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Anal. Chem. 1996, 68, 366-370
Samarium(III)-Selective Electrode Using Neutral Bis(thiaalkylxanthato)alkanes Didarul A. Chowdhury, Takashi Ogata, and Satsuo Kamata*
Department of Applied Chemistry and Chemical Engineering, Faculty of Engineering, Kagoshima University, 1-21-40 Korimoto, Kagoshima 890, Japan Kousaburo Ohashi
Department of Chemistry, Faculty of Science, Ibaraki University, Bunkyo, Mito 310, Japan
Samarium(III)-selective electrodes constructed on the basis of bis(alkylxanthato)alkanes and bis(thiaalkylxanthato)alkanes are described. As a class, both types of compounds showed quantitative responses toward Sm(III) ion. Among the ionophores tested in this work, the neutral carrier, 1,4-bis(3-thiapentylxanthato)butane, provided an ion-selective electrode almost ideal for determination of Sm(III) in aqueous solution. This electrode exhibits a Nernstian slope of 20 mV per concentration decade, with a detection limit of 5.0 × 10-7 M Sm(NO3)3. Acceptable selectivity was obtained for Sm(III) determination against many of the transition, alkali, alkalineearth, and some of the lighter rare-earth metal ions. Only Cu(II) and Fe(III) interfere seriously. Samarium is an important member of the rare-earth family of elements and is known to constitute strong magnetic material; it has thus found particular use in the production of permanent magnets. The available methods for low-level determination of rare-earth ions in solution include spectrophotometry,1,2 ICPMS,3,4 and ICP-AES.5 Isotope dilution mass spectrometry,6 neutron activation analysis,7,8 X-ray fluorescence spectrometry,9 etc. are also used in some laboratories. These methods are either time consuming, involving multiple sample manipulations, or too expensive for most analytical laboratories. Neutral carrier-based ion-selective electrodes (ISEs) can offer an inexpensive and convenient method of analysis of rare-earth ions in solution, provided acceptable sensitivity and selectivity are achieved. Although the neutral carrier-type ISEs have been successful for determination of a wide variety of metal ions, including the alkali, alkaline-earth, transition, and some other heavy metal ions,10-12 the use of such electrodes in the determination of rare-earth ions has yet to be realized in a practical sense. Only a few reports are (1) Gladilovich, D. B.; Kuba´n ˜, V.; Sommer, L. Talanta 1988, 35, 259-265. (2) Hrdlicka, A.; Havel, J.; Moreno, C.; Valiente, M. Anal. Sci. 1991, 7, 925929. (3) Houk, R. S.; Fassel, V. A.; Flesch, G. D.; Svec, H. J. Anal. Chem. 1980, 52, 2283-2289. (4) Shibata, N.; Fudagawa, N.; Kubota, M. Anal. Chem. 1991, 63, 636-640. (5) Mazzucotelli, A.; DePaz, F.; Magi, E.; Frache, R. Anal. Sci. 1992, 8, 189192. (6) Masuda, A.; Nomura, N.; Tanaka, T. Geochim. Cosmochim. Acta 1973, 37, 239-243. (7) Marsh, S. F. Anal. Chem. 1967, 39, 641-645. (8) Mccown, J. J.; Larsen, J. P. Anal. Chem. 1961, 33, 1003-1007. (9) Cornell, D. H. Pure Appl. Chem. 1993, 65, 2453-2464.
found in the literature on the preparation of rare-earth ISEs. These electrodes are either based on solid-state sensors of rare-earth oxide13,14 or rare-earth compound15 membrane types or contain conventional liquid ion exchange membranes derived from dinonylnaphthalenesulfonic acid,16 bis(2-ethylhexyl)phosphoric acid,17 and tributyl phosphate.14 Some of those electrodes are reported to be highly sensitive to particular rare-earth ions but often suffer from the lack of selectivity and range of linearity required for the practical determination of individual rare-earth ions in a multicomponent solution. In fact, publications dealing with the use of neutral carrier-based ISEs for rare-earth elements have yet to appear in the literature. In our laboratory, a number of sensor materials containing the bis derivatives of alkyl- or thiaalkylxanthates have been synthesized, some of which were used in solvent extraction of metal ions.18 These compounds are composed of two parallel chains of either OS or SOS donor sets, connected by an alkylene chain of variable length. In the usual sense, lanthanoid cations possessing marked A-type character, as defined by Pearson, do not form compounds with ligands containing soft donor atoms but can be forced to do so under special structural and reaction conditions, as reported by Ciampolini and co-workers.19 The authors synthesized a number of lanthanoid(III) complexes with polythia macrocycles of the 18-crown-6 type. From spectral and crystallographic data, it was proved that both the oxygen and the sulfur donors in the macrocycles coordinate with the lanthanoid ion to induce a deca-tetrahedral structure to the ultimate complex. It was interesting to note that the macrocycles containing OO-S(10) Ammann, D.; Bissig, R.; Cimerman, Z.; Fielder, U.; Gu ¨ ggy, M.; Morf, W. E.; Oehme, M.; Osswald, H.; Pretsch, E.; Simon, W. In Ion and Enzyme Electrodes in Biology and Medicine; Kessler, M., Clark, L. C., Jr., Lu ¨ bber, D. W., Silver, I. A., Simon, W., Eds.; Urban & Schwarzenberg: Munich, 1976. (11) Morf, W. E.; Simon, W. In Ion-Selective Electrodes in Analytical Chemistry; Freiser, H., Ed.; Plenum: New York, 1978. (12) Se´nkyr, J.; Ammann, D.; Meier, P. C.; Morf, W. E.; Pretsch, E.; Simon, W. Anal. Chem. 1979, 51, 786-790. (13) Takasaka, Y.; Suzuki, Y. Bull. Chem. Soc. Jpn. 1979, 52, 3455-3456. (14) Zhang, Y.; Wu, J.; Wang, E. Electroanalysis (N. Y.). 1993, 5, 863-867. (15) Suzuki, Y.; Itoh, H.; Nakano, T. Rare Earths Mod. Sci. Technol. 1982, 3, 521-525; Chem. Abstr. 1995, 97, 173997m. (16) Harrell, J. B.; Jones, A. D.; Choppin, G. R. Anal. Chem. 1969, 41, 14591462. (17) Sykut, S.; Dumkiewicz, R.; Dumkiewicz, J. Zesz. Nauk Politech. Slask. Chem. 1983, 108, 145-156; Chem. Abstr. 1985, 102, 38825k. (18) Chowdhury, D. A.; Mendoza, C. S.; Kamata, S. Solvent Extr. Ion Exch. 1994, 12, 1051-1071. (19) Ciampolini, M.; Mealli, C.; Nardi, N. J. Chem. Soc. Dalton Trans. 1980, 376-382.
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OO-S donor sets formed larger number of lanthanoid complexes (Ln ) La, Ce, Pr, Nd, Sm, Eu, Ho, Yb) than those containing SSO-SS-O or SS-S-SS-S donor sets (Ln ) Eu, Sm, Yb). In addition to the macrocyclic effect, the formation of the lanthanoid complexes was also found to be dependent on the nature of the solvent used. The ligands presented in this work are not macrocyclic but contain OS donor combinations in a chain structure having enough flexibility to entrap a metal ion in a definite geometry. Unlike the cyclic ligands, the open chain structure is also believed to be free from steric constraints during coordination with a metal ion. It was thus interesting to devise rare-earth electrodes based on the bis(alkylxanthato)alkanes and bis(thiaalkylxanthato)alkanes. In this paper, we report on the use of these ionophores as components in Sm(III)-sensitive sensors, with special emphasis on 1,4-bis(3-thiapentylxanthato)butane. EXPERIMENTAL SECTION Reagents. Stock solutions (10-1 M) of samarium(III) were prepared by dissolving Sm(NO3)3‚6H2O (99.5% purity, Wako Pure Chemicals, Tokyo, Japan) in distilled, deionized water, and the working solutions were prepared by serial dilution. Tetrahydrofuran (THF), used for dissolving the membrane components, and the plasticizer bis(2-ethylhexyl) sebacate (DOS) were purchased from Fluka Chemie AG (Switzerland). The plasticizers o-nitrophenyl octyl ether (NPOE), 2-fluoro-2′-nitrodiphenyl ether (FNDPE), and the anion excluder potassium tetrakis(4-chlorophenyl)borate (KTCPB) were obtained from Dojindo Laboratories (Kumamoto, Japan). All other chemicals used in analytical determinations were of either analytical reagent or guaranteed reagent grade purity. Synthesis of Ionophores. The syntheses of bis(butylxanthato)alkanes (ligands I-VI in Table 1) have been described earlier.18 The syntheses of bis(thiaalkylxanthato)alkanes (ligands VII-XII) were accomplished in ways similar to the case of bis(butylxanthato)alkanes, except that a 2-(alkylthio)ethanol was used instead of an alcohol as one of the starting materials. The purified compounds appeared as faintly colored clear liquids possessing boiling points >200 °C and having specific gravity values of 1.11.2. The elemental analysis and spectral data for ligands I-VI were published in the literature.18 The elemental analysis data obtained for ligands VII-XII are given in the following. Ligand VII, calcd for C17H32O2S6: C, 44.31; H, 7.00. Found: C, 44.32; H, 6.98. Ligand VIII, calcd for C18H34O2S6: C, 45.53; H, 7.22. Found: C, 45.35; H, 7.17. Ligand IX, calcd for C17H32O2S6: C, 44.31; H, 7.00. Found: C, 44.42; H, 7.02. Ligand X, calcd for C18H34O2S6: C, 45.53; H, 7.22. Found: C, 45.62; H, 7.18. Ligand XI, calcd for C13H24O2S6: C, 38.58; H, 5.98. Found: C, 38.32; H, 5.82. Ligand XII, calcd for C14H26O2S6: C, 40.16; H, 6.26. Found: C, 40.13; H, 6.21. Structural identification of the newly synthesized compounds was done by using 1H-NMR and IR data. Membrane Preparation. The PVC membrane of the sensor was immobilized on a carbon rod of 4 mm diameter, obtained from Nippon Carbon Co. (Yokohama, Japan). The clean rod was tapered slightly at one end to remove sharp edges by polishing against a fine sandpaper. The ionophore (30.6 mg; 10.2 wt %), the plasticiser (181.2 mg; 60.4 wt %), PVC (86.1 mg; 28.7 wt %), and KTCPB (2.1 mg; 0.7 wt %) were dissolved in 2.5 mL of THF. A carbon rod was then dipped into the homogeneous coating solution up to 6 mm depth, withdrawn quickly, and held upsidedown for a few seconds to allow the THF to dry out. The process was repeated several times until a uniform coating was formed
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Table 1. Structure and Compositions of the Synthetic Ionophores
on the tapered end of the carbon rod. The membrane so formed was dried for 24 h at room temperature (25 ( 2 °C), and the rod was covered with paraffin film, keeping the exposed area of the membrane (5 mm) and the contact point open. Usual thickness of the membranes varied from 10 to 15 µm. Each electrode was conditioned before potentiometric measurements by immersing it in 10-3 M Sm(NO3)3 solution for 24 h. The compounds used in different trials as the plasticizer included NPOE, DOS, and FNDPE. Electrode System and EMF Measurements. Cell configurations used for potentiometric measurements were of the type
membrane-coated carbon rod electrode/sample solution/ reference electrode EMF measurements were carried out with respect to a saturated calomel electrode (Iwaki Glass, Tokyo, Model IW-067) coupled with a Orion Research microprocessor ion analyzer/901. The concentration of Sm(III) ion in the sample solution was varied from 10-1 to 10-9 M in stirred solution. The EMF values were recorded when the reading of the ion analyzer became stable. The pH values of the sample solutions were adjusted with NaOH and HNO3 and measured with a TOA Model HM7E pH meter (TOA Electric Co.,Tokyo, Japan). Unless otherwise stated, the pH of the analyte solutions was fixed at around pH 5.5. The
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Table 2. Properties of the Sm(III)-Selective Electrodes Based on Bis(alkylxanthato)- and Bis(thiaalkylxanthato)alkanes
(
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Figure 1. EMF vs aSm(III) plot for Sm(III) electrodes containing bis(isobutylxanthato)alkanes of various chain length. (O) n ) 1, (b) 2, (0) 3, (9) 4. Individual points in each curve represent the mean of three measurements made on a single electrode for each ligand. The ranges of RSD(%) values of the measurements were 3.1-5.1, 1.84.7, 2.4-4.8, and 1.7-3.3 respectively, for n ) 1, 2, 3, and 4 ligands.
activities of metal ions were based on their activity coefficient, γ, as calculated from the modified Debye-Hu¨ckel equation:
log γ ) -0.511z2[xµ/(1 + 1.5xµ) - 0.2µ] where µ is the ionic strength and z is the valence of the concerned ion. All the EMF measurements were performed at 25 ( 2 °C. Response times for the electrodes were evaluated by measuring the time required to achieve a level within 1 mV of the steadystate potential reading, following a rapid increase in the Sm(III) concentration of the analyte solution from 10-4 to 10-3 M, as recommended by IUPAC.20 RESULTS AND DISCUSSION Response Characteristics of Sm(III)-Selective Electrodes. The structures and compositions of the ionophores used in the Sm(III)-selective electrodes are given in Table 1. These compounds contain sulfur and oxygen as potential donor atoms. If we neglect the thioketo sulfur atoms because of their poor coordinating ability,21 two ethereal oxygen and two thioether sulfur atoms are left for coordination in bis(alkylxanthato)alkanes. Two additional thioether sulfur atoms are present in the bis(thiaalkylxanthato)alkanes. On the basis of the lanthanoid-sulfur coordination in lanthanoid complexes with polythia macrocycles of 18crown-6 type,19 the present ligands are expected to utilize the SOS or OS donor sets to bind with Sm(III) ion, the stabilities being defined probably by the length of the alkylene chain connecting the two xanthate groups.18 Accordingly, Sm(III)-selective electrodes containing ligands I-IV were constructed in NPOE media to evaluate the effect of the alkylene chain length on the Sm(III) ion response. The results, as shown in Figure 1, indicate that a chain distance of 3 or 4 carbon atoms (i.e., n ) 3 or 4 in Table 1) between the bridging sulfur atoms is necessary to obtain a (20) IUPAC Recommendations for Nomenclature of Ion Selective Electrodes. Pure Appl. Chem. 1976, 48, 127. (21) Akbar Ali, M.; Livingstone, S. E. Coord. Chem. Rev. 1974, 13, 101-131.
sensor
slope (mV)a
detection limit (M)b
responce time (s)b
III IV V VI VII VIII IX X XI XII
19.4 ( 0.6 19.2 ( 0.3 18.0 ( 0.3 19.2 ( 0.3 19.4 ( 0.5 19.1 ( 0.2 16.9 ( 0.7 19.1 ( 0.2 19.2 ( 0.4 20.0 ( 0.3
1 × 10-6 1 × 10-6 1 × 10-6 1 × 10-5 1 × 10-5 5 × 10-7 1 × 10-5 5 × 10-7 1 × 10-6 5 × 10-7
12 8 15 10 10 10 15 8 12 5
a The values are based on six measurements made on two separate electrodes for each sensor. b Detection limit and response time values represent the average of two measurements, each made on two different electrodes.
meaningful response for the Sm(III) electrode. Further studies were thus carried out by taking the bis-xanthates separated by propylene or butylene chains (ligands III-XII). To characterize the present ionophores in terms of Sm(III) response and to test their suitability in samarium determination, a series of membrane electrodes was fabricated incorporating each ligand with NPOE as the plasticizer. The analytical performances were evaluated for each of these electrodes from their EMF vs log aSm(III) plots. The results, as summarized in Table 2, indicate that both the bis(alkylxanthato)- and bis(thiaalkylxanthato)alkanes (n ) 3, 4) act as active ionophores to induce quantitative responses toward Sm(III) ion. Ionophores in both classes produce slopes of about 19 mV per decade of concentration change in their respective response versus activity profiles. These values are representative of the Nernstian slope values expected for the uptake of a tripositive ion into the carrier membrane. In general, the n ) 4 ligands showed better sensitivities for Sm(III) in terms of the observed detection limits or response times when compared with those of the n ) 3 ligands. This may be due to the fact that the larger size of the ligand cavity in n ) 4 ligands facilitates the easier adaptation and quicker release of the Sm(III) ion. The bis(thiaalkylxanthato)alkanes (ligands VII-XII, n ) 4) are characterized with lower detection limits and shorter response times than those of the corresponding bis(alkylxanthato)alkanes in n ) 4 compounds. This would suggest that the additional sulfur atoms introduced in the bis(thiaalkylxanthato)alkanes contribute to the binding of Sm(III) inside the membrane phase. By comparing the electrode responses for ligands VII-XII, it appears that bulky alkyl groups (viz., 2-methylpropyl, n-butyl) attached to the bis molecule would disfavor Sm(III) uptake. On consideration of the above facts and the response profiles of the ionophores studied, the ion-selective electrode based on 1,4-bis(3-thiapentylxanthato)butane (ligand XII) was determined to be the most ideal for determination of Sm(III) ion in aqueous solutions. Effect of the Membrane Medium. The potentiometric response of an ion-selective electrode based on neutral carrier ionophore is greatly influenced by the polarity of the membrane medium, which is in turn defined by the dielectric constants of the major membrane component.22 A decrease in the membrane (22) Morf, W. E. The Principles of Ion Selective Electrodes and Membrane Transport; Elsevier: New York, 1981.
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Figure 2. Effect of the polarity of the membrane medium on the response curve of Sm(III). Membranes prepared with (O) DOS, (b) NPOE, and (0) FNDPE. The extents of error observed for three measurements of potential on indifferent electrodes were in the ranges 3.1-7.0%, 2.8-4.8%, and 2.8-5.1% of the mean value for electrodes with DOS, NPOE, and FNDPE, respectively.
polarity usually results in an increased preference for monovalent over divalent cations. The effect of membrane polarity on the Sm(III) response was studied on electrodes containing three types of plasticizers having different dielectric constants. As evident in Figure 2, the EMF response of the electrode containing DOS ( ) 12) is not well defined, showing only a limited range of linearity, whereas those for NPOE ( ) 25) and FNDPE ( ) 50) provide useful linearity from about 1 × 10-7 to 5 × 10-3 M Sm(III) concentrations. The results indicate that an increased polarity of the membrane would lead to a lower detection limit for samarium determination with the present ionophores. This rather unusual effect of the plasticizer indicates that nonpolar solvents like DOS are not suitable for fabrication of ion-selective electrodes responsive to trivalent cations, viz., Sm(III). Effect of pH on Sm(III) Response. The effect of H+ ion concentration on the electrode response was studied in a 10-3 M Sm(NO3)3 solution, where the pH was adjusted with HNO3 and NaOH. As seen in Figure 3, the Sm(III) electrode based on 1,4bis(3-thiapentylxanthato)butane seems to respond only to activity of free Sm(III) ion in the acidity range below pH 4.5. Above pH 7.0, the Sm(III) signal gradually diminishes with further increases in the value of pH. This is probably because of the formation of Sm(OH)3 in the system, which has been identified as a limitation of rare-earth determination by other methods.23 In the acidic range, i.e., at pH lower than 4.5, the electrode response increased rather irregularly with increasing analyte acidity. At such high acidities, the membrane may extract H+ ions in addition to Sm(III) ions,24 or the electrode may be responsive to hydrolysis products of Sm(III) possessing net charges lower than that of free Sm(III) ion. If the Sm(III) activities are to be measured in acidic media with the present electrode, corrections must be applied to account for those factors. The useful acidity for the Sm(III) electrode under study thus falls in the range of pH 4.5-6.7. (23) Wang, W.; Chen, Y.; Wu, M. Analyst 1984, 109, 281-286. (24) Kojima, R.; Kamata, S. Anal. Sci. 1994, 10, 409-412.
Figure 3. Effect of pH on the calibration plot of Sm(III) with ligand XII in NPOE plasticizer. The points in this plot are based on three individual measurements on a single electrode. The SD values were in the range 1.8-6.2% of the corresponding mean value of the potential.
Figure 4. Selectivity coefficient (log KpotSm,B) values for Sm(III)selective electrode based on 1,4-bis(3-thiapentylxanthato)butane (ligand XII). The horizontal bars for each of the interferring ions are the averages of two measurements made on two separate electrodes.
Selectivity of Sm(III) Determination. Potentiometric selectivity coefficient (KpotSm,B) values for the present electrode were determined by the matched potential method25 by changing the concentration of the interfering ions in a 10-5 M Sm(III) solution. The selectivity testing of the electrode with ligand XII was carried out in two plasticizer media, namely, NPOE and FNDPE, as shown in Figure 4. Monovalent cations (K+, Na+, NH4+) are equally rejected by the Sm(III) electrode in both media, the log KpotSm,B values being in the range -2.81 to -3.30. The rejection of divalent cations (Mg2+, Ca2+, Ni2+, Zn2+, Pb2+) is favored more in NPOE medium than in FNDPE, with both plasticizers still showing selectivity coefficients acceptable for Sm(III) determination in the presence of those ions. Selectivity against tripositive Al3+ and Cr3+ is also in the safe region in both media. The most important characteristic of the Sm(III)-electrode based on ligand XII is its selectivity against other rare-earth(III) cations. As evident from Figure 4, the selectivity coefficient values for La3+, Ce3+, Pr3+, Nd3+, and Gd3+ are in the range -1.5 to -2.4 in NPOE. These values fall in the range -1.0 to -2.1 in FNDPE. These results (25) Gadzekpo, V. P.; Christian, G. D. Anal. Chim. Acta 1984, 164, 279-283.
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clearly demonstrate that the ionophore having the bis(thiaalkylxanthato)alkane structure can differentiate between the individual rare-earth ions, which was impossible with rare-earth electrodes based on a conventional solid-state sensor.13 It may also be noticed that the present electrode is very selective for determination of Sm(III) in the presence of the neighboring Nd3+ ion. The observed Sm(III)-selectivity of the present electrode (ligand XII) can be explained in the light of the conventional idea of the cation-cavity size relationship,26 assuming that the ionophores can adopt a cavity-like structure at the event of complexation with the metal ions. Nevertheless, other factors, such as the membrane polarity contributing to the stability of the cation-ionophore complex, play definite roles in imposing the observed cation selectivity. If we consider the lanthanide(III) ions under study, the ionic diameter (Å) increases in the order Gd3+ (1.94) < Sm3+ (2.00) < Nd3+ (2.08) < Pr3+ (2.21) < Ce3+ (2.14) < La3+ (2.28). A computer-aided molecular modeling estimation of the ionophore XII shows that a separation of ∼2.29-2.61 Å is possible between the two parallel chains containing the donor atoms, as defined by the allowed rotations along the bridging alkylene chain. If we make allowance for the actual bond lengths for Sm-S and Sm-O coordination,19 it appears that the most stable structural conformation in ionophore XII provides the optimum cavity size to accommodate Sm3+ ion, whereas the other lanthanide(III) ions are rejected according to the deviation from this optimal condition. Thus, the observed selectivity pattern of the present Sm(III) electrode against other rare-earth ions can be attributed to the structural flexibility of the ionophores. The metal ions, the presence of which might interfere with the determination of Sm(III), are the Cu2+ and Fe3+ ions and, to some extent, Pb2+ ion. These ions are known to produce interferences in other ionselective electrodes based on sulfur-ionophores12 and must be removed, preferably by complex formation prior to samarium (26) Massaux, J.; Desreux, J. F. J. Am. Chem. Soc. 1982, 104, 2967-2972.
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analysis. Similar interferences may also be expected from very soft metal cations like Ag+ and Hg2+, because of the presence of soft sulfur donors in the ionophore structure. The coated carbon rod electrodes based on ligand XII and NPOE were found to give reproducible responses for at least 2 weeks after use. The test of reproducibility was carried out on two electrodes, selected from different batches of membrane preparation. The slope values of the calibration curves were measured on each electrode at alternate days employing three concentration points, namely, 10-5, 10-4, and 10-3 M Sm(NO3)3 solution of pH 5.5. The procedure was followed for 25 days. Both electrodes exhibited slope values above 19 mV up to 15 days but deteriorated to below 18 mV at the end of 25th day. The mean value of the observed slope for measurements up to the 15th day (number of measurements ) 8 + 8) on the two electrodes came out to be 19.67 ( 0.58 mV per decade change in concentration of the analyte. CONCLUSIONS This work describes the rare-earth ion sensitivity of bis(alkylxanthato)alkanes and bis(thiaalkylxanthato)alkanes as ionophores in membrane electrodes on the basis of potentiometric data. The ISE incorporating 1,4-bis(3-thiapentylxanthato)butane may, in fact, be useful for low-level (10-3-10-7 M) determination of samarium in the absence of free Cu2+ and Fe3+ ions. However, the mechanism of Sm(III) extraction by the membrane is to be evaluated to define these electrodes theoretically.
Received for review August 10, 1995. Accepted November 4, 1995.X AC950814B X
Abstract published in Advance ACS Abstracts, December 15, 1995.
