COLLECTION
Notes on
Qualitative
Analysis
Editor’s Note: The following papers have been collected over a period of months. A similar collection has appeared in previous August issues (J. Chem. Educ., 35, 401 (1958), 36, 379 (1959), and 37, 407 (I960)). It is the Editor’s hope that collection into one place will prove more attractive to readers than the more frequently used editorial procedure of scattering items such as these through the pages of several issues. We apologize to authors who thus have had to wait to see their work in print. We commend to readers’ attention the fact that often these represent the solution of problems given to freshman students in qualitative analysis courses. We hope that publication in these pages can stimulate just, such activity. If readers merely incorporate these ideas into their own laboratory instructions, our battle is only half won. If, beyond that, these ideas suggest to the readers that they use their own ingenuity in similar fashion, we are gratified.
Alan T. Thomas and J. P.
Phillips1
University of Louisville Louisville, Kentucky
Ion Exchange Paper Chromatography
Although experiments in paper chromatography and ion exchange have become fairly common operations in chemistry courses from the high school
level up, each method offers certain nuisances for the laboratory instructor. The most serious procedural problem is that both techniques are likely to require more time than the usual laboratory period is designed
three amino acids with different isoelectric points. Figure 2 shows the same amino acid mixture developed on ordinary filter paper. While such chromatography performed with aqueous buffers instead of solvent combinations could hardly be expected to be anything or
to cope with.
While circular disc chromatography on paper is faster than strip or sheet methods, the resolution suffers. Rapid flow rates on ion exchange columns cannot be recommended. The preparation and preservation of the solvent mixtures required for paper chromatography is another difficulty, albeit one more likely to trouble the laboratory assistant than the student. Technique and developing equipment also affect the results of conventional one- or two-dimensional sheet chromatography. It is even necessary in many instances to consider the hazards involved in handling some of the required solvents, e.g., their toxicity or
inflammability. Paper chromatography
the rather recently available ion exchange resin impregnated papers2 is notably more rapid than the usual variety because the resolution of such systems is sufficiently superior to allow good separation, even on circular disc chromatograms (see Figures 1 and 2). Furthermore, only aqueous buffers are necessary for the development, and control of separation is often simply a matter of varying the pH of the buffer. Figure 1 illustrates these points well; a mixture of arginine, tyrosine, leucine, and aspartic acids gave acceptable separations on a disc chromatogram requiring only 15-20 minutes to develop. It is very easy for a student to separate two 1
on
Present address: Brown-Forman Distillers Corp., Louisville
Kv. 2 The grade used in this work was Reeve Angel SA-2, impregnated with Amberlite 1R-120 resin in the Na+ form. A bulletin (#3031) is available describing these papers and others, with a bibliography on their use. 10,
406
/
(left). Separation of arginine, tyrosine, leucine and aspartic acid (in that order from the center) with a buffer of pH 3.6 on SA-2 cation A ninhydrin dip was used to detect exchange resin impregnated paper. the amino acids,- 0 7 of each acid was used. Figure 2 (right), The same as Figure except that Whatman #4 filter paper was used instead of the SA-2. Figure
Journal of Chemical Education
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but disappointing, the comparison remains striking. Standard disc paper chromatography in Petri dishes gave these results.3
A lowering of pH in the aqueous buffers used causes a greater tendency of a given amino acid to form a cation, and thus strongly influences its migration on a cation exchange resin impregnated paper. Thus, performance of the experiment at several pH values can give widely differing separations. This experiment is also an effective lecture demonstration. 3
For
tice,
see
convenient description of disc chromatography pracHart, H., J. Chem. Educ., 36, A501 (1959).
a
