Article pubs.acs.org/jchemeduc
A Modified Recipe and Variations for the Briggs−Rauscher Oscillating Reaction Stanley D. Furrow* Department of Chemistry, Penn State Berks College, The Pennsylvania State University, Reading, Pennsylvania 19610, United States S Supporting Information *
ABSTRACT: A modified recipe for the Briggs−Rauscher oscillating reaction is suggested. The modified recipe, [H2SO4] = 0.10 M, [KIO3] = 0.020 M, [NaIO4] = 0.00062 M, [malonic acid] = 0.050 M, [MnSO4] = 0.0067 M, [H2O2] = 1.0 M with 0.01% starch, turns blue almost immediately, then oscillations start with two “slow” (∼25 s) cycles, then settle into regular approximately 12−15 s cycles. Oscillations end with a clear solution, avoiding cleanup of solid iodine resulting from the “standard” recipe, so that only neutralization of excess acid is needed before disposal. Effects of concentration variations are presented.
KEYWORDS: First-Year Undergraduate/General, Demonstrations, Physical Chemistry, Textbooks/Reference Books, Electrolytic/Galvanic Cells/Potentials, Kinetics, Mechanisms of Reactions, Oxidation/Reduction
T
he Briggs−Rauscher oscillating reaction1 with H2SO4, KIO3, malonic acid (MA, CH2(COOH)2), Mn(II) catalyst, H2O2, and starch has been a favorite with lecture demonstrators and chemical “magic” shows because of its dramatic and rapid color changes from clear to yellow-brown to blue and repeat. It has been used as a laboratory exercise. A typical recipe2−5 results in a mixture with [H2SO4]0 = 0.026 M, [KIO3]0 = 0.067 M, [MA]0 = 0.050 M, [MnSO4]0 = 0.0067 M, [H2O2]0 = 1.3 M with 0.01% starch. These concentrations are similar to those originally reported by Briggs and Rauscher. Starch is used to provide the deep blue color when I2 and I− are both present. None of the concentrations are critical so the demonstration is rather reliable. (A test ahead of time is advised.) One drawback of the above recipe is the precipitation of solid I2 when oscillations end. The reaction no longer oscillates, but oxygen is still evolved, carrying iodine fumes into the surrounding air. Furthermore, the solid iodine should be reduced to iodide before disposal. A time series from the above mixture is shown in Figure 1. Solid I2 precipitation and rapid increase in [I−] coincide with the drop in potential of the iodide-selective electrode versus Ag/AgCl double-junction reference. It is possible to retain most of the features of the demonstration above, yet avoid the precipitation of I2. The [IO3−] must be lowered to half the [MA] or less, and the [H2SO4] must be raised to keep the vivid blue part of the cycle. If the acid concentration is too low, the mixture may still oscillate (detected by a suitable electrode system), but the oscillations may not be visible. For that reason, we have not attempted to optimize the duration of oscillations, nor could that easily be done from the data referenced herein, as a majority of experiments were done in a darkened container without starch, © 2012 American Chemical Society and Division of Chemical Education, Inc.
Figure 1. Time series for “standard” Briggs−Rauscher demonstration, potential of iodide-selective electrode versus Ag/AgCl reference. [H2SO4]0 = 0.026 M, [KIO3]0 = 0.067 M, [MA]0 = 0.050 M, [MnSO4]0 = 0.0067 M, [starch]0 = 0.01%, [H2O2]0 = 1.3 M. A, H2O2 added; B, solid I2 precipitates. Blue color (from starch−iodine complex) coincides with low potential (roughly below zero) and high [I−] and [I2].
and there are regions of initial concentration where the blue starch−iodine complex would not be visible.
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BEHAVIOR WITH CLASSIC BRIGGS−RAUSCHER INGREDIENTS To avoid iodine precipitation, [MA] must exceed [KIO3] by roughly a factor of 2. That ratio is the proportion necessary to ensure that the limiting reagent for the end of oscillations is Published: September 4, 2012 1421
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Table 1. Approximate Ranges and Trends with Decreasing Concentration of Reactants Species and Rangea [H2SO4] 0.40 to 0.0060 M [KIO3] 0.030 to 0.0090 M [MA] 0.090 to 0.030 M [MnSO4] 0.020 to 0.0030 M [H2O2] 2.5 to 0.30 M
Induction period/s decrease 400 to 13 increase 55 to 220 decrease 150 to 30 increase 50 to 240 increase 20 to 340
Oscillation Length/s
Time to Transitionb/s
increase, decrease 400 to 700 to 60 below 0.025M, decrease 500 to 90 increase, decrease 170 to 700 to 250 increase, decrease 400 to 500 to 0 decrease at low [H2O2] 500 to 90
increase 500 to infinite below 0.025 M increase 1050 to infinite decrease infinite to 400 generally increase 800 to 2400 increase 340 to infinite
a
Concentration range over which oscillations were observed. bInfinite signifies a stable potential for several thousand seconds in a concentration region where the trend is toward longer times.
Figure 2. Time trace of potential of iodide-selective electrode versus Ag/AgCl reference for the reference Briggs−Rauscher reaction. [H2SO4]0 = 0.10 M, [KIO3]0 = 0.020 M, [MA]0 = 0.050 M, [MnSO4]0 = 0.0067 M, [H2O2]0 = 1.0 M. A, H2O2 added; B, induction period, 67 s; C, length of oscillations, 468 s. (Left) Plot over a 10 min time period; a type I state and (right) same plot extended to 33 min time period showing the transition from a type II to a type I state, D = 816 s.
solution had the following concentrations: [H2SO4]0 = 0.10 M, [KIO3]0 = 0.020 M, [MA]0 = 0.050 M, [MnSO4]0 = 0.0067 M, [H2O2]0 = 1.0 M. There is considerable leeway in the concentrations for which oscillations occur. Oscillations are present with [MnSO4] and [H2O2] variations over a 6- to 8-fold range and with [H2SO4] over more than 60-fold range. The concentration of each of the components was varied, oneby-one. The data were taken from an Orion iodide-selective electrode model 9453 versus Ag/AgCl double junction reference electrode filled with 10% NaClO4 solution. The solution was held in a thermostatted double-walled beaker at 25.0 °C, with light excluded. Demonstrations at ambient temperature, containing starch, and exposed to light may differ from the reported times. The oscillating solutions are known to be light sensitive. A time series with the reference solution (without starch) is shown on the left in Figure 2 (the time series with starch present is nearly the same). There is an induction period of about 1 min. The oscillations with a period of about 12−15 s last for about 8 min (as recorded with a suitable electrode pair), and the oscillations end with a clear solution. For a visual demonstration (including starch), the blue transition gradually gets weaker and is no longer visible after about 5 min.
iodate instead of malonic acid and that the principal reactive organic species are malonic acid and iodomalonic acid (IMA), not diiodomalonic acid (I2MA). Excess MA inhibits the decomposition of I2MA.6 The concentration dependence of the other reagents to prevent solid I2 is much smaller. Lowering the iodate concentration from 0.067 M in the “standard” recipe to 0.020 M so that [MA] will reach a 2.5-fold excess will prevent the formation of I2 for days, but cause an increase in an induction period. With just that change, the blue starch−iodine complex never forms. [H2SO4] can be increased to 0.10 M and the blue transition is again visible, but increase in acidity also increases the induction period. Furthermore, we have tried to lower the concentration of caustic hydrogen peroxide. Those three changes all work in the direction of increased induction time before the first blue color.
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BEHAVIOR WITH MODIFIED CONCENTRATIONS Concentration space has been examined around a reference solution, looking for the region in which the oscillations end with no solid iodine. A summary of the trends of variation of [H2SO4], [KIO3], [MA], [MnSO4], and [H2O2] is shown in Table 1; the detailed data are available in the Supporting Information. The induction period is the time from addition of H2O2 to the next rise in potential, reproducible to about ±5 s. The oscillation length is the total time oscillations were observable (by electrode potential), reproducible to about ±15 s. The time to transition is the delay from the end of oscillations to the midpoint of the potential drop at the transition. These times were erratic, varying from run to run by 200 s and sometimes much more. Whatever was causing the variation has not been identified. The reference
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TRANSITION FROM TYPE I STATE TO TYPE II STATE
With the reference Briggs−Rauscher reaction, the amplitude of the oscillations gradually diminishes and the oscillations end with the potential relatively high, that is, low [I−]. By analogy with steady states in flow reactors, Vanag7 has described this state as type I (low [I−]) and the previous state shown in Figure 1 as type 1422
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II, that is, low potential and high [I−]. From 15 to 30 min after end of oscillations, there is a transition from type I to type II and the solution suddenly turns blue again. This is shown in on the right of Figure 2. The [I2], however, does not exceed the solubility limit. The [I−] and [I2] slowly decrease again by the following reactions:
recipe is given in the Supporting Information using 3% H2O2 instead of 30%.
H 2O2 + 2H+ + 2I− → I 2 + 2H 2O H 2O2 + I 2 + 2CH 2(COOH)2 → 2CHI(COOH)2 + 2H 2O
Much of the iodate has depleted by the end of the oscillations. The type I to type II transition obviously comes with too long a delay to be directly included in an oscillating solution demonstration. It may, however, be left standing and will certainly be a surprise if observed by the audience.
Figure 3. Time trace of potential of iodide-selective electrode versus Ag/AgCl reference for modified Briggs−Rauscher reaction with NaIO4. A: H2O2 added. The color is blue (from starch−iodine) when the potential is below approximately zero volts.
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ELIMINATING THE INDUCTION PERIOD The induction period after mixing, during which time the solution remains colorless or pale iodine color, can be either an awkward pause or an opportunity for the demonstrator to explain some features about the reaction. A qualitative explanation of the reaction is available; however, a quantitative explanation is still elusive. On addition of the last component, usually [H2O2], to the other components, there is an immediate rise in potential corresponding to an abrupt rise in [HOI] or [HOIO] or both. Most of the induction period is the slow drop in potential (increase in [I−]) and sometimes a minor delay until the next potential rise, after which the oscillations are more or less regular. That slow reduction of HOIO or HOI is inhibited by increased [MA] or surprisingly, by increased [H2SO4]. The rate of iodine production, in a system without MA, drops off steeply below [H2SO4] approximately 0.05 M,8 yet the induction period for the oscillator is very short in that range of acidity. Thus, the induction period is short even though I2 production is slow. It may be that oxidation of HOIO and HOI is increased as [H2SO4] increases. For those demonstrators who wish to eliminate the induction period, there are three possibilities. The first is to increase [H2O2]. This is effective, but is opposite to the goal of lowering of [H2O2]. The second is the addition of I− after acid, iodate, and MA, but before starch and hydrogen peroxide. This is also effective, but iodide and starch must be in separate stock solutions. After iodide is added, a waiting period is necessary for the iodine color to clear before starch is added. The extra steps take about as much or more time than the induction period that one is trying to eliminate. The third is the addition of NaIO4. This remedy is recommended. Detailed instructions are given in the Supporting Information. Three stock solutions can be prepared: one with H2SO4, KIO3, and NaIO4; one with MA, MnSO4, and starch; and one with H2O2. (NaIO3 could be substituted for KIO3.) When equal volumes of the three are mixed, NaIO4 reacts rapidly with H2O2 to form some IO3− and I2. The blue color forms almost immediately. The first two cycles are lengthened somewhat (to 25 or 30 s), then the oscillations repeat at 12−15 s intervals. NaIO4 will have reacted completely and have no further effect. Thus, the recommended concentrations after all components have been added are as follows: [H2SO4]0 = 0.10 M, [KIO3]0 = 0.020 M, [NaIO4]0 = 0.00062 M, [MA]0 = 0.050 M, [MnSO4]0 = 0.0067 M, [H2O2]0 = 1.0 M, [starch]0 = 0.01%. A time trace is shown in Figure 3. A second modified
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MECHANISMS A skeleton model has been proposed9,10 that involves radical production by the reaction H+ + IO3− + HOIO ↔ 2IO2 · + H 2O
(1)
followed by H+ + Mn II + IO2 · → HOIO + Mn III
(2)
then regeneration of MnII by Mn III + H 2O2 → Mn II + HOO· + H+
(3)
2HOO· → H 2O2 + O2
(4)
The sequence (1) + [2 × (2)] + [2 × (3)] + [2 × (4)] leads to autocatalytic formation of HOIO: H+ + IO3− + HOIO + H 2O2 → 2HOIO + H 2O + O2
Eventually I2 is produced by the net process 2H+ + 2IO3− + 5H 2O2 ( +Mn II) → I 2 + 5O2 + 6H 2O ( +Mn II)
(5)
I2 reacts with MA, with the net process 5CH 2(COOH)2 + H+ + IO3− + 2I 2 → 5CHI(COOH)2 + 3H 2O
(6)
During oscillations, the processes alternate between eq 5, when [I2] and [I−] are low but increasing, and eq 6 when [I2] and [I−] are high but decreasing. See the demonstration in ref 2 for a more complete explanation. Attempts have been made to revise the skeleton mechanism,11−15 but to date many unresolved questions remain. Reaction 1 is probably not the source of radicals. An alternative source is yet to be identified. Evidence has been presented15 that the reaction H+ + IO3− + HOO· → IO2 · + O2 + H 2O
(7)
is important. The iodinated products are reactive and their reactions should eventually be included,.6,16 The skeleton model 1423
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can simulate oscillations of about the correct frequency, but changes in [I2] and [I−] and variations with initial concentration changes are not well reproduced in the simulations. A subset of the Briggs−Rauscher oscillating reaction, the Bray−Liebhafsky17 oscillator, is based on only acidic H2O2 and KIO3. (That oscillator has convenient oscillation frequency only at elevated temperatures, 50−70 °C.) Obviously a complete model must be able to handle both systems (adjusting for temperature and concentrations). A recent impressive Bray−Liebhafsky model has steps that are not included in the Briggs−Rauscher skeleton model,18 although some of those steps may be negligible under Briggs−Rauscher conditions. One reason for inclusion of the tables with concentration variation is to provide a basis for modeling in the future. Modeling heterogeneous precipitation of iodine will be more challenging. The transition to the type II state is no doubt related to formation and decomposition of I2MA.6,7,15 Studies on that transition are ongoing.
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ASSOCIATED CONTENT
* Supporting Information S
Specific recipes for the demonstration using 30% H2O2 or 3% H2O2; tables with variation of properties with concentration. This material is available via the Internet at http://pubs.acs.org.
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AUTHOR INFORMATION
Corresponding Author
*E-mail:
[email protected]. Notes
The authors declare no competing financial interest.
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REFERENCES
(1) Briggs, T.; Rauscher, W. J. Chem. Educ. 1973, 50, 496. (2) Shakhashiri, B. Chemical Demonstrations; A Handbook for Teachers of Chemistry; University of Wisconsin Press: Madison, WI., 1985; Vol 2, pp 248−256. (3) Summerlin, L. R.; Ealy, J. Chemical Demonstrations, 2nd ed.; American Chemical Society: Washington DC, 1988; Vol 1, p 113. (4) Roesky, H. W.; Mökel, K. Chemical Curiosities; VCH: New York, 1996; p 264. (5) Wang, M. R. J. Chem. Educ. 2000, 77, 249. (6) Onel, L.; Bourceanu, G.; Wittmann, M.; Noszticzius, Z.; Szabó, G J. Phys. Chem. 2008, 112, 11649. (7) Vanag, V. K. J. Chem. Biochem. Kinet. 1992, 2, 75. (8) Cooke, D. O. Int. J. Chem. Kinet. 1980, 12, 683. (9) Noyes, R. M.; Furrow, S. D. J. Am. Chem. Soc. 1982, 104, 45. (10) DeKepper, P.; Epstein, I. R. J. Am. Chem. Soc. 1982, 104, 49. (11) Plath, P. J.; Wiegel, H. Z. Phys. Chem. (Leipzig) 1987, 268, 33. (12) Turanyi, T. React. Kinet. Catal. Lett. 1991, 45, 235. (13) Vukojevic, V.; Sorensen, P. G.; Hynne, F. J. Phys. Chem. 1996, 100, 17175. (14) Szabó, E.; Sevcik, P. J. Phys. Chem. A 2010, 114, 7898. (15) Furrow, S. D.; Cervellati, R.; Amadori, G. J. Phys. Chem. A 2002, 106, 5841. (16) Furrow, S. D.; Aurentz, D. J. J. Phys. Chem. A 2010, 114, 2526. (17) (a) Bray, W. C. J. Am. Chem. Soc. 1921, 43, 1262. (b) Bray, W. C.; Liebhafsky, H. A. J. Am. Chem. Soc. 1931, 53, 38. (18) Schmitz, G. Phys. Chem. Chem. Phys. 2011, 13, 7102.
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