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Anal. Chem. 1988, 60, 1694-1699
of the PD chemistry of these and other types of negative ions. Another obvious extension of the work reported here will be to determine the hardware and, in particular, light sources that might be most appropriate for a given application of the PDM-ECD.
ACKNOWLEDGMENT We thank Norman Dovichi for a helpful suggestion concerning the treatment of some of the data reported here. LITERATURE CITED (1) Lovelock, J. E.; Maggs, R. J.; Wade, R. J. Nature(London) 1973, 247,
194-196. (2) Wllknlss, P. E.; Lamontagne, R. A,; Larson, R. E.; Swinnerton, J. W.; Dickson, C. R.; Thompson, T. Nature (London),t%ys. Sci. 1973, 245, 45-47. (3) Lovelock, J. E. Nature (London) 1971, 230,379. (4) Grimsrud, E. P.; Miller, D. A. Anal. Chem. 1978, 5 0 , 1141-1145. (5) Miller, D. A.; Grimsrud, E. P. Anal. Chem. 1979, 57,851-859.
(6) Phillips, M. P.; Goldan, P. D.; Kuster, W. C.; Sievers, R. E.; Fehsenfekl, F. C. Anal. Chem. 1979, 57, 1819-1825. (7) Valkenburg, C. A.; Knighton, W. B.; Grimsrud, E. P. HRC CC, J . High. Resolut. Chromatogr. Chromatogr. Common. 1988, 9 , 320-327. (8) Mandl, A.; Hyman, H. A. Phys. Rev. Leff.1973, 370,417-419. (9) Mandl, A. Phys. Rev. A . 1978, 74, 345-348. (10) Stelner, B. Phys. Rev. 1968, 773, 136-142. (11) Clodius, W. B.; Stehman, R. M.; Woo, S. B. Phys. Rev. A 1983, 27, 333-344. (12) Berry, R. S.;Reimann, C. W.; Spokes, G. N. J. Chem. Phys. 1962, 37, 2278-2290. (13) Webster, C. R.; McDermid, I. S.;Rettmer, C. T. J . Chem. Phys. 1883. 7 8 . 646-651. - - -(14) Dizak, P. S.;Brauman, J. I.J. Am. Chem. S O ~1982, . 704, 13-19. (15) Gygax, R.; McPeters, H. L.; Brauman, J. I. J. Am. Chem. SOC.1979, 10 7 . 2567-2570. (16) Zimmerman, A. H.; Brauman. J. I. J. Am. Chem. SOC. 1977, 99, 3565-3568. (17) Richardson, J. H.; Stephenson, L. M.; Brauman, J. I. J. Chem. Phys. 1973, 59,5088-5076.
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(21) Herbst, E.; Patterson, T. A.; Lineberger, W. C. J . Ch8m. Phys. 1974, 67,1300-1304. (22) Wetzel, D. M.; Brauman, J. I . Chem. Rev. 1987, 87, 607-622. (23) Corderman, R. R.; Lineberger, W. C. Annu. Rev. Phys. Chem. 1979, 30. 347-378. (24)Berry, R. S. Chem. Rev. 1969, 69, 533-542. (25)Drzaic. P. S.;Marks, J.; Brauman, J. I.I n Gas Phase Ion Chemistry, Vol. 3 , Ions and Light; Bowers, M. T.. Ed.; Academic: New York,
1984;pp 167-211.
(26) Mead, R. D.; Stevens, A. E.; Lineberger, W. C. I n Gas Phase Ion
Chemistry, Vol. 3 , Ions and Light; Bowers, M. T., Ed.; Acedemlc: New York. 1984;pp 213-248. (27) Dovlchi, N. J.; Kelter, R. A. Anal. Chem. 1983, 55, 543-549. (28) Aue. W. A.; Siu, K. W. M. J. Chromatogr. 1987, 203,237-244. (29) Gobby, P. L.; Grlmsrud, E. P.; Warden, S. W. Anal. Chem. 1980, 52,
473-482. (30)Grlmsrud, E. P.; Warden, S. W. Anal. Chem. 1980, 52, 1842-1844. (31) Wentworth, W. E.; Chen, E. C. M. I n flectron Capture, Theory and Practice in Chromatography: Zlatkls. A,. Poole, C. F., Eds.; Elsevier: New York, 1981;pp 27-68. (32) Conner, J. J . Chromatogr. 1980, 200, 15-34. (33) Warden, S.W.; Crawford, R. J.; Knighton, W. B.; Grimsrud, E. P. Anal. Chem. 1985, 57,859-665. (34) Siegel, M. W.; McKeown, M. C. J. Chromatogr. 1976, 722, 397. (35) Smith, D.; Adams, N. G. I n Physics of Ion-Ion and Electron-Ion Coilisions; Broulllard, F., McGowan, J. W., Eds.; Plenum: New York,
1983;pp 501-531. (36) Mitchell, J. B. A.; McGowan, J. W. I n Physics of Ion-Ion and flectron-Ion Collisions; Broulllard, F., McGowan, J. W., Eds.; Plenum: New York. 1983;p 279. (37) Blondl, M. A. Comments At. Mol. Phys. 1973, 4 , 85. (38) Slegel, M. W.; Fite, W. L. J. Phys. Chem. 1978, 8 0 , 2871-2881. (39) Knighton. W. B.; Grimsrud, E. P. Anal. Chem. 1982, 5 4 , 1892-1893. (40) Melhuish, W. H. J. Opt. SOC.A m . 1962, 52, 1256. (41) Zimmerman. G.; Chow, L.; Paik, U. J. Am. Chem. SOC. 1958, 8 0 ,
3528-3531. (42) Malmstadt, H. V.; Enke, C. G.; Crouch, S. R. Nectronics and Instrumentation for Sclentisrs ; Benjamin/Cummlngs: Menlo Park, CA, 1981; p 31. (43) Grimsrud, E. P.;Kim, S. H. Anal. Chem. 1979, 57, 537-541.
RECEIVED for review December 17,1987. Accepted April 15, 1988. This work was supported by the Chemical Analysis Division of the National Science Foundation under Grant No. CHE-8711618.
Separation of Silver from Other Metal Cations Using Pyridone and Triazole Macrocycles in Liquid Membrane Systems R. M. Izatt,' G. C. LindH, R. L. Bruening, Peter Huszthy, C. W. McDaniel, J. S. Bradshaw,* and J. J. Christensen Departments of Chemistry and Chemical Engineering, Brigham Young University, Prouo, U t a h 84602 Selectlve transport of Ag' over other metal ions in competitive experiments in bulk and emulslon liquid membrane systems has been accompllshed by uslng macrocycles of the protonionlrabie pyrldone and trlaroie types. The transport of Ag' by the pyrldone macrocycles Involves the cotransport of an anion, whlle transport by the triarole macrocycles can involve either co-anion transport or the counter transport of H'. I n partlcular, the affinity of Ag' for the trlazole moiety, the excellent fit of Ag' Into an 18-crown-6 sized cavity, and the presence of only one proton-Ionizable site per macrocycle molecule comblne to produce hlghly selective transport of Ag+ over Pb2+ and TI+ with trlazoio-18-crown-6 derivatives in membrane systems contalnlng acid recelvlng phases. Transport of alkali-, alkaline-earth-, and several transitionmetal cations Is minimal In slmliar membrane systems and, hence, edectlve transport of Ag' over these catlons is also expected.
Silver has long been an important metal in commerce. It
is valued for its resistance to corrosion and for its use in alloys and jewelry. The recovery of silver from waste solutions has commercial importance, and improved methods for accomplishing this are of interest. In addition, legal limits have been placed on silver concentrations in municipal water supplies. Improved means of lowering silver concentrations are needed, since the acceptable level, 50 ng/mL ( I ) , is exceeded in many culinary water supplies. The analysis, recovery, and/or removal of Ag+ at low concentrations from solutions containing other cations a t high concentrations requires particularly selective reagents. The selectivity of certain macrocycles for Ag+ over other metal ions has been observed (2). Macrocycles of the 15crown-5 and 18-crown-6types containing one or more nitrogen donor atoms have been particularly effective. Incorporation of macrocycles of this type into liquid membrane systems has made possible the separation of Ag+ from other metal cations (3, 4 ) . The main analytical interest in these molecules is in their incorporation into both polymer and liquid membrane ion-selective electrodes (5-11). Recently, we have incorporated proton-ionizable macrocy-
0003-2700/88/0360-1694$01.50/0t2 1988 American Chemical Society
ANALYTICAL CHEMISTRY, VOL. 60, NO. 17, SEPTEMBER 1, 1988 BULK LlOUlD MEMBRANE Source phase
+\
1695
0
1 1,n.O 2 , n z t
Organic m m b r a n e
3, n i 0, R, = H, R, = oclyl
6, A 7, A
I i
benzo cyclohexano
4, n i 1, R1 = H, R, i oclyl 5, n = I , R~ 3 octyl, R, i H
Figure 2. Macrocycles used in this study.
EMULSION LIOUID MEMBRANE
Source phase Orpanic membrane containlng carrier eceiving phase
Figure 1. Schematics of the membrane systems studied.
cles having pyridone (12,13) and triazole (14) moieties into liquid membrane systems. Alkali-metal cations are extracted into the membrane and transported in these systems only when the source-phase pH is greater than the pKa values (10 to 12) of the macrocycles. Thus, alkali cation-macrocycle interaction occurs only when the macrocycle is negatively charged due to the loss of a proton. Transport in these systems is by a proton-coupled mechanism. Alkaline-earth cations are not extracted and transported by these ligands at any source-phase pH values where these cations are soluble. We hypothesized that the interaction of Ag+ with these nitrogen-containing macrocycles might result in a complex of sufficient thermodynamic stability that protons would be dissociated from the macrocycle at pH values well below the macrocycle pK, value. The ability to carry out H+-M+ coupled transport a t pH values in the neutral to slightly acidic region is important in designing separation processes, since Ag+ and most other transition- and post-transition-metal cations precipitate as hydroxides in slightly alkaline solution. Data from this and earlier studies (12-14) will be used to demonstrate that membrane systems can be designed to effect the selective transport of Ag+ over alkali-, alkaline-earth, and several transition- and heavy-metal cations.
EXPERIMENTAL SECTION Bulk liquid membrane transport experiments were carried out as described previously (15-17). The bulk membrane system uses small quantities of carrier and, hence, is a good system for screening carrier properties. Each cell (Figure 1)consisted of a 3.0-mL membrane phase (CH2C12,B&J Chrompure and EM Omnisolve, 1.0 mM in carrier, stirred at 120 rpm by a magnetic stirrer) interfaced to both a 0.8-mL source phase (consisting of either 1.00 M total cation or an equimolar cation mixture of known pH) and a 5.0-mL receiving phase (consisting of either distilled deionized water or a 0.031 M HN03 solution of pH 1.5). Both types of receiving phases were studied so that receiving-phase pH and anion effects on transport would be elucidated. Compounds 1-7 (Figure 2) were used as carriers and were prepared as reported previously (18-20). Reagent grade HN03 (Fisher, Mallinckrodt, Ashland) was used to prepare the acidic receiving phases. The sourcephase salts studied were AgN03 (Baker),Pb(N0J2 (Baker), (Mallinckrodt), Fe(N03)3 (MCB), Ni(N03)2(Chemical Mfg. Corp.), and Cd(N03)2(B&A). The source-phase pH values were 6.9 (AgN03),3.8 (Pb(N03& 4.8 (Zn(NO&J, 1.4 (Fe(NO3I3), 3.7 (Ni(N03)2),4.5 (Cd(NO,),), and 4.0 (AgN03and Pb(N03)2). The receiving phase was sampled after 24 h and analyzed for cation concentration by using a Perkin-Elmer Model 603 atomic absorption spectrophotometer, and for NO3- concentration, by
using a Dionex Model 2010 ion chromatograph. The pH values of the aqueous solutions at the beginning and end of the 24-h period were measured with a Sargent Welch miniature combination pH electrode. All aqueous solutions were prepared with distilled deionized water. Emulsion liquid membrane (Figure 1)experiments were performed with 1 and 5 as carriers, as described previously (21). The experiments were performed by using a system containing 0.001 M AgN03 in the source phase, 0.03 M 1 in toluene (Fisher and EM Omnisolve) as the membrane, and either H20,0.1 M MgS203, (Fluka-Garantie), or 0.031 M HN03 (pH 1.5) as the receiving phase. Experiments involving 5 contained one or two of the salts AgNO,, Pb(N03)2,and TlN03 (Aldrich)at 0.001 M as the source phase, 0.03 M 5 in phenylhexane (Kodak) as membrane, and either 0.1 M MgS203or 0.031 M HN03 (pH 1.5) as the receiving phase. All of the emulsions contained 3% (v/v) sorbitan monooleate (Span 80,IC1 Americas) as surfactant and were stirred at 600 rpm. The initial and final source phases were sampled and analyzed for cation, anion, and H+ content. Emulsion experiments were performed with particular systems to confirm the results of the bulk membrane experiments. Solvent extraction experiments were performed to determine the equilibrium constants (KeJfor the following reactions:
where M = Ag or Pb, L = 5 , and phenylhexane is the organic solvent. Equal volumes of the aqueous and organic phases were stirred at 600 rpm. The time required to reach equilibrium was determined by performing the experiments as a function of time. It was assumed that equilibrium was reached when the cation, anion, and H+ concentrations of the aqueous phase were constant with time. The aqueous phase was 0.001 M in M(N03), where M = Ag+ (n = 1)or Pb2+(n= 2), and the organic phase was 0.03, 0.01, or 0.005 M in 5 for the Pb2+experiments and 0.002,0.001, 0.0005, or 0.00025 M in 5 for the Ag+ experiments. The changes in the aqueous phase NO3- and H+ concentrations were used to determine the amount of Ag+ extraction due to each specific extraction mechanism (eq 1 and 2). The extraction of Pb2+ involved the extraction of two NO3- ions. The K,, values and 1:l stoichiometry of the reactions were determined and calculated according to the method of Ouchi et al. (22). All experiments were performed at least in triplicate. The standard deviations from the mean among values in the bulk membrane measurements are less than 125%,and those in the emulsion membrane and solvent extraction measurements are presented in the Results and Discussion section. Experiments under conditions identical with those described in this section but without the macrocycles present were also performed. In these "blank" experiments, cation fluxes were less than 0.3 X mol.s-1-m-2in the bulk membrane experiments and source-phase cation disappearance was undetectable in the emulsion membrane and solvent extraction experiments. However, in the emulsion membrane blank experiments containing SzO?- in the receiving phase, appreciable Ag+ transport (40%) was observed.
RESULTS AND DISCUSSION In the following material, it will be shown that Ag+ transport with the pyridone macrocycles occurs by a neutral transport
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ANALYTICAL CHEMISTRY, VOL. 60, NO. 17, SEPTEMBER 1, 1988
Table I. Neutral or Co-Anion (A-) a n d Proton-Coupled Carrier (L)-Mediated Transport Mechanisms a n d Associated Driving Forces for a M"+ Cation
type of transport
source
proton-coupled M"+
mechanism membrane receiving
ML
-+
driving force"
Table 11. Comparison of Single a n d Competitive Ag+ a n d PbZ+Fluxes" in a Bulk Liquid Membraneb System Containing Pyridone a n d Triazole Macrocycles a n d Neutral or Acidic Receiving Phases
carrier
1 1 2
M"+
[M"+],/[H+l," [M"+l,/[H+l,"
= source Dhase: r = receiving rhase.
mechanism (neutral macrocycle and cotransport of an anion). Both neutral and proton-coupled mechanisms can be operative with the triazole macrocycles. These mechanisms are diagramed together with the driving force for transport via each mechanism in Table I (23-26). Highly selective Ag+ transport will be shown to occur in competitive systems when the triazole macrocycles are used as carriers and the transport driving forces are controlled by proper adjustment of selected system variables. Transport by 1-7 of Ag+ and Pb2+,generally, was much greater than that of the other cations studied. Macrocycle selectivity between two cations cannot be determined from single-cation systems when both significant extraction and transport of both cations are observed. This is particularly true in comparing cations of differing valency under neutral mechanism transport conditions, since different concentrations of the cotransporting anion are present in the source phase. Hence, competitive systems were studied in detail for Pb2+ vs Ag+ transport. Single-cation experiments were performed with 1-7 for Zn2+,Cd2+,and Ni2+and with 3,5, and 6 for Fe3+ in the bulk liquid membrane system. Fluxes for these systems were 55 X mol-s-1.m-2 or less, except for Cd2+with 3 as carrier, where fluxes were 97 X and 143 X mol-s-1.m-2 for the H20 and acid-receiving phases, respectively. Many of the fluxes were 10 X lo4 mol.s-1.m-2 or less. The transport of alkali- and alkaline-earth-metal cations by 1-7 has been shown to be minimal when neutral and acidic source phases are used (12-14). Hence, selective transport of Ag+ and Pb2+ over these cations by 1-7 is expected. In this assessment, the lack of transport of these other cations is attributed to poor extraction into the membrane phase. Ag+ and Pb2+Transport Using Pyridone Macrocycles. Transport of Ag+ and Pb2+by the pyridone macrocycles (1 and 2) in bulk liquid membrane systems (Table 11) is seen to be greater with a H 2 0 (pH 7) than an acid (pH 1.5) receiving phase. This result is consistent with a neutral transport mechanism (Table I). The presence of NO, (HN03) in the receiving phase reduces the concentration gradient in AgN03 or Pb(NO& between the aqueous phases compared to that for a pure HzO receiving phase. If the transport were proton-coupled, the presence of H+ in the receiving phase would be expected to enhance transport. Transport of Ag+ and Pb2+ by a neutral mechanism using 1 and 2 as carriers was substantiated further by measuring the amount of NO3- cotransport in the single systems containing pure H 2 0 receiving phases. The NO3- transport was sufficient, within experimental error, to indicate a 2:l and 1:l NO3-to cation transport ratio for PbZ+and Ag', respectively. This indicates that Pb2+ and Ag+ transport by 1 and 2 proceeds quantitatively by a neutral mechanism. The effective transport of Ag+ and Pb2+from neutral source phases contrasts with the transport of alkali-metal cations by
2 3 3 4 4 5 5 6 6
7 7
initial receivingphase pH 7 1.5 7 1.5 7 1.5 7 1.5
I 1.5 7 1.5 7 1.5
single systems Ag+ flux Pb2+flux 508 267 641 268 65 533 515 1230 182 750 61 182 269 1125
competitive systems Ag+/Pb2+fluxes
73 15 776 478 394 446 1356 868 246 340 2 2.5 322 326
38118 15213 1551536 100/209 5611 45212 420187 1303173 149/4 60118 47/
