A Spectroelectrochemical Demonstration of a Modified Belousov-Zhabotinskii Reaction Jeffrey Rosenthal University of Wisconsin-River Falls, River Falls, WI 54022 Introduction Recent developments in Chaos (1)have sparked renewed interest and improved understanding of some interesting oscillating reactions. One of t h e most popular, the Belousov-Zhabotinskii (BZ) reaction, involves reduction of bromatc bv c e n u m ~ l lthrough l~ a complic~tedmechanism to form bromide and cenum(1V1. CeriumrIV, is convcncd back to cerium(II1) by malonic acid, and the cycle repeats itself, or oscillates, many times. The following demonstration, which measures both the electrochemical potential and the solution transmittance, was based on a modified version of the BZ reaction (2). The modification replaces cerium with ferroin, a n iron tris(orthophenanthro1ine)comdex.. in order to imnrove the visual indication of the oscillations. Reference2 wntainsadiscussim ofthemechanism of rhe BZ reaction a s well as references to mom dctailcd descriptions. Reference 3 includes instructions for a demonstration of the cerium BZ reaction that follows the electrochemical potential.
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Experimental The oscillating reaction recipe was obtained fmm reference 2. Four solutions were prepared from reagenbgrade chemicals.
The mixture was swirled in a wvered flask until the orange (bromine) disappeared from both the solution and the gas. A 1-mL aliquot of solution D was added to the mixture and stirred until the solution was a homogenous red. This solution was then poured into a 150-mL beaker and placed on a magnetic stirrer a s shown in Figure 1. The electrochemical potential of the iron(II)/(III)tris (orthophenanthroline) redox couple was monitored by measuring the potential difference between a platinum disk electrode and AgIAgC1 reference electrode. Note that a different reference electrode may be preferred because this reactionis very sensitive to chloride. The chloride leak from the reference electrode was negligible; reproducible oscillations were observed for more than a n hour. The light from a slide projector (the arrow in Fig. 1) was directed between the two electrodes and was filtered by a piece of red glass placed in front of the photodiode. Thus, the ~hotodiodemeasured the transmittance of the solution ab&e 600 nm. Hecause the irond11complex was red (high transmittanceabove 600nm1and the Iron! 1II)comolexwas blue (high absorbance above 600 nm), the transmittance indicated the iron (II)/(III)concentration ratio. The electrochemical potential and photocurrent were both recorded by a Data Translation DT 2801 analog-todigital hoard interfaced to a DEC Rainbow 100 computer.
D. 2 5 m ferroin ~
To ensure that no chloride was present, as it kills the oscillation, the ferroin was prepared from ferrous ammonium sulfate a n d a stoichiometric amount of orthophenanthroline (3 mol of orthophenanthroline to 1mol of iron). The oscillating solution was obtained by mixing 14 mL of solution Awith 7 mL of solution B and 2 mL of solution C.
Pt Electrode
rv
AA
Ag/AgCI Reference Electrode
Figure 2. Diagram of the circuit used to amplifythe photocurrent. The op-amps are available from Datel Internil, and the photodiode from Motorola Semiconductor Products, Inc. The electrochemical ootential tvoicallv was 0.9-1.0 V and was recorded directl; ~ h o t o c u k n (akps, t was converted t o a voltage and amolified. We used a s i r n. ~ l e+ o ~ ..a m ~ c i r c u i t (see Fig. 2). The software, written in C, graphically displayed in real time both signals a t sampling rates up to 20 Hz. The data were plotted on an HP 7475 digital plotter. Results and Discussion
'R
Magnetic Stirrer
Figure 1. Diagram of the demonstration setup. 794
Journal of Chemical Education
The data plotted in Figures 3 and 4 were typical. As Figure 3 shows, oscillations were quite regular. The spurious points in the photocurrent were caused by carbon dioxide (from oxidation of malonic acid) bubbling through the solution in front of the photodiode. The expanded time scale of Figure 4 shows the correlation between electrochemical potential and photocurrent.
T l m lierondsl
Figure 3. Photocurrent and redox potential for twelve cycles of the described reaction.
Figure 4. An expanded plot of one cycle, showing the correlation between photocurrent and redox potential.
The electrochemical potential may be used to calculate the iron(II)/(III)concentration ratio from the Nemst equation.
and the photocument was a t a minimum, indicating a low transmittance of the solution above 600 nm. The blue iron(II1) complex was the predominant species. At the minimum ele&rochemical the photocun-ent was at a maximum because the red iron(I1) complex was the predominant form.
[Fe" (o-phen) I EmlI= 9 - 0.5916log
[Fel" (o-phen) I
ECelris the measured potential, and
IP is
the standard reduction potential for the iron(II)/(III) o-phen complex redox couple (0.950 V us SCE) [Fe" (o-phen)
1 and [Fel" (o-phen)? I
are the complex ion concentrations. For the maximum electrochemical potential (1.0 V) the concentration ratio was 0.14. I n other words, there was approximately 7 times more iron(II1)complex than iron(I1) complex. At the minimum potential (0.90 V) the concentratinnntio was 7.0: the iron(II1 comvlex concentration was ~---approximately 7 times that'of the ibn(111) complex. These results corresvonded with the obsewed vhotocurrent. At the maximum'eledrochemical potential ~
Conclusion The visual affedwas very useful. This demonstration has been presented a t our "Science Day" program, a biannual event a t the university to which local high school science teachers are invited to bring their classes. Both the students and their teachers attend a varietv of discussions and demonstrations presented by several departments. Beyond the colors. however. there is a lot of chemistw that mav be discussed'by comp&ng the electrochemical &d transmittance data. Literature Cited
~
[Fe" (o-phen)p 1 < [Fen' (0-phen)? I
1.Dleick, J. Chaos M o k i w o New S&nn;VikingPenguin: NmYork, 1987. 2. Epteul.1.R. ChemEngNelus, 1987,1Msreh,301,2P36. 3. Hamis, D. C. Quontitotiw Cbmieol Analysis. 2nd ed.: W.H.Freeman: New Yorh, 1987; p p 3 4 5 3 4 7 .
Directions for "Supermarket Chemicals" Experiments Available The authors of the article "Qualitative Analysisof Eleven Household Chemicals" [Solomon, S.; Fulep-Poszmik, A.; Lee. A. J.Chem. Edne. 1991,68,3281, which was featured on the April 1991 cover, have received a number of requests for their laboratory dirretmns. They will send 8 copy of there directrun* to anyone who requests rhem. Thcy are also availablr i n a Maemtovh furmar, which will he prowded to anyonesendingn disk. Requests rhould bedmckd to: Sally Solomon, Department ofChemisty, 32nd and Chestnut Srrrrts, Drexel I..nwersity, Philadelphia, FA 19104.
Volume 68 Number 9 September 1991
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