sdlled by
GEORGE L.
GILBERT
hrnison University Granville. Ohio 43023
Adapting a pH Meter for Lecture Demonstrations: Construction of an Auxiliary Scale for Overhead Projectors Submitted by:
Checked by:
Joseph A. Stanko University of South Florida Tampa, Florida 33620 Charles G. Haas and J o h n R. Lotz T h e Pennsylvania State University University Park, 16802 Wayne L. Felty The Pennsylvania State University Wilkes-Barre
We have found lecture demonstrations which utilize p H measurements of solutions to he invaluable in illustratinr the concepts of weak acids, ionization equilibrium, titration c&es and buffer action when these subjects arise in a general chemistry course. However, to be effective, such demonstrations usually demand that the p H meter scale be able to be While this no doubt viewed andread easily by ~arg~classes. can he achieved by the use of a closed-circuittelevision camera in the lecture hall, we have found that it may also be satisfactorily, and less expensively, accomplished by equipping an available p H meter with a duplicate auxiliary scale suitahle for projection with a conventional overhead projector. The purpose of this note is to provide details on the construction of such an auxiliary projection-scale for a specific p H meter and offer euidelines for anolication to other twes. .. .. Figure 1 shows an auxiliary meter movement which adapts a Fisher Arrumet Model 120 DH meter for ~roiectionuse at USF. A duplicate of the main meter movement was purchased from the manufacturer for 530. This came convenientlv mounted in a transparent plastic case. The white, opaque pH-mV scale was easily removed and replaced with an accurate replica on transparent acetate produced by our Graphics Department using a Kodalith film-positive transfer process for $1. The only additional adaptations of the auxiliary meter movement were four plastic dowels for corner legs to support the meter scale in th; horizontal plane on the o;erhead brolector, and two electrical extension leads. Klectrically, the auxiliary meter is connected to the main p H meter at the lecture bench via the set of extension leads which plug into the jacks on the meter which are normally used for recorder output. Figure 2 is a photograph of the projected image of the scale which can easilv be read to tenths of a DHunit bv-Dersons . in a 200-seat room. In order for the ~roiectionmeter t o function ~ r o ~ e rwhen lv it is connected to iherecorder terminals, it is necessary that the p H meter output be a signal intended for driving a current-type recorder. Surh asignal is often provided in thecircuit desirn placinr the recorder terminals in series with the . by .. instrument's meter movement. Most Beckman instruments, including the older Zeromatic models, and the Fisher Accumet Model 120 described herein fall in this category and the auxiliary meter gives a very faithful rendering of the reading on the main meter scale. If the recorder signal is intended only to be used to drivea
Figure 1: Photographof an auxiliary pH meter movement adapted fw ovemead proienian use in conjunction with a Fisher Accumel Model 120 meter. Meter dimensions are 4.5 X 4.5 X 2.5 in.
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Figure 2: Photograph of the projected image, 6 foot square, of the meter scale shown in Figure 1.
potentiometric-type recorder, the extra meter movement will give a large loading of the recorder circuit, and would likely ruin the circuit calibration. (The popular Corning Model 7 p H meter, for example, falls in this category.) Thus, direct connection of the external meter to the recorder output will not give satisfactory operation. For such instruments it will be necessary to tap the main meter circuit internally. For the p H meter which does not allow direct use of the recorder outout. there are two simole wavs in which it can he adapted for'dekonstration purposes. o n e possibility is to install a DPDT switch and a connection socket t o allow a duplicate projection movement t o he substituted in place of the original movement in the instrument. The demonstrator would, of course, lose the readout on the front panel, but he presumablv could look at the proieded image. The instrument could still be used in the nor&lfashion when the switch was This note is based on a presentation at the annual "My favor it^ I.erture Demonrtrahn" Meeting uf the Tampa Bay Subsection oi the Florida Srctiun of the American Chemical Society. 1)ecemher. 1973. Volume 54, Number 6, June 1977 / 365
positioned to disconnect the external meter. Alternately, if one were willine to sacrifice a DH meter for this numose. . . . the meter movement could be removed from its mounting and usrd as the nroiection meter. The unlv modification needed would be to-prepare the transparent k a l e and to install extension leads. Finally we note that the projection scale is also used for lecture demonstrations of EMF of elertrochemical rells since the millivolt scale is standard to most p H meters. Notes on several suitable lecture demonstrations are available from the authors on request.
White to Rose and Return: A Multipurpose Demonstration Submitted by: Edwin S . Olson South Dakota State University Brookings, 57006 Checked by: George Wollaston Clarion State College Clarion, Pennsyluania 16214 A slightly yellowish liquid resembling a white wine is poured from a c l e i bottle into agoblet, whereupon it changes & a r& color. After a few seconds the color returns to pale yellow. The . . bottle may he shakm to conwrt the remaining liquid ton&. It too returns to pale yellow. The secret ofthe transformation is revealed by eiplaining the reaction of the colorless leuco form of a dye with oxygen when shaken with air to give the rose dye (oxidized form). The oxidized rose dye reacts with the aldehyde groups of glucose to give back the reduced leuco dve eluconic acid. The similar demonstration with , olus .. methylene hlue is well known. '['his demonstration might he used to illustrate any of the
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366 1 Journal of ChemicalEducation
following: properties of vat dyes, a simple redox system, the ease of oxidation of aldehvde - erouos - .to the acid. or the reactions of reducing sugars versus nonreducing kgars. Other aldehvdes will work. although the less water soluble ones of course form a separate layer which slows the whole process down. The recipe for the system is 10 g - of -glucose. 10 g of sodium hydroxidk, 500 ml of water and a tiny pinch of phenosafranine. An even smaller pinch of safranine bluish may be added to increase'redness. Another variation of the above is to hold up a flask containing a pink liquid, explaining "it has the ability to infect nearby white wines, converting them to ros6." When the pink flask &d another ~olorlessflask are shaken simultaneously, the colorless one turns to pink and the pink one tums to purple ("hack to grape juice"). The purple liquid fades to pink than colorless; the pink liquid fades to colorless. In this demonstration both flasks contain the pink safranine bluish system and one has in addition a little methylene blue. The one containing the methylene blue is shaken slightly heforehand to activate the pink color. This demonstration is useful for illustrations of reversible redox reactions and for relative reduction potentials. The leuco safranine bluish dve has a lower reduction notentid than leuco methylene hlue; thus any oxidized colorkd methylene blue formed hv reaction with oxveen is converted to the leuco form with concomitant oxidationof the leuco safranine hluish to the pink form. Methylene Blue (oxid.) + Safranine Bluish (leuco) * Methylene Blue (leuco) + SafranineBluish (oxid.) Only after the safranine hluish is all oxidized, do we see the color of the oxidized methylene blue in addition to the pink. This demonstration might also he used in an elementary biochem course to introduce a discussion of the electron transport chain.
