J. N. Demur and D. Diemente University of Virginia Charlottesville, 22901
I
An Oscillating Chemical Reaction
with a Luminescent Indicator
Oscillatine chemical reactions. esneciallv the Belousov reaction ( t h i cerium-catalyzed oxidition of malonic acid by hromate ion), have received much recent attention ( I 6). The Belousov reaction has been adapted for a lahoratory experiment (3) and a lecture demonstration (2). In this adaptation, ferroin, tris(l,lO)phenanthroline)iron(II) ion [Fe(phen).?+], is added as a redox indicator. The Fe(phen)?+(red)-Fe(phen)3sf (hlue) color change signals the oscillations in the Ce4+-Ce3+ ratio. We wish to report a modification which makes the demonstration more dramatic and much easier to view in large lecture halls. One of our present research activities concerns luminescence in metal complexes. From these studies we were aware that the deep orange ions tris-(2,2'-hipyridine)ruthenium(I1) [Ru(bipy),Z+) and tris(1,lO-phenanthroline). ruthenium(I1) [Ru(phen)3z+] are highly luminescent in aqueous solution, emitting bright orange when excited by uv light. In addition, hoth complex ions are readily oxidized to the corresponding nonluminescent ruthenium(II1) species. The oxidations are reversible with formal reduction potentials of -1.25-1.30 V for the Ru(bipy)32+Ru(hipy)33+ and R~(hipy),~+-Ru(phen)33+ couples (7). The Formal reduction potentials for Fe(phenI32+-Fe( p h e n ) P and Ce3f-Ce4+ are -1.1 V and -1.44 V, respectively (7). Since a t one extreme in the oscillation of the Belousov reaction, the cerium is to a significant degree in the oxidized form, it is clear that the solution potential rises during the oscillation to a t least -1.4 V. Since the ferroin indicator is predominantly in the reduced (red) form a t the other extreme in the oscillation of the modified Belousov system, the potential during this phase must fall below -1.1 V. These extremes straddle the reduction potentials of the Ru(bipy)aZ+-Ru(bipy)33+ and Ru(phen)~Z+-Ru(phen)33+couples, SO it occurred to us that the ruthenium species might oscillate between their reduced and oxidized forms if either of them replaced the ferroin indicator in the modified Belousov reaction.. On trying the demonstration using the reagent concentrations given by Field (2), but substituting R ~ ( b i p y ) ~ Z + in place of ferroin, rhythmic oscillations were observed, although the color change under room light was not easily discernible. On turning out the lights and irradiating the solution with uv radiation, however, the oscillations were most dramatically revealed by a red-orange luminescence
Concentrations for the Oscillatina Reaction
Compound Ru(bipy)aClz Malonic Acid KBrOa H&O.O
Minimum 2.0 X 0.1 0.045 0.4
Concentrations Optimum Range
Maximum
(0.8-1.3)x 10-4 0.2-0.3 0.045-0.07
1.3 x 0.4 0.09
1.0-1.4
20
Concentrated HzSOI was assumed to be 18 M. which gradually grew in intensity and then abruptly extinguished; the period was about 20 sec. This reaction is an excellent demonstration even for large lecture halls, so long as the room can he almost completely darkened. Further consideration led to an improvement. Since the cerium ion may absorb uv light and reduce the energy available to the indicator, it was felt desirable to miuimize the cerium concentration. Since a wide variety of metal ions with oxidation states differing by one electron are capable of catalyzing the reaction (inter alia MnZ+Mn3+, Fez+-Fe3+, Ce3+-Ce4+, and even Fe(phen)32+Fe(phen)2+ (2, 6)) it seemed reasonable that R ~ ( h i p y ) 3 ~ + might he able to function as hoth the catalyst and the indicator, replacing ferroin and cerium ion in the demonstration. We repeated the demonstration with the suggested substitution. It was necessary to adjust the reagent concentrations to sustain regular oscillations (vide infra), hut the reaction is far superior for visualization by luminescence than when cerium ion is present. The emission is bright enough that it can be seen easily in a faintly lighted room, althoueh it is. of course. more dramatic if the room is quite biack. For the excitation we used a 15-in. Sears blacklight fixture with a Westinghouse F15TBIBLB lamp. Tbis fixture has an efficient reflector and is capable of efficiently irradiating solutions in wide, tall graduated cylinders. The Ru(bipy)aCl~.6HzOwas bought from G . Frederick Smith Chemical Company. This complex is rather expensive (-$lo per g) hut so little is used for the demonstration that a 1-g bottle should last for years. For maximum luminescence intensity, the sample should be kept as close to the lamp as possible, and it must he efficiently stirred to prevent spatial oscillations. The lamp has a significant hlue output which can partial-
Volume 50, Number 5, May 1973 / 357
ly obscure the sample luminescence. This is easily removed with a sheet of orange or red cellophane or plastic placed between the reaction vessel and the viewers. The best color and thickness of material can only he determined by experiment; materials emitting noticeable visible light when exposed to the blacklight are, of course, unsatisfactory. We find that two layers of thin red cellophane wrapped around the lamp-sample-stirrer assembly works well. Since the blacklight is then invisible even when viewed straight on, the lamp can he positioned to face the audience directly providing a viewing field greater than 180". The cellophane red-shifts the emission somewhat; using orange or yellow sheets would give a more accurate presentation. Concentration ranges of the reactants which sustain 0scillations are listed in the table. The optimum ranges are also given. The tahle is not exhaustive; weak oscillations occur with concentrations outside of the indicated ranges. At the higher concentrations of Ru(hipy),z+ (28 X M ) , the oscillation is easily seen under the room light as the catalyst-indicator cycles from the orange reduced form to the green oxidized form. This change is not as attractive as the red-to-blue ferroin oscillation. We usually performed the reaction a t room temperature, where the period is about 30 sec. Gentle heating
358 /Journal of Chemical Education
shortens the period, hut too high a temperature terminates visible oscillations. The period and the intensity of the luminescence are also quite sensitive to the H2S04 concentration; the higher concentrations give shorter periods and weaker emission intensity. Solutions also show the luminescence oscillation if Ru(hipy)?+ is replaced with Ru(phen)32+. Under our trial conditions, however, the luminescence is weaker. We found no noticeahle increase in the luminescent intensity when our solutions were deoxygenated with nitrogen. Classroom discussion following the demonstration can stress the importance of considering formal reduction potentials in the selection of the catalyst-indicator for this reaction. In addition, comments can be made on the thermodynamics (1, 2) and kinetics (5) of oscillating reactions. For more advanced students, the demonstration provides an amusing introduction to the discussion of excited states, molecular luminescence, and photochemistry. Literature Cited
