Anal. Chem. 1987, 59, 1513-1517
1513
Lithium Separation and Enrichment by Proton-Driven Cation Transport through Liquid Membranes of Lipophilic Crown Nitrophenols Hidefumi Sakamoto, Keiichi Kimura,* and Toshiyuki Shono
Department of Applied Chemistry, Faculty of Engineering, Osaka University, Yamada-oka, Suita, Osaka 565, Japan
Protondriven cation transport through supported liquid m e m branes containing IIpophRk crown nitrophenolderlvathres has been studled. The membranes of lipophilic 14trown-4 nitrophenoi derlvatives were found to transport Li+ efflclentty against the ion concentration gradients. Extremely high Li+ seiectlvities were realized in competltlve transport of alkali metal Ions through the membranes, reflecting the catloncomplexing properties of the 14trown-4 derlvatives. Effects of the pH in the aqueous phases, the kind of membrane solvents, and the initlal cation concentration on the Li+ trarisportablitly were also examlned. Simultaneous separation and enrichment of Li' from aqueous solutions of Li+/Na+ mlxtures (1/1 and 1/100) are demonstrated, using a transport cell system In which a hlgh-volume source phase and a iowvolume recelving phase are divided by the liquid membranes of the crown nltrophenols.
Membrane cation transport mediated by ionophores or ion carriers is a most attractive procedure for separation of alkali and alkaline-earth metal ions. It is, however, not very easy to separate Li+ and other alkali metal ions, especially Na+, because of the strong hydration of Li+ and the similarity of Li+ and Na+ in the chemical property. There also seems to be no naturally occurring ionophores that can differentiate Li+ from other alkali metal ions significantly, although many natural ionophores specific to K+ and Na+ have emerged. Several synthetic, electrically neutral ionophores for Li+, such as polyether amide derivatives (1-4) and crown ether derivatives (5-9)) have been reported and they are candidates of ionophores for Li+ separation by ionophore-facilitated membrane cation transport where the transport is driven essentially by the ion concentration gradients between two aqueous phases. Coupled membrane transport in which cation transport is driven by counter transport of proton possesses great advantages over the ionophore-facilitated passive transport, In the ionophore-facilitated cation transport, not only the cations but also appropriate counteranions are transported through the membranes to counterbalance the positive charge. Alternatively if ionophores carry negative charges in the molecule, then there would be no need for the counteranions. This is true for proton-driven membrane transport by ionophores bearing potential anionic moiety (10-13). Also, since cations are transferred through membranes against the cation concentration gradients in the proton-driven membrane transport, cation enrichment based on the uphill transport is feasible. There are several synthetic Li+ ionophores bearing easily ionizable moieties that are applicable to proton-driven membrane transport of Li+. Hiratani et al. (14) have reported synthesis of polyether carboxylic acid derivatives and their Li+ selectivities in proton-driven membrane transport of alkali metal ions. Bartsch et al. (15) have synthesized crown-4 derivatives possessing a carboxylic group, some of which were found to extract Li+ selectively. We have also designed easily
ionizable Li+ ionophores that bear 14-crown-4 cycle and nitrophenol groups. We recently communicated the extremely high Li+ selectivities of the crown nitrophenol derivatives in proton-driven membrane cation transport (16). In this publication we report the detail of the competitive proton-driven transport of alkali metal ions through supported liquid membranes of which ionophores are lipophilic 14crown-4 phenols 1-5 (Figure 1)and also crown phenols with the larger ring sizes for comparison. Simultaneous separation and enrichment of Li+ by the membrane transport systems are also described. EXPERIMENTAL SECTION Chemicals. Lipophilic crown phenol derivatives, 1 through 7, were synthesized according to the procedure reported elsewhere (17). Microporous polypropylene f i i (Duragard 2500, maximum pore size 0.04 X 0.4 pm, thickness 25 gm) for the liquid membrane support was kindly supplied by Polyplastics, Inc. The membrane solvents, o-nitrophenyl octyl ether (NPOE) (18), o-nitrophenyl phenyl ether (NPPE) (19),and o-fluorophenyl o-nitrophenyl ether (FPNPE) (ZO), were prepared as described in the literature. Commercially available bis(Zethylhexy1) sebacate (DOS),dibutyl phthalate (DBP), dioctyl phthalate (DOP), and tris(2-ethylhexyl) phosphate (TEHP) were purified by distillation. Dioctyl phenylphosphonate (DOPP) (Dotite) was used as received. Metal salts,tetramethylammonium hydroxide (TMAOH) (10% aqueous solution), hydrochloric acid, and phosphoric acid were of analytical grade. Ethanolamine was purified by vacuum distillation. 3(N-Morpho1ino)propanesulfonic acid (MOPS), 2-(Nmorpho1ino)ethanesulfonic acid (MES), and tris(hydroxymethy1)aminomethane (Tris) were purchased. Water was deionized and distilled. Competitive Membrane Transport. A transport glass cell, shown schematically in Figure 2, was generally employed. In the glass cell two compartments are divided by a supported liquid membrane. A disk of 5 cm diameter was cut out from a micromol L-' porous polypropylene film and then soaked in a 4 X crown ether solution for impregnation. The membrane impregnation was completed in several seconds and the longer soaking did not change the transportability. The membrane solvent was NPOE unless otherwise specified. After the impregnation the external solution on the membrane was carefully wiped off with filter paper. The resulting membrane, which generally contained about 30 pL of the membrane solution, was fixed to the glass cell by using silicon rubber packings and clips. The exposed area of the membrane was about 5 cm'. Equal volumes (40 mL) of aqueous solutions for the source and receiving phases were placed concurrently into the two compartments. The whole setup for the transport experiments was kept at 25 "C by using a circulating thermostated bath. The aqueous phases were stirred magnetically at a rate of 400 rpm during the cation transport. Aliquots (100 pL) of the aqueous solutions in the both phases were occasionally taken up and then diluted 100 times. The ion concentrations were then measured by flame photometry with a Jarrell-Ash AA-781 flame photometer. For the competitive transport of alkali metal ions which were carried out for the comparison of the crown ether derivatives and the membrane solvent in the cation transportability and the Li+ selectivity,transport system I was employed (Chart I). Transport system 11,which contains only Li+ and Na+ as the alkali metal ions, was employed on examining the dependence of pH and initial
0003-2700/S7/0359-15 13$01.50/0 0 1987 American Chemlcal Society
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ANALYTICAL CHEMISTRY, VOL. 1
R=
59,
NO.
11,
JUNE 1, 1987
ONoi
Chart I Transport System I
OH
2 a = -Q
6
OH NO2
"=:,A=
GN0' OH NO,
3 R=
@OZ
LiOH. NaOH. KOH, RbOH.
OH NO2 L R=
aN%
a n d CsOH
I
Membrane
II
KOH. RbOH. and CrOH
(ZxlO-' mo1.L-I each)
NO?
5 a=
LiOH. NaOH.
$j) OH
Flgure 1. Crown ether derivatives employed in this study.
T ~,~
5
T
Aqueous Source Phase
t28+204
4 0 rnl
Aquwus Receiving Phase
6 Transport System I1
0
T i m N 1
II
4 0 mL
LiCl and NaCl
LiCl and NaCl
(1xlO~'rnol.C' each)
( 1 ~ 1 0 'rnoLL-' ~ each)
Membrane a b u f f e r (basic)
a buffer (acidic)
\ Phose
PO mL
\3 Aaueous Source Phase
Flgwe 2. Glass cell employed for cation competitive transport: (1) aqueous source phase: (2) aqueous receiving phase; (3) supported liquid membrane; (4) stir bar with star-shaped fins (1.5 cm dlameter); (5) ground glass stopper; (6) glass tube for sampling. The dimension is given in millimeters. cation concentration of the aqueous phases on the cation transportability. The pH for the source phase was varied while keeping the pH for the receiving phase constant (1 X lo-' mol L-I HCl) and vice versa (the source phase was 1.1X lo-' mol L-' TMAOH). The pH in the aqueous phases was adjusted to pH 2.0-3.7 by H,PO,-TMAOH buffers, pH 5.0-6.0 by MES-TMAOH buffers, pH 6.0-7.0 by MOPS-TMAOH buffers, pH 7.5-9.2 by Tris-HC1 buffers, pH 9.5-10.8 by ethanolamine-HC1 buffers, and pH 11.1-12.8 by TMAOH. In the experiment for the initial cation concentration dependence, the source and receiving phases in transport system I1 contained 1.1X lo-' mol L-' TMAOH and 1 X lo-' mol L-' HC1, respectively, and the Li+ and Na+ concentrations were varied while keeping both concentrations the same. The initial fluxes were determined as average flux of cation transport from t = 0 to t = 10 h. The selectivity ratios of Li+ and the other alkali metal ions were computed from the initial fluxes. Lithium Separation and Enrichment. A cylindrical cell shown in Figure 3 was employed for this membrane cation transport. The membrane was a polypropylene film disk of 9 cm mol L-' crown diameter containing about 100 WLof 4 x dinitro-2-phenol(3) solution. The exposed area of the membrane was about 20 cm2. Both of the cell compartments have a volume of 140 mL. The source phase solution was circulated between the source phase compartment and an external reservoir (1000 mL) at a rate of 15 mL m i d by a peristaltic pump. The total volume for the source phase was therefore 1140 mL. Both of the aqueous phases in the compartments were stirred magnetically at a rate of 300 rpm. The cation transport was performed at 25 O C and transport system I11 was applied here. One of the artificial samples for the source phase contained Li+ and Na+ in 1 x mol L-' each and 1.1X mol L-' TMAOH. The other contained 6.3 X mol L-' Li', 6.5 X lo-' mol L-' Na+, and 1 X mol L-l KOH.
RESULTS AND DISCUSSION Competitive Proton-Driven Membrane Transport of Alkali Metal Ions. Ionophores for proton-driven membrane cation transport should be endowed with three main com-
I
40mL Aqueous Receiving Phase
Transport System 111
lembrane
Phase
1140 mL Aqueous Source Phase
I
I
140 mL Aqueous Receiving Phase
2
Flgure 3. Cylindrical glass cell employed for Li' separation and enrichment: (1) aqueous source phase; (2) aqueous receiving phase; (3) supported liquid membrane; (4) stir bar with star-shaped fins (1.5 cm diameter); (5) sampling tube; (6) peristaltic pump; (7) reservoir for source phase: (8) silicon rubber tube. The dimension is given in millimeters. ponents, that is, a complexing site to bind certain cations selectively, a proton-dissociable anionic site to counterbalance the positive charge of the complexed cation, and a highly lipophilic part to have the ionophore stay in the membrane phase. Crown nitrophenol derivatives, 1-4, were expected to be Li+ ionophores for proton-driven membrane transport inasmuch as they consist of a 14-crown-4cycle specific to Li+,
ANALYTICAL CHEMISTRY, VOL. 59, NO. 11, JUNE 1, 1987
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Scheme I
lot
Aqueous Source Phase
1
I
Membrane Phose
(Basic)
Aqueous Receiving Phase (Acidic)
AqueousReceiving Phase (Acidic)
/OLi
1.4
Flgure 5. Comparison of crown phenol derivatives in cation transportabilities. 1.2
2
1.0
r l
E 0.8
0.6}
1
0.4
OO
Aqueous Source P h a s e (Basic)
20
10
Time (hr)
Flgure 4. Competitive protondriven transport of alkali metal ions through membrane of crown p-nbophenol 1. [M+],and [M'],, stand for catlon concentrations at tlme t and 0, respectively. For details see Experimental Section.
an easily ionizable nitrophenol group (pK, (acidity constant) in 50% dioxane-H,O: 9.64 for 1, 10.74 for 2, 4.92 for 3, and 4. 71 for 41, and a lipophilic dodecyl group. For instance, crown nitrophenol 1 is considered to transport Li+ selectively from an aqueous source phase (basic) to an aqueous receiving phase (acidic) with the counter transport of proton as illustrated in Scheme I. At the basic interface the nitrophenol group loses a proton and concurrently the adjacent crown ether moiety binds a Li+. The crown-Li+ complex that is electrically neutralized intramolecularly is diffused to the acidic interface through the membrane. A t the acidic interface the crown complex is forced to release the Li+ by the protonation of the anionic site and, therefore, the loss of the negative charge. Figure 4 gives a typical profile for the competitive proton-driven transport of five alkali metal ions through the supported W O E membrane of crown nitrophenol 1, showing extremely high Li+ selectivity of the ionophore. The membrane clearly exhibits uphill Li+ transportability. Li+ can be transferred efficiently through the membrane from the basic to acidic phases. In contrast, the other alkali metal ions moved through the membrane only at such low transport rates that the cation transport was barely detected. Of course, the NPOE membrane without crown ether did not transport an appreciable amount of Li+ as well as the other metal ions. The Li+ selectivity on the membrane cation transport is dramatic, the selectivity ratio of Li+ and Na+ being 50. For comparison, other crown phenol derivatives 2-7 were also employed as the ionophore for the proton-driven membrane transport under the transport conditions identical with those given in Figure 4. Such high Li+ transportabilities and selectivities as seen in the membrane of crown p-nitrophenol 1 were observed with those of crown o-nitrophenol2 and crown dinitro-%phenol3 (Figure 5). The membrane of crown phenol
005
WP
DBP
NPOE
NPPE FPNPE
TEHP
DOPP NPOVTEHP ( 3 / 1 )
Figure 6. Effect of solvent in membrane phase on alkali metal ion transportability of 1. 5, however, did not afford proton-driven transport of alkali metal ions because the phenol proton is hard to dissociate (pK, > 14). Very interestingly, Li+ transportability for crown dinitro-4-phenol4 is decreased significantly as compared to that for crown dinitro-2-phenol3, although 4 is still Li+ selective. It is probably because the 4-phenoxide anion of 4 does not very easily interact intramolecularly with Li+ complexed by the crown moiety for the charge compensation. The cation complex formation accompanying the intramolecular charge counterbalance as shown in Scheme I is, therefore, essential to the proton-driven membrane Li+ transport by the crown nitrophenol derivatives. This is also supported by the fact that the NPOE membrane containing both 6-dodecyl-6methyl-14-crown-4 (neutral Li+ ionophore) (7) and 2,6-dinitro-4-nonylphenol does not permit the proton-driven Li+ transport at all. If the intermolecular interaction between Li+ complexed by the neutral crown ether and 2,6-dinitro-4nonylphenoxide anion were stable in the membrane phase, the membrane would transport Li+ more or less under the proton-driven transport condition. Employment of 13-crown-4 and 16-crown-5cycles instead of 14-crown-4decreased the Li+ transportability and selectivity in the competitive protondriven membrane transport of alkali metal ions as observed in the membranes of 6 and 7. Even Na+ selectivities were found with the membrane systems. Especially in the membrane of 6 , very high Na+ selectivity was attained, the selectivity ratio of Na+ over Li+ being 18. Evidently, the excellent Li+ selectivities of the crown nitrophenol derivatives 1-3 in the proton-driven membrane transport are primarily based on the 14-crown-4 cycle. Effects of Transport Conditions. Various membrane solvents were applied to the competitive proton-driven membrane transport of alkali metal ions using crown p-nitrophenol 1 as the ionophore. The comparison among the membrane solvents can be seen in Figure 6. In the diester-type solvents,
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ANALYTICAL CHEMISTRY, VOL. 59, NO. 11, JUNE 1, 1987
";;e 5
.-
4,y -1
PH
Effect of pH in aqueous phases on Li+ transport through nembranes of (a)crown p nitrophenol 1 and (b) crown dinitm2phenoI 3: ( 0 )pH change for source phase: (0)pH change for receiving phase. i
,
,
5
10
Cation Concentrat ion (10-2mot.L-')
PH
Flgure 7.
0
Effect of initial ion concentration on Li+ transport through membrane of 1. Figure 8.
U c
DBP, which has a relatively high dielectric constant (e = 6.4), is an excellent membrane solvent for the proton-driven membrane cation transport. Use of DOP and DOS, which possess the lower dielectric constants ( t = 5.1 and 4.0, respectively), resulted in remarkable decrease in Li+ transportability. Specifically, the DOS membrane hardly transported any alkali metal ions. The phenyl ether type solvents with high dielectric constants, W O E (e = 24), NPPE (e = 29), and FPNPE (e = 50), are also prominent membrane solvents for the membrane cation transport, allowing high Li+ transportability and selectivity. This dependence of Li+ transportability and selectivity on the dielectric constant of solvent may suggest that the Li+-crown ether complexes are partially solvated in the membrane phase. The phosphate- and phosphonate-type solvents, TEHP and DOPP, which possess some coordination property, do not seem very good as the membrane solvent. The cation coordination of the solvents might compete with the Li+ complexation by the crown ethers. A mixture solvent of NPOE and TEHP (3/1) is superior to TEHP as the membrane solvent but does not improve the Li+ transportability for the NPOE membrane. Thus the phenyl ether type membrane solvents are generally suitable for the proton-driven Li+ transport using the crown nitrophenol del ivatives. The pH in the aqueous phases is an important factor governing the Li+ transportability in the proton-driven membrane transport. Competitive proton-driven membrane transport of Li+ and Na+ through NPOE membranes of the crown p-nitrophenol 1 and dinitro-2-phenol3 was carried out by varying the pH values of the source and receiving phases independently (Figure 7). In the membrane system of 1, Li+ transport hardly proceeds with the source phase up to pH 11. Increasing pH of the source phase above pH 11 enhances Li+ transport drastically by promotion of the Li+ complex formation at the interface for the source phase. On the contrary, increasing pH of the receiving phase above pH 6 lowers Li+ transportability by depressing the cation release of the complex at the interface for the receiving phase. When the pH values of the source and receiving phases are the same, the Li+ transport ceases completely as anticipated from the transport mechanism of Scheme I. The similar pH dependence on the Li+ membrane transport was observed in the membrane system of 3, but the pH-flux plots shifted to the lower pH. The shift in the transport profile is definitely derived from the pKa difference between 1 and 3. In the system of 3, some decrease in the Li+ flux was caused by increasing pH in the basic phase above pH 11. This "apparent" decrease in the Li+ flux is attributed to competitive formation of the crown-Li+ complex and an ion pair of the uncomplexed crown ether anion and tetramethylammonium ion in the membrane phase.
b
:: ec s -+r
3
;?
O n
cb
0 .e
e
# n sourcc phase
L
C U
0
V
0 '
10
20
30
Time (hr)
Flgure 9. Time-transport profile on experiment for Li+ separation and enrichment from aqueous solution of Li+ and Na+ mixture (111). For details see Experimental Section.
The initial cation concentration in the aqueous phases also affects the membrane cation transport. Figure 8 shows the relationship between the initial Li+ concentration and Li+ flux in the competitive proton-driven transport of Li+ and Na+ through the NPOE membrane containing crown p-nitrophenol 1. The Li+ flux is increased with the initial cation concenmol L-' and then levels off at the tration up to about 4 X higher concentrations. That is to say, the Li+ flux is proportional to the initial cation concentrations below the crown ether concentration in the membrane phase. Above the crown ether concentration, most of the crown ether molecules bind Li+ in the membrane phase, so the Li+ flux does not depend on the initial cation concentration any more (11). Simultaneous Lithium Separation and Enrichment by Membrane Transport. One of the most intriguing aspects for the proton-driven membrane Li+ transport by the crown nitrophenol derivatives is in simultaneous separation and enrichment of Li+ from aqueous solutions. The source phases were aqueous solutions containing Li+ and other alkali metal ions which were adjusted to high pH values by TMAOH or KOH. An aqueous HC1 solution that did not contain any metal ion was chosen as the receiving phase. In this case, if the volumes of the source and receiving phases are the same, Li+ enrichment in the receiving phase, that is, attainment of the higher Li+ concentrations than the inital one, is never realized by the proton-driven cation transport. However, if the source phase is greater in the volume than the receiving one, it is possible to simultaneously separate and enrich Li+ from the source phase in the receiving one (21). We, therefore, designed a transport cell system, as shown in Figure 3, which possesses a high-volume source phase compartment and a low-volume receiving phase one. A t first, selective Li+ enrichment from an artificial sample containing an identical concentration of Li+ and Na+ was
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ments. With three successive transport experiments, the Li+ concentration in the resulting solution exceeded the Na+ one and also the Li+ concentration in the starting solution employed for the first transport experiment. Thus Li+ in the aqueous solutions containing a large excess of Na+ was successfully separated and enriched. The greater volume difference between the source and receiving phase would bring about the more efficient Li+ enrichment. In conclusion, the proton-driven cation transport through the supported liquid membranes of the crown nitrophenol derivatives is quite promising for Li+ separation and enrichment from aqueous samples of various metal ions. The membrane transport system might be applicable to Li+ recovery from natural waters.
LITERATURE CITED
0
1
2
3
Number of Transport Experiment (time)
Figure 10. Change in catin concentrations of source phase resulting from repeated membrane transport using U+ and Na+ mixture (l/lOO). Transport time was 30 h.
attempted by using a supported NPOE membrane of crown dinitro-2-phenol3. Figure 9 clearly demonstrates the selective Li+ enrichment from the aqueous solution by the protondriven membrane transport. In 30 h of transport, the Li+ concentration in the receiving phase was about 5 times higher than the initial concentration in the source phase. On the other hand, the Na+ concentration in the receiving phase is still below the initial cation concentration in the source phase. An attempt was also made to separate and enrich Li+ from an aqueous solution containing about 100 times higher concentration of Na+ than that of Li+ by the proton-driven membrane transport through the 3 membrane. In the sample employed for the source phase, the concentration ratios of Na+ and K+ over Li+ were about 103 and 1.6, respectively. In the first proton-driven cation transport, the concentration ratios of Na+ and K+ over Li+ were reduced to about 11 and 0.2, respectively, although Li+ enrichment was not yet observed. Repeat proton-driven cation transport was carried out by using mol L-' KOH the resulting receiving phase, to which 1 X was again added, as the source phase and 1 X lo-' mol L-' HC1 as the receiving phase. Figure 10 depicts the concentration changes of Li+ and Na+ after each of the transport experi-
(1) Zhukov, A. F.; Erne, D.; Ammenn, D.;Guggi, M.; Pretsch. E.; Simon, W. Anal. Chim. Acta 1981, 131, 117-122. (2) Metzger, E.; Ammann, D.;Schefer, U.; Pretsch. E.; Simon, W. Chimia 1984, 3 8 , 440-442. (3) Margalit, R.; Shanzar, A. PfUgerS Arch. 1982, 395, 87-92. (4) Gadzekpo, V. P. Y.; Hungerford, J. M.; Kadry, A. M.; Ibrahim, Y. A,; Christian, G. D. Anal. Chem. 1985, 5 7 , 493-495. (5) Olsher, U.; Jagur-Grodzinski, J. J . Chem. Soc., Dalton Trans. 1981, 501-505. (6) Aalmo, K. M.; Krane, J. Acta Chem. Scand., Ser. A 1982, 3 6 , 227-234. (7) Kitazawa, S.; Kimura, K.; Yano, H.; Shono, T. J . Am. Chem. SOC. 1984, 106, 6978-6983. (8) Czech, B. P.; Babb, D. A.; Son, B.; Bartsch, R. A. J . Org. Chem. 1984. 49, 4805-4810. (9) Kobiro, K.; Matsuoka, T.; Takada, S.; Kakiuchi, K.; Tobe, Y.; Odaira. Y. Chem. Len. 1986, 713-714. (10) Frederick, L. A.; Fyles, T. M.; Malik-Diemer, V. A,; Whitfield, D. M. J . Chem. Soc., Chem. Common. 1980, 1211-1212. (11) Fyles, T. M.; Malik-Diemer, V. A.; McGavin, C. A,; Whitfield, D. M. Can. J . Chem. 1982, 60. 2259-2267. (12) Strzelbicki, J.; Bartsch, R. A. J . Membr. Sci. 1982, IO, 35-47. (13) Pugia, M. J.; Ndip, G.; Lee, H. K.; Yang, I.-W.; Bartsch. R. A. Anal. Chem. 1988, 5 8 , 2723-2726. (14) Hiratani, K.; Taguchi, K.; Sugihara, H.; Iio, K. Bull. Chem. SOC.Jpn. 1984, 5 7 , 1976-1984. (15) Bartsch, R. A.; Czech, B. P.; Kang, S. I.; Stewart, L. E.; Walkowiak, W.; Charewicz, W. A,; Heo, G. S.;Son, B. J . Am. Chem. SOC.1985, 107, 4997-4998. (16) Kimura, K.; Sakamoto, H.; Kitazawa, S.;Shono. T. J . Chem. Soc., Chem. Commun. 1985, 669-670. (17) Kimura, K.; Tanaka, M.; Iketani, S.; Shono, T. J . Org. Chern. 1987, 52, 836-844. (18) Allen, C. F. H.; Gates, J. W. Organic Syntheses; Wiley: New York, 1955; Collect. Vol. 111, pp 140-141. (19) Brewster, R. Q.; Groening, T. Organic Syntheses; Wiley: New York, 1943; Collect. Vol. 11, pp 445-446. (20) . . Rvba, 0.; Petrinek, J. Collect. Czech. Chem. Commun. 1984, 4 9 , 237 1-2375. (21) Uto, M.; Yoshida, H.; Sugawara, M.; Umezawa, Y. Anal. Chem. 1988, 5 8 , 1796-1803.
RECEIVED for review December 3,1986. Accepted February 24, 1987.
