Edwin J. Goller Merrimack College North Andover, Massachusetts
Cation Analysis with Thin Layer Chromatography
Reported here is an adaptation of the familiar technique of thin layer chromatography (TLC) to an undergraduate laboratory operation for cation analysis. While several such schemes employing paper chromatography have been devised (I-,$), a major portion of the work with TLC has been devoted to the separation of organic materials (5-8). One of the more comprehensive works published on cation analysis with TLC is that of Seiler, et al. (9-13). The procedure as developed here, while less comprehensive, has the advantages of being simple, rapid, inexpensive, and easily adaptable to any present undergraduate curriculum. When a judicious selection of cations is made, it is possible to analyze unknown samples without the necessity of carrying out preliminary experiments with known mixtures. Preparation of Adsorber and Chromatoplates The suspension of adsorber (AlzOa) and the preparation of chromatoplates are accomplished using a procedure similar to that described by J. J. Peifer (13): Thoroughly wet 60 g aluminum oxide G' with a minimum amount of chloroform-methanol (7:3, w/w), and dilute the mixture until the total volume of solvent is
442. / Journal of Chemical Educofion
100 rnl. The suspension may be used immediately or stored for several months in a tightly sealed container. The chromatoplates, measuring 5 X 10.5 cm, are cleaned with detergent, rinsed with distilled water, and then rinsed with 50% aqueous methanol acidified with hydrochloric acid. When dry, two plates are sandwiched together and held in place by means of thin tape strips. The strip across the top also serves to fasten a small piece of string to the plates. This string is used to withdraw the sandwiched pair after they have been immersed in the alumina suspension. When the solvent evaporates, the plates are separated, and the adsorber adhering to the edges is removed. The coated plates are then exposed to steam for approximately 30 see, and oven-dried at 110' for 1 hr.
The Experiment The developing solvent is prepared by dissolving sufficient NaBr in 1 M HCI, such that the resultant solution is 0.5 M in bromide ion. Then an equal volume of isopropanol is added to obtain a 50% (u/v) solution. 'Aluminum Oxide G is distributed by Brinkmann Instmments, Ine., New York, and contains edcium sulfate as binder.
Results for Identification of Cations
Reagent
Color
Rr
(NHM (NHM (NHM (NH,),S (NHM
Black Lt. yellow Blsck Grey-brown Black Brown Dk. yellow Black
0.95 0.65 0.60 0.60 0.4.5 0 . 4 5 (tails) 0.25 0.00
(NHM (NHM (NHM
Figure 1 (left). Beaker Plate in ~ l ~ c ~ .
and filter
p o p e r strip$.
Figure 2 Irightl.
The indicator solutions of dimethyl glyoxime (DMG) and an~moniumsulfide are applied to the chromatoplates by means of suitable atomizers. The sample solutions are prepared according to the procedure prescribed by most texts on qualitative analysis, preferably with the nitrate salts. The developing "tank" may be constructed before the chromatoplates are spotted. It consists of a 400-ml beaker containing two strips of Whatman No. 1 paper measuring approximately 3 X 26 cm. The strips are placed in the beaker so t,hat they cross a t the base and continue up the walls, each in the form of a U (Fig. 1). Finally, 35 ml of solvent is introduced, and the beaker is covered with a piece of clear plastic wrap held in place with an elastic band. An unknown sample is applied to the chromatoplate in the usual manner by means of a small capillary tube. The plate is spotted t,wice, once on each side of center. The diameter of the applied spot should not exceed in., for larger samples t,end to yield poorly defined spots after development. As soou as the spots dry, the plate is placed in the "tank" (Fig. 2) and allowed to develop for a period of 11/2 to 13/4 hours. After the plate is removed, it is put aside for --., 5 min. to dry. Half the plate is covered with a sn~all piece of paper while t,he uncovered side is sprayed with DMG. The colored areas are noted and the R, values determined (see Fig. 3). The remaining side is then sprayed with ammonium sulfide, and again the col-. ored areas noted and R, 5 values calculated. Comparison of this data with that A. given in the table enables 8 I, one to determine the composition of the unknown Figure 3. Typical results sample. A
Discussion
As is indicated in the table, the PhZ+,after development, yields a spot somewhat less than ideal. The failing however, does not render its identification difficult, since a well defined and readily detectable colored area appears a t an Rt of 0.45. The detection of antimony is dependent on the appearance of a dark yellow spot a t an Rr of 0.25. It should be noted that the development of this color after exposure to ammonium sulfide
DMG DMG IIMG DMG DMG DMG DMG DMG
... ...
Red Lt. brown Lt. brown ... . ..
...
requires at least 2-3 minutes. Finally, while the R, value of cadmium is close to those of nickel and copper, its identification in a sample containing either nickel or copper can be accomplished if care is taken when observing the sprayed plate. Trace amounts of iron and copper in the adsorbent material are detectable in the developed plates. The iron present appears as a fine gray line across the plates after they are sprayed with ammonium sulfide. Detection of iron is, however, in no way affected by its presence in the adsorber. The copper appears as a diffuse stripe across the plate, and its detection is somewhat more difficult. If the size of the sample spot applied is no larger than '/& in. in diamet,er, the developed copper spot is readily detectable. The larger the sample applied, the more difficult to detect it becomes. If desired, this prohlem may be eliminated by predeveloping the plates with a mixture of methanol-cone HCl (9:1, u/v). Finally, it is known that Rr values obtained with TLC are not as reproducible as with other chromatographic techniques. This, however, is not critical, since the separations themselves are reproducible. Positive identification of the ions present can he made when these R, values are used in conjunction with the information obtained from the color reactions. The author is grateful to Dr. Laurence D. Frizzell, who originally suggested t,he project. Literature Cited (1) RITCHIE, A. S., J. CHEM.EDUC.,38,400 (1961). (2) POLLARD, F. H.,MCOMIE, J. F. W., AND ELBEIH,I. I. M., J . Chem. Soe.. 466 (1951). F.H.; MCOMIE, F. w., A X D STEVENS, H. M., (3) POLLARD, J . Chem. Soe., 1863, (1951). (4) POLLARD, F. H., MCOMIE, J. F. W.. AND STEPHENS, H. M., J. Chem. Soc., 4730 (1952). M. H., J. CHEM.EDUC.,40, 29 (1963). (5) ANWAR, (6) ROLLINS, C., J. CEEM.EDUC.,4 0 , 3 2 (1963). (7) DRULING, L. F., J. CEEM.EDUC.,40, 536 (1963). (8) NAFF,M. B.,AND NAFF, A. S., J. CHEM.EDUC.,40, 534 (1963). (9) SEILER,'H.,AND SEILER,M., Helu. Chim. Ada, 43, 1939 (1960). (10) SEILER,H., AND SEILER,M., Helv. Chim. Ada, 44, 939 (1961). (11) SEILER,H., AND ROTAWEILER, W., Helv. Chim. Ada, 44, 941 (1961). (12) SEILER,H., AND KAFPENBERGER, T.,Helv. Chim. Ada, 44, 1282 (1961). (13) PEIFER, J. J., Mzkrochim. Ada, 529, (1962).
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Volume 42, Number 8, August 1965
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