In the Classroom
Chemistry “en Miniature” Herbert W. Roesky Institut für Anorganische Chemie der Universität, Tammannstrasse 4, D-37077 Göttingen, Germany
Chemistry “en miniature”—small but excellent—it does not matter what you call it, we are speaking of a new type of experiment in chemistry (1). Small-size experiments will be the future of chemistry. While doing practical chemical work or performing demonstrations in lectures every chemist has to be conscious of the environment. This means that chemicals have to be handled in a responsible way and there has to be an efficient means of waste disposal. The smaller the quantities of chemicals used for an experiment, the easier they can be disposed, and the lower is the encumbrance to the environment. It is relatively easy to use small quantities of chemicals as long as a student is doing practical work. He is close to the experiment and can watch it carefully even if he is using very small quantities of chemicals. It is much more difficult to perform experiments “en miniature” in a lecture hall. The well-known small-scale overhead projections are not useful in precipitation reactions, because all solids formed seem to be black. Furthermore, the smallscale method is advantageous when the demonstration involves noxious gases and liquids of high vapor pressure such as aqueous hydrogen sulfide solutions. Herein we describe the use of modern equipment such as projectors and video cameras for performing demonstrations. The Data and Video Large Scale Projector Online DV 8300 (Sanyo) furnishes a high-quality image of data and video projection in a compact and mobile instrument, which does not even require a supplementary overhead projector. Even very small details and characters appear in a clear and correct way. A camera can be attached to this instrument. We attached the Flex Cam Video-Camera to the Online DV 8300. This minicamera has a flexible “swan neck” that can be rotated through 30°. It can focus between 1 cm and infinity, which means an actual enlargement up to fiftyfold. The smallest details are caught and projected onto the screen by using a data display. All this is possible in almost every lighting condition. Thus, experiments “en miniature” can be seen even by a student who is sitting at the rear of the lecture hall. Experiments on a big scale cannot be followed in the same way, regardless of the amounts of chemicals that are used. The procedure described herein works far better than the video technique that is used to show the reaction course of an experiment on a TV screen and which we and others used for some time (2, 3). To do experiments in a lecture hall the reactions are performed in UV-quartz cells (3 mL) or in vials of comparable size. The glass receptacles are equipped with a septum and a screw cap, so that no toxic or bad-smelling vapors can escape. To fix the quartz cells and the vials we have constructed simple plastic supports (Fig. 1). The quantities of solutions used are in the range of 0.5– 1.0 mL. The solution is normally added dropwise using a plastic syringe or a micropipet. For an excellent projection the sample has to be illuminated. The light should not be too strong (15 W) and a spotlight should not be used. A
Figure 1. Two quartz cells are shown closed by a septum to avoid elimination of H2S vapors. In the right cell MnS is precipitated using a syringe for adding aqueous H2S (pH = 6). The different colors of the universal indicator paper are shown in the background of the cell. In the left cell no precipitation of MnS is observed (pH = 1).
slightly illuminated titration table can also be used. In the following section some experiments are described in detail. The sulfide precipitation of metal ions is illustrated by using solutions of manganese, cadmium, lead, and arsenic salts. One milliliter of the metal salt solution (0.1 M) is placed into the quartz cells equipped with a septum. With a syringe, 0.1 M aqueous hydrogen sulfide is added dropwise. During this procedure the metal sulfides (MnS, pink-white; CdS, yellow; PbS, black; As2S3, yellow-orange) precipitate. There is no smell of H2S. The experiment can also be done by first filling the H2S solution into the quartz cells and then adding the metal salt solutions. The only disadvantage is that four syringes have to be used for the different metal salt solutions. The color differences are clearly visible (Fig. 1). To prove that there is CO 2 in exhaled air, barium dichloride (1 mL, 0.1 M) solution is filled into a quartz cell. Exhaled breath is then pressed into the solution using a plastic tube connected with a glass capillary. After several repetitions of this procedure white barium carbonate precipitates. Even small color differences, which occur when silver salts of chloride, bromide and iodide are precipitated, can be shown by the precipitation of 0.1 M solutions of halogen ions and AgNO3 (0.01 M) solutions. To bring about the precipitations, 5 drops of silver nitrate solutions have been used. If the traditional overhead projection is used, the precipitates seem to be black.
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In the Classroom Summary Using the procedure described above, a spectator can see much better the fascinating color changes and precipitations produced than would be possible by using the current presentation of experiments. In a lecture hall with 500 spectators these experiments are much more perceivable than a reaction made on a large scale using 100 or 200 mL of solutions. Without any danger, these reactions on a small scale can be done even in classrooms or seminar rooms. Before the lecture, using a separate fume hood, toxic or strongly smelling solutions can be filled into the quartz cells equipped with a septum. Furthermore the amounts of chemicals used are drastically reduced. The amount of waste is reduced by a factor of 100 to 200. So the chemical demonstrations in lectures contribute to the protection of the environment. This procedure saves resources and decreases waste. It is an excellent example for modern chemical instruction within the meaning of “sustainable development”. Acknowledgment Financial support of the Fonds der Chemischen Industrie is gratefully acknowledged. Literature Cited 1. Roesky, H. W. Chemie in unserer Zeit 1995, 29, 133–134. 2. Roesky, H. W. Kontakte 1984, 1, 18–25; 1984, 2, 42–37; 1993, 1, 35–43; 1993, 2, 18–25. 3. Roesky, H. W.; Möckel, K. Chemical Curiosities; VCH: Weinheim, 1996.
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