginsky, Talanta, 1969, 16, 1277. 7. P. K. Spitsyn and V. S. Sharev, Zh. Analit. Khim., 1970, 25, 1503.
8. S. Kotrly and L. &ha,
Handbook of Chemical Equilibria in Analytical Chemistry, p. 205. Hotwood,
Chichester, 1985.
?‘&nta, VoI. 37, No. 9, pp. 885-888, 1990
PergamonPresspk
Printed in Great Brimin
EXTRACTION SYSTEM BASED ON TWEEN 4&SALT-H,O WITHOUT ORGANIC SOLVENTS IA BUHAI
and &&~aRIGAN
Department of Chemistry, South-central College for Nationalities, Wuhan, Hubei, 430074, People’s Republic of China (Received 15 September 1989. Revised I4 February 1990. Accepted 21 February 1990)
&nmary--An aqueous solution of Tween 40 forms liquid and solid phases in the presence of various salts, depending on the Tween 40 concentration, the identity and con~tration of the salt, and the solution acidity. The distribution of Zr(IV), U(VI), Fe(III), Pb(I1) and some organic photometric reagents between the Tween 40 phase and aqueous phase containing salt was examined. The quantitative extraction, separation and determination of Zr(IV) or U(M) in the presence of Pb(II) was achieved by controlling the solution acidity in the system. The extraction mechanism has been tentatively studied.
extraction with common organic solvents, only neutral species are normally transferred from the hydrophilic to the hydrophobic phase. Therefore the method cannot be applied to the extraction of strongly hydrophilic species, especially those with multiple charges. Also, the solvents are often volatile, flammable (or even explosive) and toxic. It is known that aqueous solutions of poly(ethylene glycol) (PEG) can separate into two aqueous phases in the presence of certain inorganic salts.‘**This two-phase aqueous system has been applied to the extractive separation of metal ions.3T4 Recently, we found that aqueous solutions of poly(~nylpyrro~done~ (PVP), Tween 80 @oIyoxyethyleue sorb&an mono-oleate) and other water-soluble polymers form liquid and solid phases on addition of various salts. Under suitable conditions some water-soluble photometric reagents and their complexes with metal ions can be transferred into the solid phases formed by the polymers. Some metal ions can be separated with this system. We have called the system a liquid-solid extraction system.$ It has an advantage over the two-phase aqueous system, because it is more convenient (not requiring a separatory funnel) and is more rapid. In this paper the liquid-solid extraction system based on Tween 40--salt-H,U is reported. 1n liquid-liquid
EXPmIWTAL
Appamm
A Shanghai model 721 spectrophotometer was used for photometric meas~ements. A
Shanghai model pHs-2 pH-meter and a shaking machine were used. Reagents
An aqueous solution of Tween 40 (polyoxyethylene sorbitan palmitate) was prepared by dissolving 50 ml of Tween 40 in 200 ml of distilled water. Standard 1.O-mg/ml solutions of U(VI), Pb@) and Fe(EII) in 0.151cInitric acid, and Zr(IV) in 10% v/v hydrochloric acid were prepared frum UO,(No,), - 6H@, pure lead and iron, and ZrOCl, - 8H,O respectively. More dilute solutions were prepared by appropriate dilution as required. All other reagents were of analytical grade. Procedure
Given volumes of Tween 40 aqueous solution, photometric reagent solution and metal ion solution were put into a 25-ml tube fitted with a stopper. The acidity of strongfy acidic solutions was adjusted with hydrochloric acid, and in the pH range 1.5-6.5 by use of monochloroacetic acid or acetic acid buffers. The solution was then diluted to 10.0 ml with distilled water, a given weight of solid salt was added and the mixture was shaken for 3 min. After standing for several minutes, the solution separated into liquid and solid phases. The liquid phase was carefully decanted. The Tween 40 solid phase was washed two or three times with l-2 ml of a saturated solution of the same salt, and the washings were combined with the separated liquid phase. The washed Tween 40 phase was dissolved with distilled water. The photometric
885
886
LI BUM and
MENG RIGAN
reagent or complex in the dissolved solid or liquid phase was determined spectrophotometritally. The degree of extraction (E%) was calculated as the ratio of the amount of species extracted by the solid phase (Tween 40 phase) to the amount initially added to the system. The conditions used for determination of the metal ions with arsenazo III as photometric reagent are given in Table 1. RESULTS AND DISCUSSION
Phase separation solution
conditions for
0
Tween
40
Figure 1 shows the phase diagrams for the system based on Tween 40 and various salts, namely sodium citrate, ammonium sulphate and sodium dihydrogen phosphate. With constant Tween 40 concentration, the salt concentration necessary for phase separation depends on the identity of the salt. Because most metal sulphates are water-soluble, (NH&SO, is preferred for use in the phase separation. The necessary salt concentration increases as the Tween 40 concentration decreases. The salt concentration necessary also depends on the pH. In the pH range 0.0-2.0 it decreases with increase in pH and remains unchanged when the pH value is further increased. The reason is probably that at low pH the sulphate ion is protonated to HSOi and the salting-out ability is reduced. Distribution of organic reagents
The distribution of some water-soluble reagents between the Tween 40 and aqueous phases was examined at different pH values from 0.0 to 6.5. The organic reagents chosen, containing different chelating groups, were arsenazo III, Bromopyrogallol Red, Chromazurol S and Xylenol Orange. The E% values of these reagents were found to be independent of the pH. E% for the first three reagents was from 95 to lOO%, and for the last about 70%. It is possible to use them as extractants and extraction-photometric reagents in this system. Extraction of metals in the presence of arsenazo III E&et
v
4
5
c mu t M
Fig. I. Phase diagrams of the Tween 40-aqueous salt system: a, Na,C,H,O, (pH 6.6-7.2); b, (NH&SO, (pH 3.8-3.9); c, NaH,PO, (pH 2.9-3.1).
Pb(I1) in the Tween 40 phase on the pH value in the presence of arsenazo III. The stability of the complexes of Zr(IV), U(V1) and Fe(II1) can be increased by raising the pH from 0.0 to 3.0, so the E% is correspondingly increased. The E% for these metals is maximum at pH 3.0, and then decreases with further increase in pH, probably owing to hydrolysis and other side-reactions of the metals at high pH. In contrast to Zr(IV), U(VI) and Fe(III), for Pb(I1) E% dereases as the pH increases from 0.0 to 3.0, and then increases at higher pH. Effect of arsenazo III concentration. Figure 3 shows that at fixed pH the degree of formation of the complexes of the metals can be increased by increasing the arsenazo III concentration up to I .29 x 10v4M; the E% values for Fe(II1) and Pb(I1) continue to increase, but those of Zr(IV) and U(V1) decrease slightly with further increase in arsenazo III concentration. Without arsenazo III present, the E% values for the four metals are all zero. This means that
s 60 6 40
of PH.
dence of E%
Figure 2 presents the depenfor Zr(IV), U(VI), Fe(II1) and
Table 1. Spectrophotometric Zr(IV) Wavelength, nm 624 Acidity O.lM HCI
determination of metals Uol’I) 648 pH 2.0
Fe(II1)
Pb(II)
610 pH 3.5
646 pH 5.0
Fig. 2. Relationship between E% of metals and pH: CT_, 40 = 12%; Cuew, ,,1= 1.29 x lo-‘M; CcNHlbSOI = 2.27M; amount of metals added: Zr, 40 fig; Pb, 50 pg; U, 40 pg; Fe, 20 pg.
Liquid-solid extraction system
887
Table 2. Recoveries in extractive separation of U(W) from Pb(II)
$
WI) added, fig
60
ru40
0
0.5
1.0
1.5
2.0
2.5
3.0
3.5
Fig. 3. Effect of arsenazo III concentration on E% for the metals: extraction acidity, Zr, pH 1.0; Pb, pH 5.0; U and Fe, pH 3.0; other conditions as for Fig. 2.
the uncomplexed metal ions cannot be extracted into the Tween 40 phase. Eflect of (NH&SO4 concentration. When the (NH&SO, concentration is increased, the E% of Pb(I1) is almost constant, but the values for Zr(IV), U(W) and Fe(II1) increase to a maximum and then decrease, as shown in Fig. 4. As the salt concentration increases, the Tween 40 is salted out and E % is therefore increased. When the Tween 40 is completely salted out the salteffect of the excess of ammonium sulphate decreases the stability of the complexes and hence decreases E %. Recovery tests for quantitative separation
Figure 2 shows that at pH 3.0 the E% values for Zr(IV) and U(V1) are in the range 96100%. At the same pH the E% for Pb(I1) is only about 1l%, so after the first extraction with arsenazo
70
G 60
Fe ^
40
Pb
t
I
I
1.5
,
2.0
I
I
2.5 30
Csott,
I
3.5
1
4.0
M
Fig. 4. Relationship between E% and CNHlhso,: extraction conditions as for Fig. 3. 37,%-C
40.0
100.0 100.0 500 1000 5000
Apparent WI) recovery, %
No. of extraction operations
39.0 101.0 60.8 4.6 5.3 5.4
97.5 101.0 101.3 92 106 108
2 2 2 2 2 3
III and the Tween 4O-(NH&SO,,-H20 system, Zr(IV) or U(W) can be quantitatively transferred into the Tween 40 phase, and a large fraction of the Pb(I1) remains unextracted. To separate the residual Pb(II), the salted-out Tween 40 phase is dissolved in 10 ml of water, and then salted out with 3.0 g of ammonium sulphate; two or three such operations should result in satisfactory separation of Zr(IV) or U(V1) from Pb(II), as shown in Tables 2 and 3. Extraction mechanism for Tween 40phase Composition of complex of U(VI) with arsenazo ZIZ extracted into Tween 40 phase. The
continuous-variations and molar-ratio methods were used to determine the composition of the complex, which was found to be 1: 1 U(V1): arsenazo III (Figs. 5 and 6). Effect of cationic and anionic surfactants on
E% for U(vI). To study further the extraction mechanism for the Tween 40 phase, various concentrations of cationic or anionic surfactant were added to the solutions of the extraction system containing the complex of U(V1) with arsenazo III, and the E% of U(W) was determined. Figure 7 shows that at pH 3.0 E% is little affected by the anionic surfactant added (dodecylbenzene sodium sulphonate, DBS) (a in Fig. 7), but is decreased by increasing concentration of cationic surfactant (cetylpyridinium chloride, CPC) (b in Fig. 7). At lower pH (pH 1.O), the situation is quite different. The E% of U(V1) is decreased by increasing DBS Table 3. Recoveries for extractive separation of Zr(IV) from Pb(I1)
50
TAL
40.0
100.0 60.0 5.0 5.0 5.0
WI) found, Mr
4.0
Corr*nozo m t 10-4M
c
Pb(I1) added, Mr
Zr(IV) added, lrg 40.0 100.0 40.0 5.0 5.0 5.0
Pb(I1) added, PkY 40.0 100.0
200.0 500 1000 2500
Zr(IV) found, fig 39.0 94.0 40.0 4.7 5.0 5.0
Zr(IV) recovery, %
No. of extraction operations
97.5 94.0 100 94 100 100
2 2 2 2 2 2
LI BUHAIand MENGRIGAN
888
b’
Fig. 5. Job plot. Extraction acidity, pH 3.0.
0’
L 0
I
I
I
I
1
2
3
4
Cs”rtactont . 10 -2nf
Fig. 7. Effect of cationic and anionic surfactants on E% of UfVI): a, b, pH 3.0; a’, b’, pH 1.O.
P 0
I
0.5
I
I
I
I
1.0
1.5
2.0
2.5
I 3.0
CF?l/CMl Fig. 6. Molar-ratio plot. Acidity for Fig. 5.
concentration (a’ in Fig. 7), but is not affected by CPC (b’ in Fig. 7). As discussed above, U(V1) and arsenazo III form a 1: 1 complex, which is extracted into the Tween 40 phase. Arsenazo III is a weak polyacid, H, L, and there is considerable uncertainty about the values of the dissociation constants. At pH 1.0, however, the dominant species is probably H,L-, which would give a positive charge to the 1: 1 complex with UO:+, for which dodecylbenzene sulphonate might act as counter-ion to give a neutral species. At pH 3.0, however, the dominant species is probably’ H,L3-, which would give a negatively charged uranyl complex, which could give a neutral species with a cetylpyridinium counter-ion.
Under both conditions, E% for U(W) is decreased, indicating that only the charged complex can be extracted into the Tween 40 solid phase, and not the uncharged species. This mechanism is at present only speculative and requires experimental examination. It is in contrast to that of liquid-liquid extraction systems, in which only uncharged and hydrophobic substances can be extracted from the aqueous phase.
REFERENCES T. I. Zvarova, V. M. Shkinev, B. Ya. Spivakov and Yu. A. Zolotov, Dokl. Akad. Nauk SSSR, 1983, 273, 107. I&m, Mikrochim. Acta, 1984 III, 449. Li Buhai, Sun Xiaomei and Yang Shengcai, Gaodeng Xuexiao Huaxue Xuebao, 1989, 3, 293. Li Buhai, Sun Xiaomei and Wang Shoucheng, Fenxi Shiyanshi, 1989, 6, 10. 5. Li Buhai and Wang Kefu, Gaodeng Xuexiao Huaxue Xuebao, 1990, 4, 1207.
6. B. Budiginsky, Talanta, 1969, 16, 1277. 7. P. K. Spitsyn and V. S. Sharev, Zh. Analit. Khim., 1970, 25, 1503.
8. S. Kotrly and L. &ha,
Handbook of Chemical Equilibria in Analytical Chemistry, p. 205. Hotwood,
Chichester, 1985.