Anal. Chem. 1986, 58,2714-2723
2714
ACKNOWLEDGMENT We thank Texas Instruments, Inc., for the donation of a sample of the Ge-Sb-Se glass used in this study and Pylem, Inc., for supplying authentic standards of the dyes that we believed to be present in the dye mixture. We also gratefully acknowledge the loan of the fx-6200 spectrometer and diffuse reflectance accessory by the Analect Instrument Division of Laser Precision, Inc.
(3) Zuber, G. E.; Warren, R. J.; Begosh, P. P.; O'Donneii, E. L. Anal. Chem. 1984, 56, 2935-2939. (4) Lloyd, L. B.; Yeates, R. C.; Eyring, E. M. Anal. Chem. 1982, 5 4 , 549-552. (5) White, R. L. Anal. Chem. 1985, 57, 1819-1822. (6) Fuller, M. P.; Grifflths, P. R. Appl. Spectrosc. 1980, 34, 533-539. (7) Brimmer, P. J.; Griffiths, P. R. Anal. Chem., in press. (8) Kuehl, D.; Griffiths, P. R. J. Chromatogr. Sci. 1979, 17, 471-476. (9) Kuehi. D. T.; Griffiths, P. R. Anal. Chem. 1980, 52, 1394-1399. :IO) Kaiasinsky, K. S.;Smith, J. A. S.;Kalansinsky, V. F. Anal. Chem. 1985, 57, 1969-1974.
,
LITERATURE CITED (1) Percival, C. J.; Griffiths, P. R. Anal. Chem. 1975, 4 7 , 154-156. (2) Fuller, M. P.; Griffiths, P. R. Anal. Chem. 1978, 50, 1908-1910.
RECEIVED for review May 15, 1986. Accepted July 7 , 1986.
Aqueous/Aqueous Extraction by Means of a Liquid Membrane for Sample Cleanup and Preconcentration of Amines in a Flow System Gudjon Audunsson Analytical Chemistry, University of Lund, Box 124, S-221 00 Lund, Sweden
A new conflguration for sample cleanup and enrlchnlent of amines In a flow system Is suggested In which the sample passes a liquid membrane whereupon the analyte of Interest Is released and trapped In a stagnant acceptor phase on the other slde. The resulting plug of analyte Is then swept from the membrane separator to detectlon. The mass transfer across the ItquM membrane Is discussed theoretically as well as the Influence of transport on the acceptor concentratlon profile. The effect of sample volume, support matrix, types of lmmoblllred solvents, donor flow rate, and partition coefflclents of analytes between donor phase and membrane phase on enrichment factor and separatlon Is demonstrated and the results are compared wlth theoretical predlctlons.
ture. Thus a preconcentration is achieved. When the same has passed the membrane, a definite fraction of the analyte is contained in the small volume of the acceptor chamber. This volume plug may be analyzed directly by some flowthrough detector or, more appropriately for a mixture of amines, injected by means of a suitable interface onto a chromatographic column. The technique is well suited for automation and on-line coupling to analytical systems. Using different compositions of the membrane phase and/or the acceptor and donor phases, one should be able to more-or-less selectively clean up most amines. Application of the technique to analytes other than amines is obvious, most straightforward to acidic compounds.
Many analyses of organic compounds in liquid samples require a selective cleanup and preconcentration step. A direct on-line coupling of the sample preparation procedure to the analytical system is favorable since it minimizes sample handling and thereby the risk for contamination or loss of the analyte. In addition, on-line coupling makes automation of the whole process possible, resulting in more reproducible and economical analyses. Over the past 10 years liquid membrane separations have been demonstrated (1,2) mostly concerning carrier facilitated metal ion separations (3-5) and to a lesser degree separations of organic substances (6-9). Very little appears to have been done for analytical purposes. This work describes an alternative method using liquid membrane separation for sample cleanup and preconcentration of amines, sine trace analysis of amines has been a subject of interest at this laboratory for some time (10). Other preconcentration methods for amines have recently been discussed (11). The alkalized sample stream passes a hydrophobic liquid membrane in which nonionic components of the sample dissolve. The membrane is installed in a dialysis module having stagnant acidic solution on the acceptor side which solely traps the basic constituents of the sample mix-
Equipment. A schematic of the flow system is shown in Figure 1. To facilitate independent variation of flow rates, two peristaltic pumps (I) were used, FIA-08 (Bifok, Sweden) and Ismatec m p 13 GJ-4 (Zurich, Switzerland). Standard PVC manifold pump tubings (Elkay Products, Inc., USA) were used throughout and the various parts were connected with Teflon tubing and Altex screw fittings. Both the sample inlet valve (11) and the switching valve (111) were manually operated four-way Kel-F slider valves (Cheminert, Laboratory Data Control (UK),Ltd.). The mixing point (V) and the bypass loop coupling (VI) were made of Teflon. The channels meet at a 60° angle. The membrane separator (IV) was machined from blocks of Teflon (acceptor side) and Titan (donor side) by cutting two U-formed grooves on the opposite faces of the two blocks. In most of this work both grooves were 0.25 mm deep and 1.5 mm wide. The total groove length was 150 mm. The membrane was clamped tightly and evenly between the planar surfaces of the blocks by 10 screws. An O-ring, encircling the grooves outside the membrane, provided an additional seal to prevent solvent seepage. Titan was used in the construction of the membrane separator to make it more rigid, as Teflon is easily deformed under mechanical strain. To make the Teflon part of the separator still more rigid, it was backed up by an aluminum block (6 mm thick) in which the threads for the clamping screws were machined. One could of course have used solely Titan or other sufficiently chemically inert and mechanically hard material, e.g., Kel-F, but
EXPERIMENTAL SECTION
0003-2700/86/0358-27 14$01.50/0
0 1986 American Chemical Society
ANALYTICAL CHEMISTRY, VOL. 58, NO. 13, NOVEMBER 1986
2715
Table I. Matrix Supports Investigated
type of matrix support
support material
av pore size, Nm
av total thickness, wm
Fluoropore FG Fluoropore FH Fluoropore FA Fluoropore FS Mitex LS Mitex LC Fluoropore FH Duraoore GV (white)
PTFE with polyethylene backing
0.2 0.5 1.0 3.0 5.0 10.0 0.5 0.22
175 175 145 200 125 125 60 125
WATCI
PTFE Dolv(vinv1idene fluoride)
fi
Flgure 1. Experimental setup.
these are in general much more expensive than Teflon. The porous hydrophobic membranes used as solvent supports in this work are listed in Table I with specifications reported by the manufacturer (Millipore Corp., Bedford, MA). The analytes under study were monitored at 254 nm by a Spectra-Physics spectrophotometer (Model 770, Schoeffel Instrument Corp., USA) equipped with Servogor 210 recorder (Goerz Electro Ges. M.B.H., Austria). Chemicals. Four organic solvents, isooctane (Fluka AG), n-hexadecane (Fluka AG), n-undecane (Merck), and 1-decanol (BDH Chemicals, La.) were tested as organic phases in the liquid membrane. To simplify the detection, only analytes with a chromophorewere used both with aromatic and aliphatic nitrogen. These were o-methylaniline and m-methylaniline (Fluka AG), p-methylaniline (Hopkins and Williams, Ltd.,Essex, England), aniline (BDH Chemicals, LM.), pyridine (Mallinckrodt Chemical Works, St. Louis, MO), and 2-phenylethylamine (Fluka AG). All other chemicals were purchased from Merck except sodium hydroxide pellets, which were obtained from EKA (Bohus, Sweden). All chemicals were of analytical grade or better, except benzyl alcohol, phenylethylamine, and benzylanine, which were of puriss grade. The water was purified with a Milli-Q/RO-4 unit (Millipore, Bedford, MA). Procedure. The liquid membranes were prepared by immersing the polymer membranes in the chosen solvent for about 15 min. After the membrane had been installed in the separator, excess solvent on the membrane surface was removed by forcing water through both channels of the separator by a syringe. For membranes with polyethylene backing, the Teflon face was exposed to the donor phase. Operation of the system is best described with reference to Figure 1. Sample is introduced into the system by valve I1 where either sample or water can be selected. The sample volume is determined by time and flow rate. While the sample is being introduced, the switching valve I11 is in the bypass position, as shown in Figure 1, so the acceptor stream in the separator is stagnant. The sample is mixed with alkaline buffer in the mixing coil (VII) (100 cm, 0.5 mm i.d.). When the sample has passed the membrane on the donor side, the valve (111) is switched, transferring the analyte that has been accumulated on the acidic acceptor side to the detector. To prevent formation of gas bubbles, it was necessary to purge all the solutions with helium. Calculations. Numerical calculations were made on a table-top computer (ABC-806, Luxor, Sweden). First and second moments of the experimental and simulated peaks were calculated by using standard procedures (12). Simplex fittings were made with a
thickness exclusive backing l/@m 60 60 60 60 125 125 60 125
typical porosity
c
0.70 0.85 0.85 0.85 0.60 0.68 0.85 0.75
program taken from the literature (13).
THEORETICAL SECTION A theoretical model quantitatively accounting for the underlying mass transfer processes in the system described above sheds light on how various parameters are expected to affect the system and thereby facilitating optimization of the analysis. The concentration profile in the acceptor channel will be developed and the influence of the transport of this profile in the tubings to the detector will be considered. Development of a Concentration Profile in the Acceptor Channel. The transfer of unprotonized amine from the donor stream (phase A) to the acidic solution in the acceptor channel (phase C) via the membrane phase (B) may be thought of as occurring in two steps. Firstly, the uncharged amine migrates from the bulk donor stream toward the membrane phase. Secondly, the amine passes the membrane phase. Provided that the walls of the donor channel will not affect the mass transfer, the mass transfer coefficient in the donor stream, kA is best accounted for by the penetration theory (14) k A = ( - ) 2DJ 3aax
(see the list of symbols at the end of this paper) while the mass transfer coefficient in the membrane phase, kg, is adequately presented by the film theory given by kg
= D,/1
(2)
The development of the concentration profile rests on the following assumptions: (1)The amine is protonated as soon as it reaches the acceptor phase, (pH > pK, (amine). For pH values a t which the amine is partly hydrolyzed, the partition coefficient in eq 9 is simply replaced by the distribution coefficient, but only conditions where the amines are uncharged will be considered in this work. Increased salt concentrations will generally enhance the extractability of organics from water solutions to organic
2720
ANALYTICAL CHEMISTRY, VOL. 58, NO. 13, NOVEMBER 1986 1.0 I
2 '
'
'
"
"
I
08 2
E.
06
>:
?.
04
04
I-
/\
EC
02 1.
00
I 0
1 ~
1
2
3
4
5
0
20
60
40
100
80
t,s
CNaOH'M
Figure 6. Enrichment per sample volume at different NaOH concentrations in the donor phase and different donor flow rates: 0.49 mL/min (0); 0.57 mL/min (X); 0.73 mL/min (0);1.0 mL/min (+); 1.4 mL/min (A);1.6 mL/min (X); 1.8 mL/min (0);2.0 mL/min (m). V,,, = 0.84 mL of 20 ppm 2-phenylethylamine. Acceptor phase was 0.95 mL/min of 0.05 M H,S04 (uD," 60 WL).
Figure 7. Experimental peak with Simplex curve fitting by eq 10: 1.4 s ' ' ~coracceptor phase, 0.46 mL/min of 0.05 M H,S04 (u* responding to oD,"= 55 pL); donor phase, 0.47 mL/min 0.1 M NaOH; 8, = 2 min of 20 ppm 2-phenylethylamine; membrane, Fluoropore FG impregnated with n-undecane. Adaptation of t', r, y,and t resulted in the following values: t' = 23.2 s, T = 12.0 s,y = 5.89 X lo-* s-', and t' = 6.47 s.
solvents and NaOH is particularly suitable for amines (23). Thus K p can easily be varied for one particular amine under conditions where eq 13 is valid by altering only the NaOH concentration. Influences on DA upon changes in viscosity of the donor phase can be corrected for in eq 7 , as physical properties of aqueous NaOH solutions are well-documented (24). If such corrections can be experimentally verified, it would be a substantial support for the model presented in the Theoretical Section Eliminating Kp in k , of eq 13 by inserting k l into k2, one gets
k z / k l = (7.7
X
102)bV,'/2~A'12/R
(17)
where the Wilke-Chang equation (25) has been used for estimating D A for 2-phenylethylamine in water solution. This last equation implies that if V , and R are unaffected by CNaOH, k , plotted against klvA1/2should be a straight line with the slope (7.7 X 102)bV/,'12/R. For this purpose 1 mL of 20 ppm 2-phenylethylamine was injected into a water stream (1.0 mL/min) and subsequently mixed with NaOH solutions of various concentrations (0.1-10 M) at different flow rates (0.1-1 mL/min). Maximum peak heights were plotted as a function of CNaoHfor the different donor stream flow rates. Some of these curves are shown in Figure 6 where it is seen that peak heights increase with increasing CNsOH as a result of larger K,. Each curve has a maximum after which the peak height declines as increased viscosity obstructs diffusion in the donor phase. For 14 values of CNaOH between 0.1 and 3.4 M, expression 13 was fitted to experimental values of peak height and donor flow rate taken from Figure 6. A simplex procedure was used and kl and kz were obtained, giving a straight line as expected where the 14 points resulted in correlation coefficient of 0.9979 and a slope with relative standard deviation of 2%. The line does not pass through the origin (the intercept with 95% confidence interval being (1.2 f 0.1) X lo-' ~ ms-li'). ~ This / ~ discrepancy is probably a consequence of all approximations. If b in eq 17 is taken as the geometric depth of the acceptor chamber (0.25 mm), then the mean volume of the acceptor concentration profile, V,, calculated from the slope is less than expected by a factor of about 4. When b is estimated from the parameters y and T obtained by fitting eq 10 to an experimental peak, it turns out to be ca. 4 times less than the geometric depth while the statistical moments and t* indicate that V , is only slightly less than expected. Such fitting is shown in Figure 7 and is probably the most satisfactory way of assessing information about the
I,, m l n
SAMPLE lHTRODUCTlON TIME 9- / m i "
Figure 8. In A the peak maxima of 20 ppm benzyl alcohol (A),20 ppm 2-phenylethylamine (0),and a solution of both 20 ppm benzyl are shown for different alcohol and 20 ppm 2-phenylethylamine (0) time intervals between closing of the sample inlet valve (11) and activation of the bypass valve (111); see Figure 1. In B the maximum peak height of 20 ppm 2-phenylethylamine (0)with t , = 1.0 min and 20 ppm benzyl alcohol (A)with t , = 0.5 min are shown for different sample introduction times. The curve through the points for benzyl alcohol in B is obtained by fitting eq 18 to the points. Acceptor phase, 0.32 mL/min of 0.05 M H,SO,; donor phase, 0.37 mL/min of 0.1 M NaOH; membrane, Fluoropore FG impregnated with n-undecane.
system. Additionally, when the geometric depth of the acceptor chamber was reduced from 0.25 mm to 0.15 mm, the latter gave somewhat smaller peak heights than the former, although theory predicts an increase by a factor of 1.7 as a result of decreased chamber volume. This low value of the apparent channel depth may probably be explained by an additional mass transfer not accounted for by the model, possibly as a result of agitation caused by a fluttering membrane, which is possibly also to some degree pressed into the acceptor channel during accumulation. Washing t h e Trapped Analyte from Matrix Constitue n t s Distributing between the Phases. Only uncharged bases in the donor phase will be concentrated in the acceptor phase. Charged compounds will not pass the membrane while uncharged, nonbasic compounds will equilibrate between donor phase, membrane phase, and acceptor phase and consequently be washed from the acceptor phase by a sample-free donor stream. This is illustrated in Figure 8 for 20 ppm benzyl alcohol and 20 ppm 2-phenylethylamine, benzyl alcohol being chosen as an example since it dissolves quite well in the acceptor phase as well as in the solvent of the membrane. Figure 8A shows how the behavior of the peak maxima for both benzyl alcohol and 2-phenylethylamine behave with the time
ANALYTICAL CHEMISTRY, VOL. 58, NO. 13, NOVEMBER 1986
2721
Table VI. Influences of Some Interferents on the Response (Cpmas/ V , , , mA /mL) of 10 ppm Benzylamine
Acceptor: 0.65 mL/min, 0.05 M H2S04.Membrane: Fluoropore FG impregnated with n-hexadecane. Membrane: Fluoropore FG impregnated with n-hexadecane
sodium acetate" isopropylamine" 2-propanol" ammoniab n-propylamineb
0
10
100
500
concentration/ppm 1000 2500 5000
22.4 19.1
19.6
21.5
32.4
35.2
27.3
27.3 39.7
22.6 28.8 43.5
22.8
22.8 26.7 28.3
31.0
10000
50000
100000
22.4
23.1
27.0
29.2
27.0
42.2c 22.2 25.7
39.6
e
57.7c
" The analyte and the interferent were dissolved in 0.1 M NaOH and then allowed to pass the membrane without addition of any other chemicals. Donor stream flow rate was 0.24 mL/min. *The analyte was dissolved in water and after injection mixed with 5 M NaOH, making the donor stream 0.8 M in NaOH. Donor stream flow rate was 0.29 mL/min. Negative dip appeared before rising of the benzylamine peak. interval t, between the closing of the injection value (11) (see Figure 1)and activation of the bypass valve (111),whereupon the accumulated sample is swept from the membrane separator. First there is an increase in peak height of benzyl alcohol as the bypass valve is activated before all the sample has reached the membrane. The maximum peak height for benzyl alcohol corresponds roughly to the moment where the entire sample has reached the first part of the membrane. Thereafter the peak height decreases as the pure donor stream washes the benzyl alcohol out of the membrane and acceptor channel. 2-Phenylethylamine on the other hand reaches a plateau a t the moment when all the sample has passed the donor channel. Since benzyl alcohol is being washed out by the donor phase a t the same time as the acceptor chamber is being swept, the peaks are considerably more tailing than are the amine peaks. The maximum peak height of benzyl alcohol in Figure 8A increases with increasing contact time between sample and membrane. Eventually it reaches a plateau corresponding to the equilibrium distribution of benzyl alcohol between donor phase and membrane/acceptor phase; i.e., Cl,, = DC;,Figure 8B. This saturation may be described by a simple mass balance, assuming negligible dispersion before the sample reaches the membrane which gives
C' = CLAT(l- e-kes)
(18)
For the washing out of a nonbasic component, similar mass balance considerations result in
C' = CLAT(l- e-kaO)e-kd
(19)
where 6 is the time after all the sample has reached the membrane. Equations 18 and 19 work quite satisfactorily in practice for liquid membranes but also when the acceptor phase is an organic solvent. Both k, and k , increase with the square root of donor flow rate in a similar manner as kA of eq 1. At the same conditions as in Figure 8, both have been found to be 0.6 min-' at a donor stream of 0.4 mL/min giving half-life in washing of about 1.2 min or 0.5 mL. The equality in magnitude of k, and k, indicates that the membrane is the main barrier to mass transfer. The washing step offers the possibility of obtaining quite pure and well-defined fractions of amine in the acceptor phase. The purification has been proven to be effective for any size of sample volume as well as large concentrations of interferents. Influence of the pH of the Acceptor Phase and Selectivity Enhancement by Means of Changes in the pH of Acceptor and Donor Phases. Assumption 1, that protonation in the acceptor phase is instantaneous, means that the efficiency for stagnant and nonstagnant acceptor phases should be equal. In practice the efficiency for stagnant acceptor phases has been found to be greater than 90% of the
efficiency observed for nonstagnant acceptor phases and independent on acceptor flow rates, tested in the range 0.02-2.0 mL/min of 0.05 M HzS04with donor stream 0.5 mL/min 0.1 M NaOH. This somewhat greater efficiency for nonstagnant acceptor phases can presumably be explained by enhanced mass transfer caused by agitational effects of the fluttering membrane, which is set into more motion when both phases are flowing. pH or buffer capacity in the acceptor chamber has not been observed to affect either mass transfer rate or efficiency in the extraction if pH is kept well below pK, under the experiment, a condition easily fulfilled when using the system for trace analysis. Separation and analysis of amines that differ sufficiently in pK, may be accomplished in a simple fashion by a suitable choice of pH in both donor stream and acceptor phase, together with the washing procedure discussed above. Thus aromatic amines have been separated from aliphatic amines into two well-defined fractions by coupling two membrane separators in series. The donor stream of the first was kept at pH 7 where the aliphatic amines are protonized while the aromatic amines are not and thus extracted into the acceptor phase which was 0.5 M HzS04. The pH of the donor stream was then increased to 11by means of a stream of NaOH. The aliphatic amines were extracted into the acceptor phase of the second separator at pH 7 in which the unprotonated aromatic amines distributed as the efficiency of the first extraction was not complete. After the aromatic amines were washed from the aliphatic fraction, both types of amines could be analyzed separately. Interferences. Changes in physical or chemical properties of the sample may cause changes in the mass transfer from donor phase to the acceptor phase. Thus the technique does not necessarily exclude the need for calibration in the matrix under study. Some of the results obtained when examining the system for interferences are collected in Table VI. Increased salt concentrations usually result in larger partition coefficients and increased viscosity as seen for NaOH in Figure 6 and sodium acetate in Table VI. Influences due to differences in ionic strength may be minimized by either diluting the sample or adding salts to the donor stream in concentrations much higher than those found in the sample. Change in viscosity of the donor stream was studied by using increasing amounts of urea in the samples as urea dissolves only slightly in he hydrophobic liquid membrane. At the same conditions as NH, in Table VI no significant change in the signal for benzylamine was observed for urea concentrations up to 1.0 M and below. At 2.0 M urea concentration the signal had decreased by 6%. With increased concentrations of both isopropylamine and n-propylamine the signal for benzylamine increased while it
2722
ANALYTICAL CHEMISTRY, VOL. 58, NO. 13, NOVEMBER 1986
decreased only slightly for 2-propanol. This effect was not studied further. Comparison with FIA Extraction. In comparison with solvent extractions in FIA (flow injection analysis) (26-29), there seem to be several advantages in favor of the liquid membranes technique presented here also when the stagnant acceptor phase consists of organic solvent. Many of the serious experimental drawbacks of FIA extractions are avoided such as the need for segmentation of the sample with the organic phase, the subsequent phase separation, and the dispersion caused by these moments. The use of organic solvents in large amounts is unnecessary, making the analysis more economical and, in some cases, less hazardous. Available pump tubings are often not compatible with the organic solvent of best choice, requiring special devices for pumping (30). Besides simplicity, the liquid membrane procedure seems to offer greater possibilities for obtaining large enrichment factors in a single step than FIA extractions do, as well as increased selectivity. Slow kinetics in the extraction process might pose some problems using the liquid membrane technique. Also, some types of sample matrices might damage the liquid membrane, a limitation partly shared by the FIA extraction technique. The application of the liquid membrane technique to more complicated matrices like urine and serum are under study at this laboratory together with automation of the technique in combination with on-line coupling to chromatographic systems and the results are very promising. Work on enhanced sensitivity and selectivity of the system should make the method a valuable tool for sample preparation in trace analysis of amines as well as other groups of compounds.
ACKNOWLEDGMENT The author wishes to thank Jan Ake Jonsson, Analytical Chemistry, University of Lund, Sweden, for his valuable advice and assistance in the DreDaration of this Daper. LIST OF SYMBOLS- total area of the membrane in contact with donor A or acceptor phase cross-sectional area of channels in the membrane a separator (cm') b depth of donor channel (cm) concentration of analyte at an arbitrary position C in the donor channel accumulated concentration of analyte at an arbicACC trary point in the acceptor channel concentration of analyte in the sample CO concentration of analyte in the original sample c0,s concentration of analyte at an arbitrary point of CP the peak concentration of nontrapped substance in the CSAT membrane/acceptor phase in equilibrium with the concentration in the donor phase, C$ concentration of nontrapped substance in the acC' ceptor/membrane phase concentration of nontrapped substance in the C0' sample maximum accumulated concentration in the acCO* ceptor channel concentration a t peak maximum Cpma= distribution ratio of nontrapped substance between D membrane/acceptor phase and donor phase diffusion coefficient of analyte in donor phase DA (cm2/s) diffusion coefficient of analyte in acceptor phase Dc (cm2/s) effective diffusion coefficient of analyte in the liquid De membrane (cm2/s) diffusion coefficient of analyte in the organic solDln vent of the liquid membrane volume flow rate of donor stream (cm3/s) f volume flow rate of acceptor stream (cm3/s) F
constant of proportionality in eq 14 overall mass transfer coefficient for transfer between donor phase and acceptor phase (cm/s) mass transfer coefficient in the donor phase (cm/s) mass transfer coefficient in the liquid membrane (cm/s) grouping parameters in eq 13 partition coefficient of analyte between organic phase (in the liquid membrane) and donor phase apparent mass transfer coefficient for the passage of nontrapped substance into the membrane/ acceptor phase (cm/s) apparent mass transfer coefficient for the passage of nontrapped substance out of the acceptor/ membrane phase (cm/s) thickness of liquid membrane (cm) the permeability of the unprotonated amine in the liquid membrane (cm/s) fractional decrease in the mean height of the concentration profile after transport to the detector arbitrary time after start of transport from acceptor channel to detection parameter of integration time interval between the closing of valve I1 and activation of value I11 in Figure 1 residence time of concentration profile in tubings between membrane separator and detector time length of acceptor channel ( VcH/F) (s) mean volume of concentration profile in the acceptor channel volume of acceptor channel (cm3) volume of sample that passes the liquid membrane (cm3) volume of sample introduced to the system (cm3) position in either channel of the membrane separator (cm) time after all the sample has reached the membrane porosity of the liquid membrane matrix tortuosity factor of the liquid membrane matrix variance (second moment) of dispersion function in outlet tubings (2) =r2/24Dc (9) viscosity of the donor phase (cP) contact time between sample and liquid membrane at an arbitrary position in the channels (s) sample introduction time grouping parameter grouping parameter Registry No. Pyridine, 110-86-1;aniline, 62-53-3;o-methylaniline, 95-53-4; m-methylaniline, 108-44-1;2-phenylethylamine, 64-04-0; benzylamine, 100-46-9; sodium acetate, 127-09-3; isopropylamine, 75-31-0;2-propanol, 67-63-0;ammonia, 7664-41-7; n-propylamine, 107-10-8.
LITERATURE CITED Schultz, J. S.;Goddard, J. D.; Suchdeo, S . R. AIChE J . 1974, 2 0 , 417. Way. J. D.; Noble, R . D.; Flynn, T. M.; Sloan, E. D. J. Membr. Sci. 1982, 12, 239. Lee, K.; Evans, D. F.; Cussler, E. L. AIChE J . 1978, 2 4 , 860. Harada, M.; Yamazaki, f.; Adachi, M.; Eguchi, W. J. Chem. Eng. Jpn. 1984, 17, 527. Inokuma, S.; Yabusa, K.; Kuwamura, T. Chem. Lett. 1984, 6 0 7 . Hikita, H.;Ishlkawa, M.; Murakami, T.; Hata. M. PACHEC '83-Proc. -Pac. Chem. Eng. Congr. 3rd 1983, 1 , 321. KUO,Y.; Gregor. H. P. Sep. Sci. Techno/. 1983, 18,421. Stehle. R. G.; Higuchi, W. 1. J . Pharm. Sci. 1967, 5 6 , 1367. Behr, J.; Lehn, J. J. Am. Chem. SOC. 1973, 9 5 , 6108. Audunsson, G.; Dalene, M.; Jonsson. J.; Lijvkvist, P.; Mathiasson, L.; Skarping, G. I n t . J. Environ. Anal. Chem. 1985, 2 0 , 8 5 . Richard, J. J.; Junk, G. A. Anal. Chem. 1984, 5 6 , 1625. Grushka, E.; Meyers, M. N.; Schettler, P. D.; Giddings, J. C. Anal. Chem. 1969, 4 1 , 889. Caceci, M. S.;Cacheris, W. P. Syte 1984, May, 340. Cussler, E. L. Diffusion, Mass Transfer in Fluid Systems ; Cambridge University Press: Cambridge, 1984; pp 47-54, 284-286. Taylor, G. I . Proc. R . SOC.London, A 1953, 2 1 9 , 186. Taylor, G. I. Proc. R . SOC. London, A 1954, 2 2 3 , 446. Aris, R. Proc. R . SOC.London, A 1956, 2 3 5 , 67. Sternberg, J. C. Adv. Choromatogr. 1964, 2 , 205. Grushka, E. J , Phys. Chem. 1972, 7 6 , 2586.
Anal. Chem. 1986, 58, 2723-2726 (20) Kreft. A.; Zuber, A. Chem. Eng. Scl. 1978, 33, 1471. (21) Schlfreen, R. S.; Hanna, D. A.; Bowers, L. D.; Carr, P. W. Anal. Chem. 1977, 4 9 , 1929. (22) Cussler. E. L. Diffusion, Mass Transfer in fluid Systems; Cambrldge University Press: Cambrldge, 1984; pp 52-54. (23) Audunsson, 0.;Mathlasson, L. J . Chfomatogr. 1983, 267, 253. (24) Handbook of Physics and Chemistty; 53rd ed.;The Chemical Rubber Co.: Cleveland, OH, 1972; p D-214. (25) Wiike, C. R.: Chang, P. C. AIChE J . 1955, 1 , 264. (26) Karlberg. B.; Thelander, S. Anal. Chim. Acta 1978, 98, 1. (27) Nord, L.; Karlberg, B. Anal. Chim. Acta 1980, 718, 285.
(28) (29) (30) (31)
2723
Cantwell, F. F.; Sweilek, J. A. Anal. Chem. 1985, 57, 329. Kawase, J. Anal. Chem. 1980, 52, 2124. Karlberg, B.; Thelander, S. Anal. Chim. Acta 1980, 714, 129. Adamson, A. W. phvslcal Chemlsby of Surfaces; Wiley: New York, 1976.
RECEIVED for review March 25,1986. Accepted June 9,1986. The financial support of the Swedish Work Environment Fund is gratefully acknowledged.
Effect of Sidearm Length upon Competitive Alkali Metal Solvent Extraction into Chloroform by Lipophilic Crown Phosphonic Acid Monoalkyl Esters Michael J. Pugia,Grace Ndip, Han Koo Lee,' 11-Woo Yang? and Richard A. Bartsch*
Department of Chemistry and Biochemistry, Texas Tech University, Lubbock, Texas 79409-4260
A homologous serles of sym -bl~4(5)-terl-butylbenzo~16crownJoxyalkylphosphonkacld monoethyl esters has been synthesized and utlllzed In competltlve solvent extractlon of alkall metal catlons from water Into chloroform. The change from Na' extractlon selectlvlty for the lonlzable crown ethers with short sldearms to LI' extractlon selectlvlty for longer armed macrocycles Is Interpreted as a shlft from predominant catlon complexation wlthln the polyether cavlty for the former to predomlnant assoclatlon with the carboxylate group for the latter. Compared wlth a closely related crown ether carboxylk ackl, the extractlon of alkall metal catlons from acMk aqueous solutlons by the crown ether phosphonlc acld monoethyl esters exhlblts much hlgher efflclency.
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Lipophilic crown ether carboxylic acids, such as 1 and 2, are novel reagents for the solvent extraction of alkali metal cations from water into chloroform and toluene (1-3). For solvent extraction, such ionizable crown ethers possess the distinct advantage over neutral crown ether compounds in that movement of the metal cation from the aqueous phase jnto the organic medium does not involve concomitant transfer of the aqueous phase anion (4). In earlier work, we have established that lipophilic group attachment is required to retain crown ether carboxylic acids in the organic phase during solvent extraction (1). Also variation of the lipophilic group attachment site, e.g., in 1 and 2, has been found to influence the selectivity and efficiency of the solvent extraction process (3). Another potentially important structural parameter is the length of the arm that connects the polyether ring to the ionizable group in these metal-ion complexing agents. Unfortunately, systematic structural variation of the pendant arm length for crown ether carboxylic acids presents certain synthetic difficulties. However, a homologous series of crown ether phosphonic acid monoethyl esters 3-6 which have the same polyether and lipophilic group components as crown ether carboxylic acid 2 is more accessible. We now describe Choongwae Pharma Corp., Seoul, Korea. Department of Chemistry, Korea Military Academy, Seoul, Korea. 0003-2700/86/0358-2723$01.50/0
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the preparation of 3-6 and results for the competitive solvent extraction of alkali metal cations from water into chloroform which establish the appropriate arm length for maximal extraction selectivity and efficiency.
EXPERIMENTAL SECTION Apparatus. Alkali metal cation concentrations in the aqueous phases were determined with a Dionex Model 10 ion chromatograph. Concentrations of the organic complexing agent in the CHC13 phases were measured with a Cary Model 17 ultravioletvisible spectrophotometer. Measurements of pH were performed with a Fisher Accumet Model 620 pH meter using a Corning No. 476193 combination electrode. Melting points were taken with a Fisher Johns melting point apparatus and are uncorrected. 'H NMR and IR spectra were determined with a Nicolet MS-X infrared spectrophotometer and a Varian EM-360A nuclear magnetic resonance spectrometer,respectively. Mass spectra were obtained with a Hewlett-Packard 5595B GC/MS. Elemental analysis was performed by Galbraith Laboratories (Knoxville,TN). Reagents. The sources of reagent grade inorganic chemicals and the methods of solvent purification for the extraction experiments were the same as before (4). Tetrahydrofuran and pentane were distilled from LiAlH,. Monoethyl iodomethylphosphoric acid ( 5 ) , sym-hydroxybis[4(5)-tert-butylbenzo]-16crown6 (7) (21,and tetrahydropyranyl-protectedethylene chlorohydrin (6)were prepared by published procedures. Allyl @ 1986 American Chemlcal Society
