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Uphill, Rapid, and Selective Transport of Picrate through Dichloromethane Membrane
MICROCHEMICAL JOURNAL ARTICLE NO.
58, 192–196 (1998)
MJ971548
Uphill, Rapid, and Selective Transport of Picrate through Dichloromethane Membrane A. Safavi1 and L. Fotouhi Department of Chemistry, College of Sciences, Shiraz University, Shiraz, 71454, Iran Received August 8, 1997; accepted October 31, 1997 The transport of picrate anion against its concentration gradient by coupling it with movement of thiocyanate ion in the opposite direction is reported. The membrane containing toluidine blue separated two aqueous phases, one containing picrate and borate buffer (pH 9) and the other containing picrate, potassium thiocyanate, and borate buffer (pH 9). The picrate anion was accumulated against the concentration gradient in the aqueous phase containing thiocyanate. The rate of picrate transport increased with increasing toluidine blue concentration in the membrane; in the absence of toluidine blue, transport of picrate did not occur. The effect of thiocyanate counterions such as potassium, sodium, and ammonium was examined. No change in the rate of transport was observed by changing the counterion. The effect of other anions on the rate of picrate transport was examined. The energy of this movement comes from the simultaneous countertransport of thiocyanate. q 1998 Academic Press
Research on the phenomena of transport through membranes has been extended in recent years (1–11), especially in the biological field (12–14). The studies in biology are directed particularly toward the transfer of cations. On the other hand, less attention has been paid to the transport of anions across liquid membranes (15, 16). The transport of picrate anion against its concentration gradient has been achieved both by applying a redox reaction-driven transference system (15, 17) and by using a membrane containing a potassium ionophore (18). However, the transport process is time consuming and is not quantitative. In this paper, we report that a novel type of carrier, toluidine blue, can transport anionic substrates through a liquid membrane effectively and specifically. Uphill transport of picrate anion through the bulk dichloromethane membrane is possible. In this system, the transport of picrate is coupled with countertransport of thiocyanate anion. EXPERIMENTAL
Apparatus The apparatus used for measuring the transport of picrate consisted of a cylindrical glass cell (4.0-cm i.d.) holding a glass (2.0-cm i.d.) that separated two aqueous phases. The membrane phase in which carrier was dissolved lay below two aqueous phases, and bridged them. This membrane phase was stirred constantly with a magnetic stirrer. 1
To whom correspondence should be addressed. 192
0026-265X/98 $25.00 Copyright q 1998 by Academic Press All rights of reproduction in any form reserved.
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FIG. 1. Effect of carrier concentration on the flux of picrate through membrane.
Materials All chemicals were reagent grade and subsequently not purified. Procedure The source phase (SP) consisted of 10 ml of an aqueous solution containing 5 1 1005 M picrate anion, the pH of which was adjusted to 9 with borate buffer. The receiving phase (RP) consisted of 10 ml of an aqueous solution containing 5 1 1005 M picrate anion and 3.5 M potassium thiocyanate, the pH of which was adjusted to 9 with borate buffer. The membrane phase (35 ml) consisted of 6 1 1005 M toluidine blue as the carrier. An appropriate amount of toluidine blue was dissolved in a minimum amount of distilled water, followed by addition of 35 ml of borate buffer, pH 9. The solution of toluidine blue prepared as above was extracted into 35 ml of dichloromethane. The organic phase was then separated and used as the liquid membrane in the cell. The concentration of picrate in both aqueous phases was measured at regular intervals. With a Perkin–Elmer 35 spectrophotometer by measuring the absorbance of picrate at 420 nm. RESULTS AND DISCUSSION
The effect of toluidine blue concentration on the rate of picrate transport was examined while the pH values of the aqueous phases were maintained at 9. Figure 1 shows that the rate of picrate transport increases with increasing toluidine blue concentration up to about 6 1 1005 M. The effect of pH on the rate of transport was examined at pH 7–11. At pH lower than 7, a large blank is observed, and also, picrate is found partly as picric acid. At pH larger than 9, picrate is not completely extracted into the organic phase. Increasing
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SAFAVI AND FOTOUHI
FIG. 2. Effect of pH on the flux of picrate through membrane.
the pH from 7 to 9 causes an increase in the flux of picrate transport. However, a sharp decrease in the rate of transport is observed at pH greater than 9 (Fig. 2). When the concentration of thiocyanate in the RP was increased to 3.5 M, the rate of transport of picrate increased, while at higher concentrations of thiocyanate, no further increase in the transport rate was observed (Fig. 3). When the ionic strength of the SP was adjusted to the same value as for the RP with NaCl, the rate of picrate transport was not affected at all. It was also observed that different counterions of thiocyanate, such as potassium, sodium, and ammonium, have no influence on transport rate. This means that the transport of picrate is not affected by thiocyanate counterion.
FIG. 3. Effect of KSCN concentration on the rate of picrate transport.
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TRANSPORT OF PICRATE THROUGH MEMBRANE
195
FIG. 4. Time–transport curve of uphill transport of picrate. Concentration of picrate (j) in the receiving phase and (l) in the source phase.
The presence of anions such as chlorate, sulfate, sulfite, nitrite, fluoride, bromide, chloride, oxalate, citrate, and phosphate in the SP (0.01 M of each anion), in the presence of 1004 M picrate, had no effect on the transport rate, and only perchlorate decreased the transport by 10%. In the above study, transport of picrate through a dichloromethane membrane containing no toluidine blue was not achieved. Picrate was transferred only in the presence of thiocyanate in the aqueous phase and toluidine blue in the membrane. The rate of picrate transport increased with increasing toluidine blue concentration, suggesting that picrate anion in SP was dissolved in the dichloromethane phase, forming an ion pair with toluidine blue cation and then liberated into the RP, by the exchange of picrate with thiocyanate. Active transport of picrate was observed when the concentrations of picrate were equal (5 1 1005 M) in both aqueous phases. Figure 4 shows that the amount of picrate increases with time in the RP, while at the same time it is decreased in the SP. Table 1 shows that the transport is quantitative. To investigate the mechanism of this transport system, the change in thiocyanate ion concentration in the source phase was followed by means of a colorimetric method (formation of the colored complex of iron thiocyanate). Also, the presence of picrate in the receiving phase was followed spectrophotometrically by measuring the absorbance of picrate anion. It was found that approximately the same number of moles of SCN0 as the picrate in receiving phase was transferred into the source phase. In conclusion, by the above method, rapid, uphill transport of picrate can be easily
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SAFAVI AND FOTOUHI TABLE 1 Results of Uphill Transport of Picrate Time (h)
Concentration of picrate in SP (M)
0 1 2 2.5
5 1 1005 3 1 1005 0.5 1 1005 0
Concentration of picrate in RP (M) 5 7.3 9 1
1 1 1 1
1005 1005 1005 1004
achieved using toluidine blue dissolved in dichloromethane as the membrane. The transport is free from the interference of most anions. ACKNOWLEDGMENT The authors acknowledge the support of this work by the Shiraz University Research Council.
REFERENCES 1. Izatt, R. I.; Hawkins, R. T.; Christensen, J. J.; Izatt, R. M. J. Am. Chem. Soc., 1985, 107, 63. 2. Vogtle, F.; Weber, E. (Eds.), Host–Guest Complex Chemistry, Vols. I–III. Springer-Verlag, Berlin, 1981 (I), 1982 (II), 1984 (III). 3. Lamb, J. D.; Izatt, R. M.; Robertson, P. A.; Christensen, J. J. J. Am. Chem. Soc., 1980, 102, 2454. 4. Mcbride, D. W.; Izatt, R. M.; Lamb, J. D.; Christensen, J. J. In Inclusion Compounds (Atwood, J. L.; Davies, J. E. and Macnicol, D. D., Eds.), Vol. 3, Ch. 16, p. 571. Academic Press, Orlando, 1984. 5. Izatt, R. M.; Clark, G. A.; Bradshaw, J. S.; Lamb, J. D.; Christensen, J. J. Sep. Purif. Methods, 1986, 15, 21. 6. Ochiai, E. J. Chem. Educ., 1988, 65, 943. 7. Hiratani, K.; Taguchi, K. Chem. Lett., 1990, 725. 8. Izatt, R. M.; Bruening, R. L.; Bradsaw, J. S.; Lamb, J. D.; Christensen, J. J. Pure Appl. Chem., 1988, 60, 453. 9. Wiank, M. M.; Slowigk, T. B.; Sudholter, E. J. R.; Reinhoudt, D. N. J. Am. Chem. Soc., 1990, 112, 797. 10. Cox, J. A.; Bhatnagar, A. Talanta, 1990, 37, 1037. 11. Hiratani, K.; Taguchi, K. Chem. Lett., 1990, 725. 12. Hiraoka, M. Crown Compounds, Their Characteristics and Applications, Studies in Organic Chemistry, Vol. 12, pp. 203, 206. Kodansha, Tokyo, 1982. 13. Yamaguchi, K.; Kuboniwa, H.; Murakami, N.; Hivuo, A.; Nakahama, S.; Yamazaki, N. Bull. Chem. Soc. Japan, 1989, 62, 1097. 14. Maugh, T. H. Science, 1976, 193, 134. 15. Shinbo, T.; Kurihara, K.; Kobatuke, Y.; Kamo, N. Nature, 1977, 270, 277. 16. Tabnshi, I.; Kobuke, Y.; Imata, J. J. Am. Chem. Soc., 1980, 102, 1744. 17. Shinbo, T.; Sugiura, M.; Kama, N.; Kobatake, P. J. Memb. Sci., 1981, 9, 1. 18. Sugiura, M.; Shinbo, T. Bull. Chem. Soc. Japan, 1979, 52, 684.
ah11$$1548
02-24-98 14:16:18
mica
AP: MCH
58, 192–196 (1998)
MJ971548
Uphill, Rapid, and Selective Transport of Picrate through Dichloromethane Membrane A. Safavi1 and L. Fotouhi Department of Chemistry, College of Sciences, Shiraz University, Shiraz, 71454, Iran Received August 8, 1997; accepted October 31, 1997 The transport of picrate anion against its concentration gradient by coupling it with movement of thiocyanate ion in the opposite direction is reported. The membrane containing toluidine blue separated two aqueous phases, one containing picrate and borate buffer (pH 9) and the other containing picrate, potassium thiocyanate, and borate buffer (pH 9). The picrate anion was accumulated against the concentration gradient in the aqueous phase containing thiocyanate. The rate of picrate transport increased with increasing toluidine blue concentration in the membrane; in the absence of toluidine blue, transport of picrate did not occur. The effect of thiocyanate counterions such as potassium, sodium, and ammonium was examined. No change in the rate of transport was observed by changing the counterion. The effect of other anions on the rate of picrate transport was examined. The energy of this movement comes from the simultaneous countertransport of thiocyanate. q 1998 Academic Press
Research on the phenomena of transport through membranes has been extended in recent years (1–11), especially in the biological field (12–14). The studies in biology are directed particularly toward the transfer of cations. On the other hand, less attention has been paid to the transport of anions across liquid membranes (15, 16). The transport of picrate anion against its concentration gradient has been achieved both by applying a redox reaction-driven transference system (15, 17) and by using a membrane containing a potassium ionophore (18). However, the transport process is time consuming and is not quantitative. In this paper, we report that a novel type of carrier, toluidine blue, can transport anionic substrates through a liquid membrane effectively and specifically. Uphill transport of picrate anion through the bulk dichloromethane membrane is possible. In this system, the transport of picrate is coupled with countertransport of thiocyanate anion. EXPERIMENTAL
Apparatus The apparatus used for measuring the transport of picrate consisted of a cylindrical glass cell (4.0-cm i.d.) holding a glass (2.0-cm i.d.) that separated two aqueous phases. The membrane phase in which carrier was dissolved lay below two aqueous phases, and bridged them. This membrane phase was stirred constantly with a magnetic stirrer. 1
To whom correspondence should be addressed. 192
0026-265X/98 $25.00 Copyright q 1998 by Academic Press All rights of reproduction in any form reserved.
ah11$$1548
02-24-98 14:16:18
mica
AP: MCH
TRANSPORT OF PICRATE THROUGH MEMBRANE
193
FIG. 1. Effect of carrier concentration on the flux of picrate through membrane.
Materials All chemicals were reagent grade and subsequently not purified. Procedure The source phase (SP) consisted of 10 ml of an aqueous solution containing 5 1 1005 M picrate anion, the pH of which was adjusted to 9 with borate buffer. The receiving phase (RP) consisted of 10 ml of an aqueous solution containing 5 1 1005 M picrate anion and 3.5 M potassium thiocyanate, the pH of which was adjusted to 9 with borate buffer. The membrane phase (35 ml) consisted of 6 1 1005 M toluidine blue as the carrier. An appropriate amount of toluidine blue was dissolved in a minimum amount of distilled water, followed by addition of 35 ml of borate buffer, pH 9. The solution of toluidine blue prepared as above was extracted into 35 ml of dichloromethane. The organic phase was then separated and used as the liquid membrane in the cell. The concentration of picrate in both aqueous phases was measured at regular intervals. With a Perkin–Elmer 35 spectrophotometer by measuring the absorbance of picrate at 420 nm. RESULTS AND DISCUSSION
The effect of toluidine blue concentration on the rate of picrate transport was examined while the pH values of the aqueous phases were maintained at 9. Figure 1 shows that the rate of picrate transport increases with increasing toluidine blue concentration up to about 6 1 1005 M. The effect of pH on the rate of transport was examined at pH 7–11. At pH lower than 7, a large blank is observed, and also, picrate is found partly as picric acid. At pH larger than 9, picrate is not completely extracted into the organic phase. Increasing
ah11$$1548
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SAFAVI AND FOTOUHI
FIG. 2. Effect of pH on the flux of picrate through membrane.
the pH from 7 to 9 causes an increase in the flux of picrate transport. However, a sharp decrease in the rate of transport is observed at pH greater than 9 (Fig. 2). When the concentration of thiocyanate in the RP was increased to 3.5 M, the rate of transport of picrate increased, while at higher concentrations of thiocyanate, no further increase in the transport rate was observed (Fig. 3). When the ionic strength of the SP was adjusted to the same value as for the RP with NaCl, the rate of picrate transport was not affected at all. It was also observed that different counterions of thiocyanate, such as potassium, sodium, and ammonium, have no influence on transport rate. This means that the transport of picrate is not affected by thiocyanate counterion.
FIG. 3. Effect of KSCN concentration on the rate of picrate transport.
ah11$$1548
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mica
AP: MCH
TRANSPORT OF PICRATE THROUGH MEMBRANE
195
FIG. 4. Time–transport curve of uphill transport of picrate. Concentration of picrate (j) in the receiving phase and (l) in the source phase.
The presence of anions such as chlorate, sulfate, sulfite, nitrite, fluoride, bromide, chloride, oxalate, citrate, and phosphate in the SP (0.01 M of each anion), in the presence of 1004 M picrate, had no effect on the transport rate, and only perchlorate decreased the transport by 10%. In the above study, transport of picrate through a dichloromethane membrane containing no toluidine blue was not achieved. Picrate was transferred only in the presence of thiocyanate in the aqueous phase and toluidine blue in the membrane. The rate of picrate transport increased with increasing toluidine blue concentration, suggesting that picrate anion in SP was dissolved in the dichloromethane phase, forming an ion pair with toluidine blue cation and then liberated into the RP, by the exchange of picrate with thiocyanate. Active transport of picrate was observed when the concentrations of picrate were equal (5 1 1005 M) in both aqueous phases. Figure 4 shows that the amount of picrate increases with time in the RP, while at the same time it is decreased in the SP. Table 1 shows that the transport is quantitative. To investigate the mechanism of this transport system, the change in thiocyanate ion concentration in the source phase was followed by means of a colorimetric method (formation of the colored complex of iron thiocyanate). Also, the presence of picrate in the receiving phase was followed spectrophotometrically by measuring the absorbance of picrate anion. It was found that approximately the same number of moles of SCN0 as the picrate in receiving phase was transferred into the source phase. In conclusion, by the above method, rapid, uphill transport of picrate can be easily
ah11$$1548
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mica
AP: MCH
196
SAFAVI AND FOTOUHI TABLE 1 Results of Uphill Transport of Picrate Time (h)
Concentration of picrate in SP (M)
0 1 2 2.5
5 1 1005 3 1 1005 0.5 1 1005 0
Concentration of picrate in RP (M) 5 7.3 9 1
1 1 1 1
1005 1005 1005 1004
achieved using toluidine blue dissolved in dichloromethane as the membrane. The transport is free from the interference of most anions. ACKNOWLEDGMENT The authors acknowledge the support of this work by the Shiraz University Research Council.
REFERENCES 1. Izatt, R. I.; Hawkins, R. T.; Christensen, J. J.; Izatt, R. M. J. Am. Chem. Soc., 1985, 107, 63. 2. Vogtle, F.; Weber, E. (Eds.), Host–Guest Complex Chemistry, Vols. I–III. Springer-Verlag, Berlin, 1981 (I), 1982 (II), 1984 (III). 3. Lamb, J. D.; Izatt, R. M.; Robertson, P. A.; Christensen, J. J. J. Am. Chem. Soc., 1980, 102, 2454. 4. Mcbride, D. W.; Izatt, R. M.; Lamb, J. D.; Christensen, J. J. In Inclusion Compounds (Atwood, J. L.; Davies, J. E. and Macnicol, D. D., Eds.), Vol. 3, Ch. 16, p. 571. Academic Press, Orlando, 1984. 5. Izatt, R. M.; Clark, G. A.; Bradshaw, J. S.; Lamb, J. D.; Christensen, J. J. Sep. Purif. Methods, 1986, 15, 21. 6. Ochiai, E. J. Chem. Educ., 1988, 65, 943. 7. Hiratani, K.; Taguchi, K. Chem. Lett., 1990, 725. 8. Izatt, R. M.; Bruening, R. L.; Bradsaw, J. S.; Lamb, J. D.; Christensen, J. J. Pure Appl. Chem., 1988, 60, 453. 9. Wiank, M. M.; Slowigk, T. B.; Sudholter, E. J. R.; Reinhoudt, D. N. J. Am. Chem. Soc., 1990, 112, 797. 10. Cox, J. A.; Bhatnagar, A. Talanta, 1990, 37, 1037. 11. Hiratani, K.; Taguchi, K. Chem. Lett., 1990, 725. 12. Hiraoka, M. Crown Compounds, Their Characteristics and Applications, Studies in Organic Chemistry, Vol. 12, pp. 203, 206. Kodansha, Tokyo, 1982. 13. Yamaguchi, K.; Kuboniwa, H.; Murakami, N.; Hivuo, A.; Nakahama, S.; Yamazaki, N. Bull. Chem. Soc. Japan, 1989, 62, 1097. 14. Maugh, T. H. Science, 1976, 193, 134. 15. Shinbo, T.; Kurihara, K.; Kobatuke, Y.; Kamo, N. Nature, 1977, 270, 277. 16. Tabnshi, I.; Kobuke, Y.; Imata, J. J. Am. Chem. Soc., 1980, 102, 1744. 17. Shinbo, T.; Sugiura, M.; Kama, N.; Kobatake, P. J. Memb. Sci., 1981, 9, 1. 18. Sugiura, M.; Shinbo, T. Bull. Chem. Soc. Japan, 1979, 52, 684.
ah11$$1548
02-24-98 14:16:18
mica
AP: MCH