Anal. Chem. 1984. 56,297-298
for this limitation, a strong solvent (tetrahydrofuran/toluene) in which solutes would have a reasonably high mobility, was used for the noncontinuous development. The Regis SB/CD Chamber should also be useful for two-dimensional TLC. While the length of the plate cannot be fully optimized, continuous development can be performed in both directions. The use of the SB/CD chamber for optimized continuous development TLC has recently been described (8). The results presented in this paper lead us to conclude that careful optimization of operating conditions for continuous development TLC using binary solvents should allow the routine analysis of moderately complex mixtures by TLC.
LITERATURE CITED Nurok, D.; Becker, R. M.; Sassic, K. A. Anal. Chem. 1982, 5 4 , 1955-1959. Thoma, J. A. Anal. Chem. 1963, 35, 214-224. Jupille, T. H.; Perry, J. A. Science 1978, 194, 288-293. Nurok, D.; Zlatkis, A. Carbohydr. Res. 1980, 8 1 , 167-172. Mottier, M.; Potterat, M. Anal. Chim. Acta 1955, 13, 46-56. Soczewinski, E.; Golkiewicz, E.; Szumila, H. J . Chromafogr. 1969, 45, 1-13.
297
(7) Nurok, D.; Richard, M. J. Anal. Chem. 1981, 5 3 , 563-564. (8) Tecklenburg, R. E., Jr.; Becker, R. M.; Johnson, E. K.; Nurok, D. Anal. Chem. 1983, 55, 2196-2199. (9) Tecklenburg, R. E., Jr.; Maidak, B. L.; Nurok, D.HRC CC, J . High Res. Chromatogr Chromatogr . Commun. 1983: 6 , 627-628. (10) Touchstone, J. C.; Dobbins, M. F. Practice of Thin Layer Chromatography"; Wiley: New York, 1978. (1 1) Gonnord, M. F.; Levi, F. J.; Guiochon, G. J . Chromatog. 1983, 264, 1-6.
.
'
Present address: Department of Medical Genetics, Indiana University School of Medicine, Indianapolis, I N 46223.
David Nurok* Ronald E. Tecklenburg, Jr. Bonnie L. Maidak' Department of Chemistry Indiana University-Purdue University a t Indianapolis P.O. Box 647 Indianapolis, Indiana 46223 RECEIVED for review August 12, 1983. Accepted November 4, 1983. This work was supported by grants from the Dow Chemical Co. and the Society for Analytical Chemists of Pittsburgh.
Chemical Pretreatment of Silver Wire Containing Copper for Preparation of Silver/Silver Sulfide Ion Selective Electrode Sir: The silver/silver sulfide ion selective electrode (ISE) has been produced in a variety of forms (1-14). While the membrane type (2-12) reportedly exhibits superior performance characteristics in comparison to the type derived from solid silver (3), the latter offers decided advantages in terms of cost and convenience. We wish to report that a microscale silver/silver sulfide ISE can be readily prepared from silver wire (0.7 mm diameter) which contains 2.5-6% Cu (w/w) by exposing the wire to a deaerated aqueous solution of sodium sulfide, ascorbic acid, and ethylenediaminetetraaceticacid (SAOB-11) (15) for 2 days. Although this procedure is similar to one reported in the manufacturers' literature (15, 16), it is significant that a successful ISE could not be prepared by this technique if pure silver (99.99%) was used or if the SAOB-I1 solution was not deaerated. Finally, the performance of electrodes obtained in this manner was basically indistinguishable from that of a commercially available electrode of the membrane type (Orion Research, Inc., Model 94-16). EXPERIMENTAL SECTION Reagents. All chemicals were of analytical grade and were used without further purification. Calibration solutions were prepared from silver nitrate or sodium sulfide nonahydrate. The standard sulfide solutions were prepared in either SAOB-I1(15) or 0.1 M NaOH which contained ascorbic acid (20 ppt). The concentration of the sulfide standards was confirmed by potentiometric titration with 0.1 M PbNO, using a commercially available ISE (see above) as the sensor in combination with a double junction calomel reference electrode (Orion Research, Inc., Model 90-02). The outer filling solution consisted of an aqueous solution of KNO, (10% w/w). Preparation of the Ag/Ag,S wire electrode. A silver wire (0.7 mm diameter) was sealed within a glass tube with epoxy cement (Elmers) so that only the end of the wire was exposed. After the cement had dried, the end of the wire was polished with fine grade emery paper. Then, the electrode was immersed in
0.1 M Na2S in SAOB-I1for 2 days. A thin uniform film of Ag2S gradually formed during this period. The nature of the surface was confirmed by scanning electron microscopy and energy dispersive X-ray fluorescence. Finally, the electrodes were rinsed with distilled deionized water and stored in saturated Ag2S solution prior to use. If pure silver wire was employed as the substrate or the sulfide solution was not deaerated, no Ag2Slayer formed even after 6 days of exposure. Procedure. All potentiometric studies were carried out with a digital voltmeter (Orion Research, Inc., Model 601A or Fluke Model SOOOA) in a double-walled thermostated cell which was maintained at 25.0 f 0.1 "C. Connections to the electrodes were made via an electrode switch (Orion Research, Inc., Model 605). The double junction reference electrode mentioned above was employed. All pH measurements were made with a common glass electrode (Sargent Model 3-30050-15C). The measurements of response to Ag+ were made upon serial dilutions of a standard 0.1 M AgNO, solution with 0.1 M NaN0,. Similarly, sulfide ion standards were prepared by serial dilution of the 0.1 M stock Na2Ssolution with 1M NaOH solution which contained ascorbic acid at the 20 ppt level. Potentiometric titrations were carried out in the conventional manner. The titrant was delivered in 0.1-mL increments with a 2.0-mL syringe buret (Gilmont).
RESULTS AND DISCUSSION Optimal electrode characteristics in terms of reproducibility and the extent of the linear relationship between potential and concentration were obtained when the electrode was exposed to the deaerated SAOB-I1 solution for no more nor less than 2 days. The requirement for adulterated silver wire suggests that the formation of the Ag2Slayer is a t least dramatically facilitated by some anodic corrosion process ( I 7). Once prepared, the electrodes can be stored for long periods under saturated silver sulfide solution or briefly ( 1 week) in air with no noticeable effect. The equilibrium potentials observed during the successive dilution of standard solutions as described above are depicted in Figure 1. Linear response extends over a t least 4 decades
0003-2700/84/0356-0297$01.50/00 1984 American Chemical Society
Anal. Chem. 1984, 56, 298-299
298
ACKNOWLEDGMENT We thank the Board of Foreign Scholarships for an award under the Mutual Education Exchange (Fulbright) Program. N.R. also acknowledges the Yugoslav-American Commission for Educational Exchanges for selection. Registry No. Ag, 7440-22-4;Cu, 7440-50-8;Ag,S, 21548-73-2; Na2S,1313-82-2;ascorbic acid, 50-81-7;ethylenediaminetetraacetic acid, 60-00-4. LITERATURE CITED -700 - f
I
4
I
I -
d
.--L-LpL-L.Ap
7
6
5
4
3
2 -log cs2-
Figure 1. Response of the wire silver/silver sulfide electrode to silver ion and sulfide ion in aqueous solution at constant ionic strength: (curve 1) response to silver ion; (curve 2) response to sulfide ions.
in each case with essentially Nernstian slopes of 57.5 mV/ decade (Ag+)and -28.3 mV/decade (S2-). These observations are in accord with previous evaluations of commercially available electrodes (17,18). Also, the influence of pH upon to potential a t a fixed analytical sulfide concentration (loV3 M) is in accord with the literature ( 2 , 19, 20). A number of divalent cations were examined as possible interferents by using a potentiometric separate solution technique (21) at constant pH. Also, a mixed solution method (22)was employed to investigate a variety of anions. Essentially, no interference was generated by the following species: Co2+,Ca2+,Zn2+,Pb2+,C1-, Br-, I-, SCN-, NO:-, S2O2-, and C,04'-. Long-term exposure of the wire electrode to Cu2+degraded both its reproducibility and lifetime. Even brief exposure to Hg2+destroyed the Ag,S layer and, hence, the ion selective response. The same effect occurs with the commercially available electrode but at a much reduced rate. In conclusion, a remarkably inexpensive yet viable Ag/ Ag,S ISE can be produced with the aforementioned procedure given the rather unexpected condition that the Ag wire employed is adulterated with copper.
Koryta, J. Anal. Chim. Acta 1972, 6 1 , 329-411. Hseu, T. M.; Rechnitz, G. A. Anal. Chem. 1968. 4 0 , 1054-1060. Light, T. S.;Swartz, J. L. Anal. Lett. 1968, 1 , 825-836. Bock, R.; Puff, H. L. 2.Anal. Chem. 1968,240, 381-386. Ruzicka, J.; Lamm, C. G. Anal. Chim. Acta 1971, 5 4 , 1-12. Vesely, J.; Jensen, 0. J.; Nicholaisen, B. Anal. Chim. Acta 1972, 6 2 , 1-13. Popescu, I.C.; Liteanu, C. Rev. Roum. Chim. 1972, 77, 1629-1633. Pipay, M. K.; Tbth, K.; Izvekov, V.; Punbor, E. Anal. Chim. Acta 1973,6 4 , 409-415. Clysters, H.; Adams, F.; Verbeek, F. Anal. Chim. Acta 1976, 8 3 , 27-38. Sekerka, I.; Lechner, J. F. Anal. Chim. Acta 1977, 9 3 , 139-144. Clysters, H.; Adams, F. Anal. Chlm. Acta 1977,92, 251-260. Gulens, J.; Jessome, K.; Macneil, C . K. Anal. Chlm. Acta 1978. 96, 23-29. Anfalt, T.; Jagner, D. Anal. Chim. Acta 1971,5 6 , 477-481. Jensen, J. B. Anal. Chlm. Acta 1975, 7 6 , 279-287. "Instruction Manual, Sulfide/Silver Eiectrbde Model 94-16"; Orion Research, Inc.: Cambridge, MA, 1977. Electrode Ihstructions E-4"; Instrunient Manufacturing Division, Fischer Scientific Co., April 1978. Gulens, J.; Ikeda, 8. Anal. Chem. 1976,5 0 , 782-787. Crombie, D. J.; Moody, G. J.; Thomas, J. D. R. Anal. Chim. Acta 1975,8 0 , .1-8. . RadiE, N.; MilisiE, M. Anal. Lett. 1980, 13, 1013-1030. Laitinen, H. A:; Hseu, T. M. Anal. Chem. lQ79,5 1 , 1550-1552. Ammann, D.;Pretsch, E.; Simon, W. knal. Lett. 1972, 5 , 843-850. Pungor, E.; TBth, K.; Hrab&zy-Pa'll, A. Pure Appl. Chem. 1979, 57, 1913-1980.
' On leave from the University of Split, Split, Yugoslavia. Njegomir RadiE' Kevin J. Mulligan Harry B. Mark, Jr.* Department of Chemistry University of Cincinnati Cincinnati, Ohio 45221 RECEIVED for review September 14,1983. Accepted November 7, 1983. This research was supported in part by the National Science Foundation, Grant No. NSF CHE 8205873.
Exchange of Comments on Estimation of Association Constants of Bimolecular Complexes Sir: A recent paper by Horman and Dreux ( I ) shows a method of estimating association constants of bimolecular complexes from NMR chemical shift data obtained from solutions prepared with equimolar quantities of the reactants. The authors illustrate their method with data observed from the complexation of s-trinitrobenzene with hexamethylbenzene. I wish to comment on this method as applied to these data and to suggest an alternative procedure. The equilibrium under consideration is A B = C, where C is the bimolecular complex. In general the reactant concentrations are initially [A], and [B], but these are set equal in this case. The chemical shift of an atom of species A observed in an equilibrated solution is 6. At the rapid exchange limit this shift is the mole weighted average of the
+
intrinsic chemical shift of uncomplexed A and the intrinsic shift 6c of A in the complex. The system behavior is represented by a set of equations which include two conservation relationships (1) [AI, = [AI + [CI
[Blo = [BI an equilibrium expression
+ [CI
(2)
(3) and the NMR chemical shift equation
[AI06 = [AIS, + [CIS,
0 1984 American Chemical Society 0003-2700/84/0356-0298$01.50/0
(4)
