edlted by GEORGE L. GILBERT Denison University Granville, Ohio 43023
Enhancement of Concentration Quenching by Micelles
The Thermite Lecture Demonstration Joseph W. Bozzelli and Robert B. B a r a t New Jersey Institute of Technology 323 High S t r e e t Newark, N J 07102
Submitted by: Submitted by:
Checked by:
Norbert Roesslert Department of Physical Chemistry University of Siegen Federal Republic of Germany Arnold George Mansfield State College Mansfield, PA
Since the catalytic effect of micelles on reactions involving ionic species has aroused considerable interest in r e s e a r ~ h , ~ it is desirable to have a direct introduction to this phenomenon. A convenient reaction to use is the concentration quenching of the fluoresceine dianion, which occurs by the formation of a non-fluorescent dimer.a According to Hartley's rules,' fluoresceine is attracted to cationic micelles such as cetyl trimethyl ammonium bromide (CTAB) because the surfactant and the dye are oppositely charaed. Fluoresceine l c the miiomcr iurfactant molecules and does 1101a ~ w c i ~with is fullv fluortwent at surfartant runcentrations hrlow the critical micelle concentration (crnc).At concentrations above the cmc practically all of the dye is adsorbed onto the micelles. When the number of micelles is small, the fluorescence is almost entirely quenched due to the high local concentration of the dye. If, at constant dye concentration, the concentration of CTAB is raised, the fluoresceine is diluted and the fluorescence reappears. Demonstration A heakercmtaining I00 ml of'a 5 X lO-'M solution 01 the sodium ;all al'fluaresceinr is s la red on R mawetic stirrer. The effect is easily seen at this concentration, ifthe ambient light is kept low and the beaker illuminated directly. Ultraviolet light is not necessary. A 5 X 10-7 M solution of fluoresceine M CTAB is added slowly enough to ensure tborin 5 X ough mixing. This can be conveniently done by letting the solution flow from a buret. When CTAB solution is added (1-5 ml), the fluorescence disappears, only to reappear gradually as the CTAB concentration increases (15-20 ml). Depending on the ambient temnerature. it mav be desirable to use warm solutions, since CTAB tendsto falfout at around 21°C.Also, a second beaker of solution can be dated aloneside to allow a direct comparison.
Becky J. Hickey Uniuersity of Washington Seattle. WA
Checked by:
The thermite reaction and lecture demonstration' is an extremely dramatic and illustrative one for the principles of exothermic reactions and activation eneraies. I t is certainly a favorite among thr studenti and lecturers at schuds where it is performed. Two m~rliticarionsu, this lecture demonstration are described here, which make its set-up and implementation simpler and more accessible to the lecture. The first modification is that only the glycerine plus KMn04 reaction is used to ignite the t h e r m k 2 There is no barium peroxide-aluminum igniter mix and no magnesium strip to light the ignitor mix required. The elimination of the BaOdAI mix and the Me strip at the top of the thermite simplifies the demonstra~ionc;msidrrnhli. 'I'he second modification is the use o t a ?',-in. clay flower pot3 in place of a similar size clay crucible. The ciay pot' eliminates the task of drilling an outlet opening in the bottom of the crucible and holds up well during the reaction. Additional strength can he obtained, should it he desired, by placing one clay pot inside a second, or by using a 3-in. pot, which has a somewhat heavier wall thickness. I t is not recommended, however, that additional thermite mix beadded to fill the larger volume. For the demonstration, 15 g of Al powder or granules are mixed with 50-55 g of Fe20s, which leaves the Fez03 in slight excess. While Al granules are satisfactory, finely divided Al powder in a well mixed ALFe203 batch are recommended for a greater reaction probability. This mixture is then scooped into a clay pot which has a layer of paper towel or filter paper placed over the bottom opening. A small cone-shaped indentation is then made in the top of the thermite mix ap-
Remarks If an apparatus for fluorescence measurements is available, this demonstration can readily be extended into a laboratory experiment to determine the cmc. The procedure is entirely analogous to an experiment previously described in This Journal
-
.- . * ....
"Reaction Kinetia in Micelles" (Editor: Cordes, Eugene),Plenum Press, New York-London, 1973. Tdrster, T., and Kdnic, E., Ber. Aumenges. Pkya. Ckem., 61,344 (1957). Hartley, G. 5,Trans.Fnrodoy Soc., 30,444 (1934). Rujimethabhas, M., and Wilairat, P., J. CHEM. EDUC., 55,342 (1978).
LPAPER Set-up for thermite bomb.
Volume 56, Number 10. October 1979 1 675
proximately 1cm deep and 1-2 cm wide at the top (see figure). This indentation is filled with approximately 20-25 g of KMn04. A second small cone-shaped indentation is then made in the KMn04 crystals. The thermite bomb is then placed in a safety glass huusing, supported by the usual metal ring-stand with a metal water can and sand bath, respet!tiv~ly, lirci~teddirectly l~rnenththe reaction vessel. Pive to six milliliters of glycerine are then added to the depression in the KMnOd and an ignition is achie\.ed in abuut 20 to 60set:. This is a very exothermic reaction and it is necessary to use a proper safety shield to contain sparks and a sand bath to catch the molten iron. The demonstration gives off some smoke and dust and may he best performed a t the end of the period. The reaction under the above conditions has failed to ignite o r o ~ e r l vonlv once in two vears a t NJIT. where a t least 20 de&onshati&s are run per ;ear. This succkss rate is probably fortuitous. and a failure rate of 10%or less mav he ex~ected. It shuuld be pointrd out that the more convent"iona1 ~g strip plus BaO../Al ieniter mix method has exhibited a much hieher failure rate than the present system. A ~l\iln04-~lyc&ine
676 1 Journal of Chemical Education
ignition of a BaO2/A1 igniter mix2 on the thermite also shows a high ignition rate. Should the reaction fail to ignite, simply add more KMn04 and glycerine a minute or two after the initial flare-up that resulted from the glycerine oxidation hy KMn04. It might be pointed out to the students that the amount of energy output from the reaction is approximately equal to the energy needed as input to form enough Al metal for one aluminum can of 12 ounce beverage type (the reverse reaction). The large quantity of energy necessary for A1 production well demonstrates the value of recycling Al cans.
'Abea, Huben, "Tested L)emmarmtmnsin Chemistry", 6th Edition, American Chemicul Society, h w w n of Chemical Education. Easton, Pa., 1965. 2Klein,D. C., J. CHEM. EDUC., 14,320 (1937). Xlay flower pots are characterized by the inside, top diameter. 'Clay flower pots are available in quantities from florist supoliers. 5Brockett,C. P., J. CHEM. EDUC., 29,527 (1952).
