A Demonstration of Adsorption GRANT W . SMITH The University of Kansas City, Kansas City, Missouri
T
HE adsorption of complex ammonia ions on silica gel offers several distinct advantages for exhibition purposes. Among these are: the speed, specificity, and magnitude of the adsorption are high; several complex ammonia ions are highly colored and the removal of the color in the adsorption process is easily observed; silica gel itself is colorless and transparent so that the adsorption process results in vivid coloration of the adsorbent; and the materials and apparatus are conveniently available in most laboratories. The apparatus (Figure 1) is mounted on a tall ring stand and is constructed as follows: The adsorbent material, activated silica gel,' is supported on glass wool
in a 25-cm. straight drying tube mounted vertically. A capillary glass tip and saew clamp assembly controls the rate of flow of liquid through the system. A carbon filter funnel is inserted through a rubber stopper in the upper end of the drying tube. The apparatus is completed by mounting a separatory funnel, preferably of the Squibb short stem type and of a t least 250-
ml. capacity, above the drying tube assembly, and placing a beaker or flask as receiver beneath the outlet. The separatory funnel is filled with a solution containing a colored complex ammonia ion, such as that of copper (blue), nickel (blue or violet), cobalt (red), or others. All such compounds are highly adsorbed.% The solutions are easily prepared in most cases by adding ammonium hydroxide to a dilute solution of the metallic salt, e. g., cupric nitrate, until a slight excess is present as shown by the dissolving of the hydroxide precipitate and the formation of a clear, colored solution. The color should not be too intense, since the decolorization may not be complete in too concentrated solutions. The solution is allowed to drip from the funnel into the carbon filter funnel until the latter is about two-thirds full, and is kept a t this level by adjusting the outflow into the receiver so that i t drips a t the same rate. An interesting feature of this demonstration is the fact that the silica gel becomes intensely colored a t the top of the adsorbent tube, and the coloration decreases continuously until in the lower region little or no coloration is detected. Since the silica gel always assumes the color of the original solution, i t is evident that the ion is adsorbed without decomposition. Simple tests on the original and the decolorized solutions may be made to prove that almost complete removal of the metallic ammonia complex is achieved. The following spot tests are convenient for this purpose. The tests for copper, cobalt, and nickel are carried out on a white surface, such as a drop reaction plate, porcelain dish, drop readion paper, or filter paper. The test for silver, a colorless solution, is best performed on a black surface or in a small test tube. Test for Copper. To one to two drops of solution, acidified with acetic acid, are added two drops of 10 per cent potassium ferrocyanide solution. A red to red-brown precipitate of cupric ferrocyanide indicates copper. Test for Cobalt. To me to two drops of solution, acidifiedwith acetic acid, are added two drops of a saturated solution of anitroso-&naphthol in 50 per cent acetic acid. A red-brown precipitate indicates cobalt. Ted for Nickel. To one to two drops of solution are added one to two drops of a one per cent alcoholic solution of dimethylglyoxime. A bright red precipitate of nickel dimethylglyoaime indicates nickel. Test for Silver. A few drops of solution are slightly acidified with HNOa, and two to three drops of 1:l HCI are added. A white precipitate of AgCl indicates silver.
Several of these systems, each one employing a different colored ammonia complex ion as solute, may be lined up side by side to make a very colorful and instructive exhibit. 'Available from The Davison Chemical Corporation, Balti- ' SWTH,I.Phys. Chem., 43,637 (1939). more, Maryland. 432
