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INTRODUCTORY EXPERIMENTS IN INORGANIC PAPER CHROMATOGRAPHY JOHN 6. SURAK and DONALD P. SCHLUETER Marquette University, Milwaukee, Wisconsin
ALTHOUGH Tswett is usually given credit for originating the chromatographic method of analysis, his work was preceded almost 35 years by that of Schoenbein. While investigating the conditions of ozone formation under the influence of electrical discharges, he observed selective adsorption of the components of a mixture by the different heights to which they rose when a strip of filter paper was dipped in the solution. He called this new .phenomenon ''capillary analysis" and predicted its u~efulnessin the field of analytical chemistry (1). This was the inception of paper chromatography. I n 1944, after over 80 years of quiescence in this field, Gordon, Martin, and Synge extended Schoenbein's technique to two-dimensional chromatography (2). Their adaption was based on the partition principle which depends on a dynamic partition or distribution of dissolved or dispersed substance between two immiscible phases, one of which is moving past the other. The separability of mixtures depends primarily upon differences among the phrtition or distribution ratios of the solutes in the nonmobile and mobile phases. Following the original work of Gordon, Martin, and Synge, a great number of variations in technique and apparatus have been published. The most widely used method employs strips of filter paper which are suspended from a trough containing water, a partially miscible organic solvent and usually a small amount of acid. A drop or streak of the solution to be analyzed is placed near the top of the strip and the paper and trough are placed in a closed container. After a suitable length of time, depending on the filter paper and solvent used, the strip is removed from the container and developed with reagents which will locate and identify the various constituents of the mixture. A very large number of both organic and inorganic substances have been separated by this method (1). Since the field of chromatography is rapidly developing to encompass all phases of industry, it was felt that there was a need to develop a simple, small-scale method of paper chromatography which could be used in freshman qualitative-analysis classes. The majority of apparatus previously reported was not suitable for class participation (5). In the following procedure, the elementary chemistry student can be introduced to this rapid and convenient method of chemical analysis with a minimum of cost and effort. The equipment consists of a 300 X 29-mm. test tube and a loop of thin glass rod, the ends of which are
inserted in a cork stopper. This glass loop prevents the paper strips from touching each other, since two strips are run simultaneously in one or more tubes. (A 300-ml. Erlenmeyer flask is used as a support for the tube.) For a large number of samples, a testtube rack is made by drilling 1'/4-inch holes in one side of a box, 36 X 6 X 6 in. About 33 holes are drilled. This is sufficient to hold one class's test tubes. The filter paper strips, 35 cm. long and 1.5 cm. wide, have a piece of glass rod inserted a t the end to prevent curling of the paper. The procedure involves depositing a small amount of the sample mixture of ions on the paper by means of a fine capillary and allowing it to dry. After drying, the strip is placed in the test tube containing 10 ml. of developing solvent. This solvent may be a mixture of water, butanol, or tertiary butyl alcohol, and acetic or hydrochloric acid. The tube is then closed loosely with a cork until the solvent has reached the top of the test tube. The chromatogram is then removed and the separated cations areidentified with suitable reagents. When dry, the strips are sprayed with liquid plastic t o prevent fading of the identifying colors. In the classroom application of this procedure the students are supplied with filter paper cut to the proper size, and the necessary constituents for their solvent mixture. Unknown mixtures ordinarily issued to the student for analysis by conventional methods are used. It must be pointed out a t this point that there are two factors which must be.carefully avoided when this method is introduced on a class basis. The air must be relatively free of hydrogen sulfide gas or precipitation of the unknown mixture will occur immediately after the sample solution is placed on the paper. In addition, the sample spot must be dry before the strip is placed in the tube or it will tend t o be diluted by the developing solvent and a very poor separation will result. Of the many separations attempted the majority of fajlures could be attributed to one or both of these factors. Migration of the developing solvent requires approximately 20 hours, after which time the strips are removed from the tube and allowed to dry. Group I cations are identified by spraying with potassium iodide or dichromate solution, while Groups I1 and IIA are identified by spraying the strips with distilled water and exposing them to hydrogen sulfide gas in a closed container for about 15 minutes. In order to simplify identification of the various cations several chromatograms are prepared for use as standards. A few strips
MARCH. 1952
are run containing the entire group of cations which might he present in the student'sunknown solution. By comparing the colors of the sulfides or iodides formed, and the extent of migration of his own strip with that of the standard chromatogram, the student is readily able to int,erpret his results. This system proved very satisfactory when introduced to a freshman chemistry class which had no previous knowledge of the chromatographic method. A large number of developing solventsand grades of filter paper were investigated before a combination which would work under almost all conditions was obtained. A complete diecussion of these experiment,^ Chrornatoglephic Seperationr Obtained by Students including investigation of various acid and pH ranges will be found in a sub- noted that the rate of migration of the various cations sequent paper. Only those solvents and papers used is strongly influenced hy the other ions which are presin the class experiments will he discussed here. ent in the sample solution. The most satisfactory solvent mixtures consist of Although this is a very simple and rapid method of 15 per cent water, 10 per cent acetoacetic ester? 75 per qualitative analysis, it is not intended to replace the cent hutanol or tertiary butyl alcohol, and sufficient conventional method now employed. The fundamenglacial acetic acid to adjust +,he pH between 3.5 and tal purpose of this small scale procedure is to introduce 4.0. Hydrion pH paper is used to measure the pH of this important analytical tool, chromatography, to the solutions. The acid is added to give more concise the future chemist. hands and prevent trailing due to hydrolysis and the Work is under way on the remaining groups and it existence of ions in complex and simple form a t the seems very likely that this same procedure will prove same t.ime (4). The acetoacetic ester functions in the successful. A laboratory manual for the chromatographic separation of the cations and certain of the same manner (5). Two grades of filter paper yielded separations whicb anions found in the conventional qualitative inorganic differed chiefly in the speed of migration rather than analysis course is in the process of preparation. in the degree of separation. Whatman filter paper Nos. 3 and 4 gave good separations, but the R, values for LITERATURE CITED the chromatograms using the No. 4 grade were slightly (1) CLEGG,D. L.,Anal. Chem.,22,48-57 (1950). higher than for the No. 3 grade. The differenceisin the (2) GORDON,A. H.,A. J. MARTIN, AND R. L. SYNGE,Biochem. J., 38,65 (1944). porosity of the papers. AND I. I. M. ELBEIH, Several examples of separations obtained by the stu- (3) POLLARD,F. H., J. F. TV. MCOMIE, Nature, 163,292 (1949). dents are shown in the figure. The lighter bands are (4) LEDERER, M., Science, 110, 115 (1949). not too distinct in the photograph but their uppermost (5) F n ~ ~ a n s o NW., J., AND M. J. ~ O N J. S CHEM. , EDUC., boundaries are indicated by a pencil line. I t will he 27, 37 (1950).
