Chromatographic Adsorption Undergraduate Qualitative Analysis JOHN A. BISFIOP Moravian College for Women, Bethlehem, Pennsylvania
T
HE undergraduate course in qualitative analysis Dahn, using 8-hydroxyquinoline as an adsorbent. In has become the framework within which is taught this method the adsorbent acts as a base-exchanger. the elementary theory underlying analytical chemistry. Both Strain (1) and Zechmeister and Cholnocky (2) I t would seem, therefore, to be the ideal point a t which discuss the use of activated alumina as an adsorbent for to introduce chromatographic adsorption, a method of ions, using developing solutions to bring out distinctive analysis which is becoming increasingly important. colors. This is especially true since the method is a t best semiIN SEPARATING CU++ quantitative compared with the traditional methods THE USE OF BHYDROXYQUINOLINE AND CD++ taught in the course in quantitative analysis. While most of the work in this field has been with organic Preliminary experiments. Feigl (3) has given a compounds, i t is possible to separate inorganic ions by method by which 8-hydroxyquinoline may be used in adsorption methods. The use of the methods outlined the detection of Cu++. This test does not depend upon here also leads to an extended idea of valence bonding the use of thereagent as a precipitant. He says nothing for the students in classroom discussion. There are about the use of this reagent for Cd++. Yoe and two authoritative books to which students may he re- Sarver (4) discuss the use of this reagent for a precipiferred in which theory and practice are discussed (1,Z). tant, and point out that it can be used for a wide variety The main problem in inorganic chromatography is to of cations if the conditions are controlled. find adsorbents on which colors will be produced when In the present method the characteristic pure green ions are adsorbed. The alternative is to use an ad- fluorescence of the Cd++ salt of 8-hydroxyquinoline is sorbent which does not itself react with a "developer" made use of. Pringsheim and Vogel (5) mention the which will bring out distinctive colors in ions which difference between this fluorescence and that produced have been adsorbed. The first case is discussed by by other ions forming fluorescent salts with this reagent. Strain ( I ) , refemng to the work of Erlenmeyer and The copper salt does not fluoresce.
The preliminary experiments involve the determination of the limiting amounts of Cu++ and Cd++ which may be found using this reagent, that is made up by preparing a saturated solution of 8-hydroxyquinoline in 3 N acetic acid. Solutions of Cu++ and Cd++ were made up, 0.008 M and 0.009 M respectively. These were tested with the reagent solution by diluting the salt solutions and adding one drop of the reagent solution. It was found that the Cuff solutions still gave precipitates when the concentration was cut to 0.00008 M , while the Cd++ solutions did not give a precipitate in any dilution. Both the Cd++ solutions and the Cu++ precipitates were tested for fluorescence using an argon glow lamp with a Wood's filter. No fluorescence was shown by either solutions or precipitates. The acid in the Cd++ solutions was neutralized in two ways: by addmg NH3 aud with NHaC2H302(3 N). In both cases precipitates were formed which exhibited fluorescence down to a Cd++ concentration of 0.0003 M. A series of ex~erimentsdesipned to test the limitations of this method of identifykg Cd++ in the presence of Cu++ was next performed. I n these, solutions of Cd++ and Cu++ were made up in which the proportions of the respective two ions varied from 1 :1 to 1:10. In each of these experiments the Cn++ was 'first precipitated by adding 1drop of the reagent to 3 ml. of the solution under test contained in a small centrifuge tube. The suspension was centrifuged and tested for fluorescence, with negative results. Then one drop of dilute NHs was added to the supernatant liquid without removing the precipitate, and the mixture was again tested for fluorescence. The test was positive in each case. I n the experiment in which the Cd++/CuC+ ratio was 1:10, the actual concentration of Cd++ was about 9 X Adsorption ex9eriments from solutions containing both Cu++ and Cd++. The adsorption experiments were petformed on solutions of Cn++ and Cd++ of the concentrations usually found in qualitative analysis (6). The test solutions were made by mixing equal quantities of solutions of the two ions and then diluting 10 times and 100 times with distilled water. Columns of 8hydroxyquinoline were prepared in a/8 inch I.D. glass tubing which had a slight constriction. Above the constriction was a short length of glass tubing the next size smaller, with a piece of filter paper just above it, cut t o fit with a cork borer. On the filter paper was put about one inch of the adsorbent, tapped down with a second filter disc above it. Two milliliters of the solutions were passed through the columns using a desk case reaspirator for suction. The Cu++ was in moved as a greenish ring a t the top of the column, there being no apparent change in the rest of the column. In the most concentrated solution the filtrate contained enough Cd++ to give a strong test with H a . The solution diluted 10 times gave a slight test with H2S, while the most dilute solution gave a poor test. This was using only two milliliters of solution. I n all cases the columns gave a pronounced fluorescence below the top ring containing the insoluble copper compound.
Use of the method in systematic analysis. In the systematic analysis of mixtures in most procedures, Cu++ and Cd++ are found in an ammoniacal filtrate from the separation of Bi+++. It is difficultto stop the neutralization at the point of exact neutrality, and the solution usually ends up acidic. The result is that some of the adsorbent dissolves, making an irregular colnmn. A longer column may be used, or a little S-hydroxyquinoline solid added to the solution before passing i t through the column. The use of an ammoniacal solution was tried, with the realization that both Cu++ and Cd++ would precipitate completely. The results were satisfactory so far as the identification of Cd++ in the presence of Cu++ was concerned. The Cu++ was distributed more or less throughout the column, predominating a t the top. With Cd++ present the column fluoresced, while in the absence of Cd++ there was no fluorescence. THE SEPARATION OF SB+++ AND SN++ BY ADSORPTION ON ALUMINA
In order to illustrate chromatographic adsorption in which no base-exchange has occurred, the separation of Sb+++and Sn++ by adsorption on activated alumina was used. The alumina used was Baker's Activated Alumina, and it was ground to a powder before being put into the adsorption tubes, which were prepared as for the 8hydroxyquinoline adsorptions. There is a danger that the tubes may be packed too tightly. The acid solution obtained after boiling off the HzS from the mixture of SbCl. and SnC14 in Group IIB is reduced with a small amount of metallic Mg and neutralized with NH. until a precipitate just appears. This is dissolved with a minimum of HC1. Samples may be diluted 10 and 100 times as above, just enough HCl being used to prevent a precipitate from forming. The solutions were drawn through the adsorption column, two milliliters being used. The filtrates were tested with H2S, and a saturated solution of HzS was drawn through the columns. The filtrates from the two more concentrated solutions gave a good chocolatebrown precipitate of SnS. The solution diluted 100 times gave no test for Sn in the filtrate. All the columns gave a more or less irregular chromatogram with H2S water, in which the orange colored SbzSsappeared a t the top, and the chocolate SnS a t the bottom. LITERATURE CITED
(1) STRAIN, H. H., "Chromatographic Adsorption Analysis,''
Interscience Publishers, Inc., New York, 1942. L., AND L. CHOLNOCRY (translated by A. I,. BACHARACH AND F. A. ROBINSON). principle^ and Practice of Chromatography," John Wiley and Sons, Inc., New York, 1943.
(2)
ZECHMEISTER,
(3)
FEIGL, FRITZ
(fra12Sbt(2tedJ , W, mmHEWS),
Tests;
Nordemau Publishing Company, New York. 1943. (4) YOB, J. H., AND 4. A. SARVER, "Olganic Analytical Reagents," John Wdey and Sons. Inc.. New York, 1941. (5) pR1~~SHBIM, H., AND M . T J ~ ~~ ~ ~ ~, ~ l,,tersci. ~ ence Publishers. Inc.. New York, 1943. (6) MELDRUM, w. B., E. W. FLOSDORF, AND A. F. DAGGETT, "Qualitative Analysis," American Book Company, New York, 1939.
~
