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A Simple, Continous-Flow Stirred-Tank Reactor for the Demonstration and Investigation of Oscillating Reactions J. M. Merino lnstituto "P. Rubia", Valladolid, 47008, Esparia Oscillatory phenomena in physics are well-known and can be emlained bv s i m ~ l elaws. Oscillatorv or ueriodic chemical reactions were fbund later. ~ x ~ l a n a k o n stheir of behavior were more difficult due to a too-rieid intermetation of the second law of t h e r m ~ d ~ n a m i c ~ trequires hat that a closed chemical system close to equilibrium must advance mevitably toward a final stable s;at.~.The phrase "close to equilibrium" is crucial. The first oscillatingchemical system was proposed by W. C. Bray (1,21,but it was Belousow's rcport (3,that sparked scrious investiaations of oscillatina reactions. Initially. such investigations were based on tGe use of bromate a d iodate ions a n d were only slight modifications of Belousow's or Bray's reactions. Moreover, they were confined to closed reactors that limited their possible variety. I n the 19709, I. R. Epstein and colleagues (15, 17-19) began a search for new chemical oscillators in an open system but with continuous flow. For this purpose, they formulated a set of necessary and sufficient conditions for periodic reactions to occur such that a chemical system might exhibit clocklike behavior. These conditions are 1. Chemical systems will oscillate only if they are far fram
their equilibrium state. 2. One step of the reaction sequence must be autocatalytic; i.e., the formation speed of a species must be pmportional
to its own coneentration. 3. The system must exhibit or possess bistability, such that s~ontaneousdevelaument mi& lead to anv one of two
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possiblcequilibriun~states,and that a pcnurbarion ofthe sysrrm enn provoke passngr from one rguil~hrurstate to another Equilibrium hcre is the aynarnic itcody stow of competing chemical reactions and nonequilibrium thermodynamics, rather than the "static" system ostensibly addressed by classical thermodynamics. Chemical oscillators have been integrated successfully into aspects of chemical education at all levels because of their spectacular visual effects, as well as for their pedagogical content based both in theory and practice. Moreover, they provide interesting analogies and correlations to oscillatory phenomena in biologic, ecologic, and sociologic/economic systems (4). Previous pedagogic reports of oscillating reactions have used bromate and iodate systems in the presence of malonic or citric acid under null flow conditions (5).Possible variations to visualize the oscillations by dramatic color changes have included use of catalytic redox p a i r s such a s Ce(IIIjICe(1V) (61, Mn(II)/Mn(III) (7) and Fe(II)/Fe(III) (81, as well as mixPresented in part at "Coloquio lnternacional de Enseilanza de la Quimica en idiomas de origen latino", Lisboa. Poltugal, November 1989.
754
Journal of Chemical Education
tures of these ions (9).Amore spectacular visual effect uses induced chemiluminiscence (10) to mark the oscillations in Belousow's system via the complex ion (2,2' bipiridyl) Ru (11).Yet other reports describe kinetic asDects of oscillatorv phenomena rl1,'and computer modelling of such reactlo& I 121. F'inallv. i t is worth commentme that it is urobablv the ~ r i ~ ~ s - ~ a i i creaction h e r (13,141,-that is th;! most ipectacular in terms of visual effects, and perhaps lends itself best to pedagogic demonstrations. All of thesystems meutioned above are removed far from the equilibrium state by means of a high initial concentration of an oxidant ion. Oscillations commence, and continue as long as the oxidant is present, although it is decreasing in concentration; oscillations will cease when it is consumed. For this reason, closed systems prohibit sustained oscillations. More interestingly, there are chemical systems that will oscillate under continuous flow conditions. For demonstrations, this is an important point. Oscillation can be induced and will continue as long as the chemical reservoirs deliver input species. Also, and unlike closed systems, oscillation ~ e r i o d sremain constant rather than increasine steadilv i n d essentially ending the demonstration prem&urely by too lona a wait between oscillations. Commerciallv available research assemblages are convenient reactor systems for the investigation of oscillating reactions (15) but require the use of peristaltic that are expensive for general use. There are other complications that can make their use more difficult for classroom demonstrations, but the assemblage described below avoids many of these problems and is both inexpensive, convenient, and easy to assemble and use. Equipment Assemblage Asimple CSTR device that does not require a peristaltic puma uses eravitv or pressure feed. and can be used to inveHtigate oicillaiing reactions is shown in Figure 1 (general desim and approximate dimensions of the overall aup a r a t u s ) r ~ ofeed-ihernicals to the reactor vessel, two (or more) of the flasks labelled A and B are required. The reactor vessel itself is shown in more detail in Figure 2. The rubber bulb P (Fig. 1)allows air to flow through in only one direction, and is used as a pump to drive chemicals from the reservoir flasks A and B to the reactor. One driver bulb P may be used to drive both reservoirs A and B, but if so, the solution densities must be roughly equal so as to ensure that the flow rates throueh the feed-tubes durine the experiment are equal (to avoid tedious experimentation with differential concentrations of reactants). To ensure that no gas bubbles remain inside the reactor during the experiment, the exit for tube C must be exactlv flush with the stopper surface in the reactor vessel (see Fig. 2). The
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F~gure3 Dfagrammat c representation of the pH trace obtamea from osc~latmg reactlon at a wpper (11) on-cata yzeo perox~aeiih~os~ltate 30 'C in the con! ~LOJS-flow st rred-tank reactor assemblage described herein. Figure 1. Diagrammatic IayoLt of the unthermostatted wntmuousflowstirred-tankreactor assemb age and chemical reservoirs: P - nrbber bulb (it is connected to both Aand B reservoirs) Aand B - two stock flasks as chemical reservoirs. a, b, and c - glass tubes (0.d. 6 mm; i.d. 2 mm) R - rubber stopper e, f, and g - polyethylene or siliwne tubes (0.d. 8 mm; i.d. 4 mm) D - stopcock from buret S - magnetic stirrer C - glass reactor ( I 5 mL) T - waste reservoir
tubes a and b, are d ~ pcated l The stoc6flasksAand B. the lnp~t-flow TOm a t e reactlon, pnme the flow tubes w ~ t hthe rLboer b ~ l bpump aev ce P, ana then when tne ex4 tow s dnpptng wrrecfly. 1 L ds w I continue to flow under gravity via a siphon action. ouMow from the reactor. and thus the inflows of reactant streams, is controlled by means of stopcock D (such a s that for a buret). All iunctions and s t o ~ ~ emust r s be tightlv sealed to preventair from leaking &, and thus flow ;atis from changing. The feed-tubes (glass is recommended) a and b should be bent as shown in Figure 2 to allow room for the magnetic stirrer bar and sensor probe (such a s a pH glass electrode), and should be a s close to the bottom of the reactor a s possible. Gas bubble formation during the experiment is inevitable, but these bubbles flow through the outflow tube without causing- any ~-problem. Continuous monitoring and control of the flow rate is necessam. Flow rate is a n i m ~ o r t a n~t a r a m e t ein r this experiment and is defined a s t i e potieht: k =input flow (mL . $8) 1 reador volume (mL)
and so, llk becomes the average residence time of the reactants in the reactor. I n this assemblage, the flow rate of reactants is ~rooortionalto the d r i rate ~ a t the exit tube D in Figure 1. I n order to obtain a constant flow, i t is necessarv that the end of the exit tube be below the level of the reactor and below both reservoir flasks Aand B. If the densities of both solutions in flasks A and B are the same, and the heights of both flasks above the reactor are the same (same diameter tubing and etc.). then the flow rate from both reservoirs will beequal, so that: L
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Inflowrate from A = inflow rate from B = (outflowfrom R)/2 The drip rate can be measured by means of a musical metronome, and then this simple system can be calibrated by using two rules. Calibration of the volume of one dmp (a buret stopcock is usually = 0.05 mI, per drop). Calibration of the stopcoek D to provide the required drip rate and thus volume flow thmugh the reador. Alternatively, the flow rate can be measured via in-line visual flow meters (rotatingindicators)in the correspondingtubes. Experimentally, we have found that use of a metronome is both convenient, sufficientlyaccurate and provides the students with yet another aspect of participation. Further refinement can be introduced by contmlling the tempertatwe; simple immersion in a suitable thermostatic bath is possible, but use of the thermostatted type of reador vessel shown in Figure 2 is preferred. Of course, this also implies that the two reservoir flasks supplying chemicals to the reador also must be themostatted. It is necessary to maintain constant geometry for the relative elevations of the reservoir A and B, the reaction vessel, and the exit stopcock D (constant siphon action). It is passible to fit the reactor vessel with additional sensors, to record the change in a variable generated by the chemical processes. For example, a simple glass electrode allows the time dependence of the pH to be determined (see Fig. 3 for a typical trace). Chemical Oscillating System The chemical oscillator used in this work is hvdroeen " peroxide and thiosulfate ion in t h e presence of small amounts (E5 x lo4 M) of copper (11) catalyst. Hydrogen peroxide is a recornmended reactant to produce chemical oscillations with an oxidant s ~ e c i e ssuch a s iodate (16). or a reductant such a s sulfide (f7),thiosulfate (18) and t h o cyanate (19). The H2O2lS2Oa=ICu(II)system, a t the available flow rates from this apparatus, exhibits large oscillations in the pH, P t potential, and the copper ion-selective electrode potential (18). The stock solutions used here are described below.
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Figure 2. The glass reactor vessel C (thermostatted model) described in Figure 1.
Volume 69 Number 9 September 1992
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