Dean 0. Skovlin Son Fernando Volley State College Northridge, California 91324
I
The Paper Chromatographic Separation of the Iom of Elements 26 through 30 A laboratory experiment
chromatography is such a powerful tool that its principles and techniques should be introduced to the student of chemistry as early as possible. This can be simply and conveniently done by introducing the paper chromatographic separation of several inorganic ions as an experiment in the general chemistry laboratory. This experiment describes the simultaneous ascending one dimensional separation of the ions of elements iron through zinc on filter paper using a solvent mixture or hydrochloric acid and Z-hutanone. The separation of the ions is based upon their tendency to form chloro complexes and they are identified by selected spot tests. The experiment can easily he completed during a 2-hr laboratory period. Before performing the experiment the students are given a brief introductory lecture on spot tests and the main types of chromatography, classified according to both operational method and mechanism of separation. Several useful sources are available for this purpose (1-5). Experimental Solutions
Individual standard solutions and a standard mixture of Fe(III), Co(II), Ni(II), Cu(II), and Zn(I1) (500 ppm each) are prepared by dissolving the chloride salts in water containing 50 ml of concentrated hydrochloric acid and diluting to 1 liter. Solutions of various combinations of the above ions are prepared as unknowns. The eluting solution is freshly prepared by adding 10.0 ml of 7.2 M KC1 and 30.0 ml of 2-hutanone to a clean dry 600-ml beaker with cover. Elution
Chromatograms of both known and unknown solutions ere developed on a 11 X 14 em sheet of Whatman No. I chromatngraphy paper which has been folded into four equal sections. The folds are parallel to the ll-em edge and form accordion type pleats. Duplicate 10-#I spots (1 em diameter) of both the standard and unknown are added to the paper by carefully touching the tip of s. Pasteur pipet to X marks located 2.0 cm from the bottom of each section. After the spots have been dried under an infrared lamp for 1-2 rnin, the folded paper is placed in the eluting solution with the spots at the bottom. The beaker is immediately sealed with plastic wrap and the solvent is allowed lo rise to within 1.0 em of the top of the paper (30 min). The paper is then removed from the beaker and partially dried in s. fume hood for 5-6 min.
' Occasionally this test produces s faint positive test for unknowns not intentionally containing iron. This should be reported by the student as a, trace constituent. The dithizone test is impaired hy the appearance of an orange deposit if the paper columns artre allowed to remain in the ammonia fumes for more than several minutes.
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Spot Tests
During the above elution the students perform several preliminary tests. To separate cups of a spot plate is added one drop of each of t,he five individual standard solutions followed by three drops of concentrated hydrochloric acid. The resulting colors of the chloro complexes of Fe(III), Co(II), and Cu(I1) are compared to the separate color spots visible on the paper in the beaker after 10 to 15 minutes of elution. Tests are next carried out by adding one drop of potassium ferrocyanide (1%) to dried 10-@1 spots of the five individual standard solutions placed on a filter paper circle. The colors developed are noted before and after the paper has been neubralised over ammonia fumes. These tests are repeated using dithizone (O.OS'%in chloroform) and dimethylglyoxime (1% in ethanol). On the basis of these tests the students are asked to select the most suitable reagents for the identification of each ion. Location and Identification of Spots
The two yellow spots on the standard and unknown sections are identified by adding 10-pl or less of potassium ferrocyanide' to each spot. The paper is next made basic by placing it in a 600-ml beaker containing a small beaker of concentrated ammonia for 1 min. The cobalt and zinc spots are located and identified by adding 1-2 drops of dithizone' just above and below the copper spot. Nickel is located by adding dimethylglyoxime dropwise to the lower untreated portion of the paper columns. The student's report should include the identity of the ions present in the unknown as well as the ratio (R,) of the distance trweled by the metal ions lo the distance traveled by the solvent front.
Results and Discussion The use of this experiment in our first-year chemistry laboratory course has shown that no difficulties are encountered in separating the ions or determining their location and identity. Average R, values are given in the table. A variation of less than two percent can be expected among student results. For an approximate model of the paper chromatographic separation described in this experiment we may envision the stationary phase as a layer of water containincr HC1 adsorbed to the uolar cellulose molecules of the paper and the moving phase as the polar 2butanone, containing HC1. During the elution process the solute species, ranging from solvated cations to anionic chloro complexes, will be distributed between these two phases. The metallic ions that form weak chloro complexes will migrate slowly since they will
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Average R, Values Ion
RI
exist primarily as cations and be partitioned preferentially into the more polar stationary aqueous phase. Those metallic ions that form stronger chloride complexes will migrate more rapidly since they will be partitioned preferentially into the less polar mobile phase as neutral chloro complexes or as protonated univalent anionic chloro complexes. Stability constant data for chloro complex formation in 2-butanone is not available; however the relative extent of complex formation in 2-butanone would be expected to be similar to that in aqueous solution except that in the organic solvent complex formation will occur to a much greater extent a t a given chloride ion concentration (6, 7). Kraus and Nelson (8) have graphically illustrated chloro complex formation of many metallic elements in aqueous solution as plots of their anion exchange distribution coefficients versus hydrochloric acid concentration. In general, their data shows that the lower the hydrochloric acid concentration a t which anion exchange occurs for a given metal the greater the extent of complex formation. The observed order of separation of the ions studied in this experiment correlates well with that predicted from a consideration of their anion exchange behavior. Other metallic ions, e.g., Cr(III), Mn(II), and Cd(I1) show the same agree-
ment between anion exchange properties and paper chromatographic separation in a ketone-hydrochloric acid solvent (3). The order in which the ions in this experiment are eluted also parallels the extent of their extraction, as neutral complex species, from aqueous halide solutions by the water immiscible Cmethyl-&pentanone (9). This work can be usefully extended by investigating the change of R, with a change in hydrochloric acid concentration or a change in solvent, e.g., acetone or 2-pentanone in place of 2-butanone. Literature Cited
(1) ABBOTT,D.. and ANDREWB, R. S., "An Introduction t o Chmmstography;' Houghtoo Mifflin Company. Boston. 1965. (2) sum^. IVOR. "Chromatog~aphic and Electrophoretic Techniques." 3rd ed.. Intersoienee Publiehers (division of John Wiley 61 Sons. Ino.). New York, 1969, Yol. I. p. 800. (3) H ~ l s I. , M.. AND MACEX.K.. "PBPBI Chromatogr&phy," Aoademic Press, New York, 1963, p. 733. (4) Bmon. R. J.. D n n ~ n a r .E. L., A N D Z W E I ~G.. , "A Manual of Paper Chromatography and Paper ELectrophoresia." 2nd ed., Academic . Press, Ina.. New York, 1 9 5 8 , ~410. H. F.. J. CXEM.EDUC.,42,477 (1965). (5) WALTON, D. A,, J . A m r . Chbm. So
