GEORGE L. GILBERT Denism University Granville. Ohio 43023
The hainbow Connection SUBMITTEDBY Bill Hutton Iowa State University Ames. IA 50011
Prepare a dropper battle containing 100 ml of a 0.2 M aqueous NaOH solution. H2SOa-Glycerin Solution Dissolve 10 ml of 18M HzSOain 20 ml of glycerol, and transfer the solution to a dropper bottle.
CHECKED BY
Wayne L. Smith Colby College Waterville. ME 04901 This demonstration illustrates rather dramatically how the color of certain organic acids or bases (acid-base indicators) can be influenced in a reversible manner by changes in pH. It is based on a "magic trick" described by Wilson, e t al.' Although it has obvious applications as a pedagogical tool, it is described here in the context of a routine which we have employed as part of chemistry demonstrations shows the Iowa State University Society for Chemistry Undergraduate Majors and ACS Student Affiliates has been presenting for anumher of years. The demonstration employs three indicators which are completely colorless in acidic solution but form the three primary colors in basic solution. Reagents Needed Indicators The following combination of indicators give the "rainbow colors" indicated. 1.5 g phenolphthalein plus 3.0 g mnitrophenol 1) RED 2) ORANGE 0.45 g phenolphthalein plus 6.0 g m-nitrophenal 3) YELLOW 6.0 g m-nitrophend 0.6 g thymolphthalein plus 6.0 g m-nitrophenol 4) GREEN 5) BLUE 1.5 g thymolphthalein 6) VIOLET 0.9 g phenolphthalein plus 0.4 g thymolphthalein Each of these indicator combinations is dissolved in 30 ml of 95% ethanol and stored in 30-ml dropping bottles labeled with the "color" indicated. Acid-Alcohol Solution Mix 250 ml of 0.05 M aqueous sulfuric acid with 250 ml of 95% ethanol. Transfer this liquid to a bottle (capacity 10 oz or more) equipped with an automatic "shot dispenser" as commonly used for alcoholic drinks, delivers 1.5 oz per "shot." (We use a 25.4-02 liquor bottleJ2 NaOH Solutions Transfer 1200 ml of a 0.012 M NaOH solution to a clear glass pitcher. (The standard 2.5-pt "pitcher" commonly used in beer halls serves very we1L3) 'Wilson, J. W., Wilson, Jr., J. W., and Gardner, T. F.. "Chemical Maaic." Chemical Magic. Box 331, Los Alamitos. CA 90720. i~'goodquality shot dispenser should be used, and the dispenser should be thoroughly cleaned immediately after use..An alternative methd. which does not reauire the use of a shot dis~enseris to use a intewais of 5 ml heavily - - -snml . . . . = -nrarlnutad . - . . . 7 7 . niwt. ~.which has aoorooriate ,. mar*& so they are more vis ole, and add'5-ml allq,ois to each tmble;. If ths proceoure is lo loweo. the ac d-alcoho solution concentration should be ncreasea. E q ~ avo Lmes of ethanol an0 0.4 Mn2SOn works well. A small mark should be made on the pitcher at a point corresponding to slightly more than the total volume needed to fill the 6 tumblers to be used In the demonstration. The oitcher should contain at least this volume of the base SOlUtiOn ~~~
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172
Journal of Chemical Education
"Doctored" Glass Tumblers Clean sin glass tumblers (approx. 12 oz). Arrange these in asingle row extending from left to right along the demonstration table. Place 2 drops of the red indicator in the center of the first tumbler; do not allow any to splash on the sides. In a like fashion, place 2 drops of the orange indicator in the second tumbler, and continue adding 2 drops of eaeh of the appropriately colored indicator solutions in a glass in the proper "rainbow" sequence-red to violet. Allow the solutions to drv. There is no hurrv about this. The tumblers can be oreoared . . a wrrk ahead cvdpornre tr, d r y I I P S . - ~ ~ ~ , O U I I hror ill. I t a heat lampor hwt g u n is ~ t turrped~rt d the drvine. care must be exercised not to decompose the indicators thermally. The result should be six tumblers eaeh with a dry film of a different indicator deposited on the bottom; this film should he practically unnoticeable to the audience. ~
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Needed "On Stage" to Present the Demonstration The six "doctored tumblers" should be arranged in sequence (red, orange, yellow, green, blue, and violet) before a white background. (We have found the added visibility provided by a light box, which illuminates the glasses from below and behind, makes the demonstration especially colorful.) In addition, have on hand
The dropper bottle of HnSOn-glycerin Stirring rod and towel The Demonstration Routine Step I . Each tumbler is picked up in succession, the inside wiped "clean" with the towel, and the tumbler returned to its proper place in the row. Of course, the indicator film on the bottom must not be disturbed in this operation. (Note: From this point on, various solutions will be added to the tumhlers. These additions should always be made in the same sequence, e.g., left to right.) Step 2. Equal volumes (1'shot,' 1.5 oz) of the acid-alcohol solution are poured from the bottle into each tumbler using the shot disoenser. In doine this. trv to direct the stream of liquid so i t d w i not fall directly nn the indicator residue; the less the residue is disturbed and the less the indirator disperses in the solution by the time the next operation is completed, the better the effect. Step 3. Using the pitcher, add to each tumbler enough of the 0.012 M NaOH solution to fill the tumbler approximately halfway. Pour the liquid slowly, directing the liquid towards the sides. If the base solution comes in contact with the indicator, a momentary "flash"of color may be observed, and one wishes to avoid this as much as possible. At this time, the added NaOH should not be sufficient to render the solution basic. Step 4. The demonstrator looks a t the six tumblers, half-filled with what appears to be a water-clear solution,
apparently observes there is enough liquid remaining in the pitcher to add more t o the tumblers, and proceeds to do so, beginning with the first tumbler in the sequence. This time, the solution is poured rapidly from the pitcher and the tumbler filled to approximately 314-in. from the top. As the liquid is added, each solution is observed to change instantly to one of the "rainbow colors." (The rapid addition of the excess base to the still-acidic solution, which by this time has solubilized the indicator, should mix them adequately and result in an almost instantaneous formation of the base-form of the indicator throughout each solution.) Step 5. From the dropping bottle, add enough of the HnS04-glycerin solution to each tumbler to return the solutions to their original acidic condition. The same amount of this glycerin-acid mixture should be added drop-wise to the center of each tumbler. Three drops should be adequate, but one should determine the exact amount bv experiment before the performance. When the last tumbler filled, squirt about two droppersful of the acid-glycerin into the pitcher in as inobtrusive a way as possible. If carried out correctly, the very dense acid-glycerin solution will sink to the bottom of the tumbler without dissolving, and the indicator should remain in its basic form. T o insure this, we prefer using the very viscous acid-glycerin solution. The same effect may be achieved with practice using a more dilute mixture (up to a 50:50, vlv, 18 M HzSOa-glycerin). Step 6. The solutions in the tumblers are then stirred raoidlv. startine with the first tumbler. and as each solution is stiried, the &or disappears. (NOTE: o n e must not wait too lone before startine to mix the solutions. else the colors begin to L d e prematureiy.) Step 7. Usine the d r o ~ ~bottle e r containing the 0.2 M N ~ O Hsolution &d a stirring rod, rapidly "titraie" the contents of each tumbler back to its original color?
Step 8. The tumblers of solution are then poured, one a t a time, in succession, back into the pitcher. The result is a colorless solution, much like the original. Safety Precautions Once prepared, the solutions are so dilute that there is virtually no safety hazard with the experiment. DIscussIon We have used this demonstration extensively in chemistry "magic" shows, where the explanation of the phenomena is left to the audience. In a classroom situation, we have concluded the demonstration by asking the students how i t was done. The presentation is always given concurrent with a lecture on acid-base indicators. Once the students are informed that only acids or bases are used in the demonstration, and thev conclude that there are indicators, other than phenolphtalein, which are colorless in acid solution and turn color in base, they generally deduce what is taking place. This deduction is arrived a t only after some rather interesting class discussion and a better understanding of such things as how indicators work and bow universal indicators or pH papers are prepared.
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A variation we have used is to pour only enough of the NaOH from the pitcher to obtain the desired color changes in step 4. (About the same volume of solution should be in the tumblers at this point.) Perform step 5, but do not add any of the giycerin-H2SO4acid solution to the pitcher. Instead of performing step 7. restore the colors by filling each glass with NaOH solution which remains in the pitcher. At this point. in as inobtrusive manner as possible the two dropperfuls of the glycerin-H2S04solution are squirted into the now empty pitcher. The demonstration is concluded as described in step 8. This routine does not require the 0.2 MNaOH, but it does require a bit more skill on the part of the demonstrator to perform it effectively.
Volume 61
Number 2
February 1984
173
