K. S. Spiegler, J. Gruenberg, Adriana Trottner, and W. Weiss Israel Institute
of Technology Haifa,
Israel
Demonstrations for the Overhead Proiector
The following five experiments form part of a complete set of demonstrations, adapted for two overhead projectors equipped with 1000-watt lamps, for a freshman general chemistry course. The class numbers about 600 students, and good visibility of all experiments is obtained on a 9-ft X 9-ft screen with no need to dim the lights in the auditorium except in the photography experiment. The first three experiments of this series use a Master Vu Graph projector1 adapted for experiments in a vertical plane. The adaptation is shown schematically in Figure 1. The instrument is dismantled, mounted on a table, and a mirror with mount added as shown. The box holding the cellophane roll is removed in order not to interfere with the projection. This arrangement permits projection of experiments with vertical containers (e.g., test tubes), whereas in the standard position the focal plane is horizontal and experiments must be adapted to Petri dishes or similar containers. The complete equipment for the experiment should cover an area of less than 10 in. X 10 in., in order not to exceed the size of the focal plane of the instrument. The last two experiments of this series use the overhead projector in the conventional way, with t'he 10-in. X 10-in. focal plane horizontal.
The body and cover of t,he electrolytic cell are made from 1/8-in. Plexiglas sheet. The outer dimensions of inches. The electrodes the cell are 15 / 8 X 1 X 2 11% are made from strips of '/spin. silver sheet, '/a in. wide. The polarity is indicated by transparent red and green circular dots and/or plus and minus signs respectively. The electrodes are held to the cover by screws, gaskets, and nuts, as shown in Figure 2. The cell is filled to about two-thirds of its height with 0.1 N AgN03 solution. tt is connected with alligator clamps to a direct current source, e.g., a battery charger set at an output of 12 volts. It is advisable to connect a transparent ammeter in series and project it simultaneously (See Fig. 2). The current is about 350 milliamperes. As soon as the ammeter needle deflects, the formation of a deposit is observed to form on the negative electrode. The lower portion of the electrode which is snbmerged in the solution widens "mysteriously" while the upper portion does not change. After an electrolysis period of about one minute this porous deposit is qnite thick. It can he dislodged from the electrode by gentle tapping. The lecturer explains that slow electrolysis from sohitions of complex silver salts is necessary to obtain smooth, well adherent deposits. Production of Iron
The main process occurring in the blast furnace is the
Figure 1. Overhead projector arrangement f o r v e r t i c d experiments (schematic dimgram).
Silver Plating
I n this experiment the deposition of a coat of silver during electrolysis can be demonstrated hy projection in less than one minute. 1
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Charles Beseler Co., East Orange, New Jersey.
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Journal of Chemical Education
F i g w e 2. Projected image o f silver ~ l a t i n gcell ofter electrolyrir period of one minute. screen sire is 9 ft. X 9 ft.
reduction of iron oxides. It is generally believed that the active reducing agent is carhon monoxide, formed by partial oxidation of the coke, rather than solid carbon. The present experiment demonstrates this reduction by showing (a) how the originally nonferromagnetic iron oxide, FezOa, is reduced to ferromagnetic iron and (b) that CO is converted to COz in the process: Fen03+3C0
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2Fe+3COz
CO is passed over FeZO,and then through a three-way stopcock which can be opened to the atmosphere or to a lime water container. When the iron oxide is heated, the iron powder produced by the reaction is seen to rise toward a magnet suspended over the square reaction tube. Simultaneously, calcium carbonate is seen to precipitate in the lime water container. Figure 3 shows the experimental sebup. Carbon monoxide is produced by dropping concentrated HlS04 from the funnel, F, into the suction flask. S, which contains 20 ml concentrated formic acid. The gas passes through glass tubing to the reaction vessal, R, which is madc from square glass tubing. The vessel contains a l/le-in. layer of FepOs which is not attracted by the mwnet, M. At the beginning of the experiment, CO is made to displace the air in the apparatus and to escape into the air. By turning the threeway stopcock T the gas is now passed through the Lims water contained in the Plexiglas container, L, and it is seen that no precipitation occurs, because the gas contains no CO1. After a few seconds the gas is again routed upwards and lighted. Since the flame cannot he seen directly in projection, it is demonstrated by igniting a small piece of filter paper and lighting the Bunsen burner, B, with it. The stopcock is turned downward immediately. A precipitate of CaCOais observed in the lime water tank. The Bunsen burner is then shut off and the iron produced by the reaction is seen to rise toward the magnet. It is desirable to use square (rather than round) vessels R and L because the reactions can be seen much better. The authors' thanks are extended to Mr. Y. Rosenhaum, Head of the Glassblowing Shop for making the square glass vessel.
Figure 3.
Demonstretion experimenh
produdion of irsn.
Pofentiomatric Titration
I n this experiment, the change of the potential diierence between a hydrogen and a calomel electrode
during an acid-base titration is shown. It is demonstrated that this potential difference changes abruptly and simultaneously with the color of an indicator when the titration endpoint is reached. A simplified scale drawing of the apparatus is shown in Figure 4. It is possible to project the titration vessel with the electrodes, the buret, and the transparent voltmeter simultaneously, since they can all be accommodated within the focal plane of the overhead projector, because the stage of the projector measures 10 in. X 10 in.
Figure 4.
Projection opporotur for potentiometris titrotion.
The titration vessel, T, is made from '/,in. Plexiglas sheets. Its internal dimensions are 2 in. X 1 3/4 in. X 5/s in. The upper section of the buret, B, is made of square glass tubing and calibrated in ml. It contains 1M NaOH solution which is delivered through 8 mm o.d.glass tubing and the "Manostat" needle valve, V , (Emil Greiner Co., New York). This buret is mounted on a support of l/rin. Plexiglas sheet, P, by means of clips C1, C2. The electrodes are also mounted on this support by wire. The support can be attwhed to two stands flanking the projector. The electrode, K, is a small Beckman 3-in. long fibertype saturated calomel electrode2 as used with the Model G pH meter of the same company. It is fastened to the support, P, by the wire, W. The hydrogen electrode, H, is made from glass, platinum wire, and platinum sheet, as shown. The size of the platinum sheet is 5 X 5 mm. It is covered with a coat of black platinum before the experiment using any efficient platinization m e t h ~ d . ~ A slow stream of hydrogen gas from a small Kipp generator is continuously bubbled through the electrode. Electrical connections Beekmm Scientific Instruments Carp., Fullerton, Calif. OSTER,G., A N D POLLISTER, A. W., Editma, "Physical Techniques in Biological Research," Academic Press, New York, 1956, pp. 3734. a
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to the electrodes are made through the contacts X and Y. Point X, which is connected to the calomel electrode, is always positive. In order to minimize current withdrawal from the electrodes, it is not advisable to connect them directly to a regular transparent voltmeter. Instead, the potential difference is fed into a vacuum tube voltmeter whose output voltage is then fed into the regular voltmeter. In the authors' experiments the 203 Microvolt-Ammeter and Amplifier4 was found suitable for this purpose. This instrument can measure circuits of input impedance of 100 megohms on the 1-volt scale and has an output for the transparent voltmeter of 1 volt at full scale deflection. The transparent voltmeter was built from a Model 7 Universal Avometer5 by removing the whole moving mechanism and enclosing it in a Plexiglas enclosure, A. The standard scale was replaced by a transparent one made of Plexiglas. The shunts were not placed into this plastic enclosure but were left in the original box which was mounted on the same table as the overhead projector. The enclosure, A, was fitted with a brass tongue, F, to fit easily into a brass bracket attached to the top of the overhead projector box. No further support of the transparent voltmeter is necessary. Connections to the shunt box are made at D. The combination of the Kin Tel voltmeter and amplifier with the modified Avometer was found to be very useful for all quantitative projection experiments in which the variable to be measured can be translated into a voltage. It is satisfactory for temperature measurements with small thermocouples. Many other voltmeter-amplifiers are available that can be adapted for this experiment either by modifying their own indicating meter or by connecting their output to an inexpensive transparent voltmeter suitable for projection. The titration vessel is filled with 20 ml 1.0 M HC1 solution; 0.5 ml of a standard solution (0.04%) of bromothvmolblue is added. The electrodes are connected & the voltmeter-amplifier and the latter to the transparent voltmeter. The range of the Avometer is set a t 1 volt dc, the calomel electrode is positive, and the reading about 0.3 v. The yellow solution is titrated with 1.0 M NaOH solution. while being stirred bv hand with a bent viece of thin &ass rod or &tinurn wl're. The voltage changes rather abruptly to about 0.9 v after the addition of 2.0 ml NaOH solution. At the same time, the color of the solution changes from yellow to blue. The rate of titration can be suited to the lecturer's preference. The authors found two minutes adequate, provided the theoretical background has been explained in sufficient detail before the experiment. It is possible to adapt the experiment for a suitable glass electrode instead of the hydrogen electrode, if the principle of the glass electrode has been explained. Photography
This experiment shows the successive stages of the production of a photographic image--exposure, develop-
ment, and fixation-by projection with a commercial overhead projector. All stages, including the emergence of the image on the plate in the developing solution, are clearly visible on the projection screen. The experiment follows the discussion of the chemistry of photography and lasts between two and three minutes. It shows the audience essentially what they would see in a darkroom. It is necessary to use highly transparent materials, because the projection method is diascopic, i.e., based on transmitted light. Photographic plates are usually translucent, but not transparent. The silver halides in t,he photographic emulsion which scatter the light beam dissolve only in the fixation process. Hence the development of the latent image cannot be cleariy seen when unmodified, conventional photographic materials are used. In order to demonstrate the experiment to large classes, a powerful light source is necessary. Ohviously, only red light can be used for demonstrating image development, the most dramatic step of the experiment. But most red filters transmit some light which affects the plate and causes it to darken nniformly rather than in the exposed area only. It was therefore necessary to prepare an insensitive plate which is almost transparent and not much affected by the amount of red light needed during the development period; to select a suitable red filter from readily available materials; and to find the optimum combination of exposure time, developer t,emperature, and developing time. It is more convenient to modify a commerical plate for this purpose than to prepare a special emulsion. A rather slow plate is used, so that it can be handled easily in the none-too-perfect. darkness of the lecture hall when the blinds are drawn. The overhead projector has an opening for an infrared filter which is necessary when the standard 500-w lamp of the projector is replaced by a 1000-w lamp. The red filter assembly was inserted in the opening provided for the infrared filter. The assembly consist,ed simply of a piece of sheet iron, 6 X 9 in. with reinforced edges to fit into the filter frame, having an opening 1 s/8 in. X 1 in. with a small frame holding the red glass from the "6.50 mp" filter of the Fisher Electrophotometer.6 The size of the red glass is about 2 in. X 2 in. When the frame is completely pushed into the instrument, the center of the filt,er should be in the optical axis of the light system. Other red filter materials can no doubt be used. The Fisher filter was found convenient, because it passes more than 85y0 of the light of wavelength larger than 660 mp and only a very minor fraction of t,he light more energetic than 620 mp. The plates used were modified unbacked Contact Lantern Plates 7781A." These plates are of low sensitivity and translucent. In order to make them reasonably transparent, part of the emulsion was dissolved before the demonstration by inserting each plate by hand for 8-10 seconds into the fixing solution and moving it hack and forth gently, washing immediately Fisher Scientific Company, Pittsburgh, Pa. Ilford Ltd., Ilford-Essex, England. Thanks are due Ilfard, Ltd., and their agent, Messrs. A. Berner Photo Supply Centre, Tel-Aviv, for supplying the plates and technical information about them. a
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Kin Tel, 5725 Kearny Villa Rd., San Diego, Calif. Uutomatic Coil Winder and Electricit1 Equipment Co., Ltd., Winder House, Douglas Street, London, S. W. 1, England.
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in water, and drying. These operations are performed in a dark room. The original size of the plate is 2 in. X 3 in. It is cut into half (2 in. X 1 3/4 in.) and returned to the box with the emulsion side facing down. The developer contains the following: Na2S03, 28 g; Na2C03, 28 g; glycine, 4.3 g; hydroquinone, 7.5 g; and KBr, 1.0 g. Dissolve the solids, in the order given above, in about 900 ml of water and make up to 1 liter. Keep in brown bottle. The fixing solution is a conventional photographic solution made up of NazSzOa, 240 g; Na2SOz, 10 g; and NaHS03, 25 g. Dissolve in warm water (50%) and make up to 1liter. Cool to room tcmperature. The drawing to he reproduced should he on a transparent material, e.g., on a cellulose acetate sheet which is a standard accessory of the projector. A solid black cross (lines drawn a t least in. thick) or a simple emblem are suitable. The acetate sheet and the plate should be of identical size. The red filter is inserted into the overhead projector and the hall darkened. A transparent cellophane sheet is placed on the projection surface and t,he drawing to be reproduced placed on top of it and shown to the class. The plate is placed on top of the drawing with the emulsion facing downward, and the red filter pulled out immediately. The plate is exposed in white light for 8-12 seconds. This operation is of course visible on the screen. The projector lamp is switched off, the red filter pushed in, and two crystallizing dishes are placed on the projection surface. One contains a 0.5-in. layer of developer solution heated to 4 5 T , the other a similar layer of fixing solution. The projection lamp is switched on. The exposed plate is placed into the developing solution, emulsion side facing up, and the dish is gently agitated by hand. After about 15 seconds, the image begins to emerge. This process is visible on the screen. The plate is left in the developer for another 5 seconds, removed with a pair of tweezers, rinsed briefly in water, and inserted into the fixing solution. The dissolution of the remaining silver halide can be seen on the screen. The image contrasts are seen to be sharpened. The red filter is pulled out 40 seconds after the plate is immersed in the fixing bath, und rhc lights i n tl~r~hnll arc turntd OII. The cornnlrtrd neea~ivris rinsed in wnrrr and nlacr~l on top of the cellophane sheet which covers the projection surface.
A standard overhead projector with a horizontal writing surface is used. For vessels of the dimensions indicated in Figure 5 the writing surface should be at least 5 in. X 5 in.
Figure 5.
Scale is marked in mm.
The solutions are contained in clear Plexiglas cells of the dimensions shown in Figure 5. The cells are closed with plastic plugs P on rubber gaskets R. The inner compartment of the unknown solution cell, 8, is 2.7 X 2.7 X 11.0 cm. Each of the wedge-shaped cavities in the wedge cell, W, for the acid and basic indicator solutions, is 10 cm. long and the widest cross section is 2.7 X 2.7 cm as in cell S. The acid indicator solution, A, is prepared by adding 5.0 ml of standard bromophenolblue solution (0.04'%) to 100 ml 0.01 M HC1. The basic indicator solution, B, is 0.01 M NaOH containing the same concentration of indicator. 5.0 ml indicator solution is also added to the solution to he tested. The latter is a buffer solution, prepared by mixing 71.5 mlO.l M citric acid solution with 28.5 ml 0.2 M NanHPOa solution. When filling the vessels and sealing them with the plastic plugs, care is taken to avoid trapping air bubbles. The two plastic vessels are placed on the stage of the overhead projector as shown in Figure 5. The acid and basic solutions are yellow and blue respectively. The buffer solution is green. The wedge cell is now turned 90" around one of its long edges. It is observed that the color of the vessel changes from yellow on the left to blue on the right with all shades resulting from the mixing of the two colors in between. It will be seen that one of these mixed colors is identical with t,hat of the buffer solution. A transparent ruler M, calibrated in millimeters, is now used to pinpoint the exact distance, d, between the left edge of the wedge compartment and the layer whose color is identical with that of the buffer solution. It is found that d = 28 mm which shows that the mixed color consists of 29y0 of the blue (basic) and 72% of the yellow (acid) form. Hence [In-] / [H In] = 0.39 and the pH by equation (1) = 3.4, since the pK value of bromophenolblue a t room temperature is taken as 3.8 at the ionic strength of the buffer.
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The Measurement of pH by Means of Indicators
The purpose of this experiment is to demonstrate that the color of a solution containing a two-color indicator is the sum of the two different colors of the indicator in the acid and basic form respectively and to demonstrate how to calculate the pH from the formula
where [In-] and [H In] are the concentrations of the basic and acid forms of the indicator respectively.
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Plastic wedge calorimeter.
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