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Controlled Experiment Patricia Aroca, Jr. and Ricardo Aroca University of Windsor Windsor, ON, Canada N9B 3P4 Experimenting with oscillating chemical reactions has proven to be a very attractive and challenging experience for undergraduate students. Chemical oscillations that occur far from equilibrium generate intriguing spatial patterns ( I ) (that can be used in class demonstrations) as a result of the interaction of autocatalytic reaction and diffusion. Chemical reactions with neriodic behavior eenerate oatterns in time rhnt may he experimentally observed by potentiometry with an x-\ recorder or a dirital voltmeter (2).Here we describe a microcomputer-controlled experiment that interfaces a "tank" reactor (150-mL beaker with the chemical mixture), a chemical sensor (ion-selective electrode), an analogue-todigital converter, and a microcomputer in order to display and study the periodicity of chemical oscillators. This experiment is versatile and has many facets that can be used to illustrate various topics in chemistry and the use of computers in chemistry. For example: 1. electrochemistry, potentiometry, and the use of ion-selective electrodes for the detection of concentration gradients of chemi-
cal soeeies. noto~ntalv&reactions and the formationof chemical oscillators, 3. chemical instabilitiesand order out of chaos (3, 4. the use of a precision time base for digital time and frequency measurements, and 5. measurements of data in the time domain and transformationto frequency domain (Fourier transformation). 2.
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The experiment presented here is part of the teaching laboratory in the instrumental analysis course, with emphasis on computer-controlled instrumentation and chemometries.
Experlment Oscillating chemical reactions are studied in a closed system: that is. there is no exchanee of matter with the surroundings and chemical reactions proceed to equilibrium. A well-stirred "tank" reactor (150 mL beaker) contained the solutions listed below, the indicator electrode, and the standard calomel electrode. Chemicals HIS% NaBrOJ Malonic acid Ce(NH,),(SO,), MnSO,
Concentrations
Ouantity
1.5 M 0.5. 0.75. 1.0. 1.5 M
25 mL
2M
.2H20
saturated sahnated
25 mL 25 mL 4 mL 4 mL
A homemade, high-impedance analogue-to-digital converter with l-mV resolution was interfaced to an IBM/F'C microcomputer using the base-board interfacing card (from Tecmar Inc.). This card has four ports, each with 24 inpuG outout lines. Since this board is used in several experiments on computer interfacing for chemistry students, a "userfriendly" self-learning program about the Tecmar hoard was written and is available onrequest. The program for on-line data acquisition and display was written in BASIC, with utility subprograms in Macroassembler. Fast-Fonrier-transform computations were carried out with a program in BASIC, compiled to be used with the 8087 math coprocessor. Dlscusslon Oscillations were observed in the well-studied BelousovZhabotinsky (BZ) reaction ( 4 ) , for which an 18-step mechanism is now accepted (5). The BZ reaction consists of the periodic oxidation and reduction of a metal ion (cerium and manganese in the present experiments) with bromate ion and malonic acid. The experiments were conducted in a closed system; they illustrate the effect of feed concentrations on the characteristics of chemical oscillations. Closed systems are very convenient for practical demonstrations;
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December 1987
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Figure 1. Oscillations in the potential of platinum elechode vs. SCE with MnSOI (Mn catalyzed), malonic acid, and different concentrationsof bromate.
Figure 2. Mn-catalyzed reactions detected with a bromide-ian-selective elecnode versus standard ceiomel electrode.
thev offer economy of materials and ease of precise control. 0n;he other hand; oscillatory hrhavior cannit be sustained in cloied systems sincr their final stare is one ofequilibrium. The two citalysts selected for the experiments, ~ n ( 1 1 and ) Ce(III), provided distinct examples of oscillating reactions with different "incubation" periods. The Mn(I1)-catalyzed reaction, as shown in Figures 1and 2, demonstrated oscillatory behavior from the moment the catalyst was added. The Ce(II1) reaction had a long incubation period, as can he seen in Figure 3. Potential of Pt and Bromide and Electrodes The background potential was measured with a P t electrode vs. a standard calomel electrode for the solution containing bromate, malonic, and sulfuric acid. This potential was used in the computer program to scale the screen disnlav. The data collection was initiated bv the addition of the ea&lyst. Typical oscillations for the M ~ ~ Isystem I) for three different concentrations of bromate are given in Figure 1.A more sustained oscillation was obtained for a bromate concentration of 0.75 M, with a Fourier frequency of 0.049 or 4.4 oscillations per minute. In the Ce(II1) system, an incubation ~ e r i o dof a~oroximatelv two minutes was observed for simi.. lar concentrations. Oscillations in potential for the hromide-sensitive electrode are shown in Figure 2. I t can also he observed that the frequency decreased with the dilution of the bromate component. Notably, the potential of the Br- electrode vs. SCE continued to oscillate long after the oscillations ended for the ~otentialsa t the ~ l a t i n u melectrode (compare Fies. 1 and The observation could be explained by the factthat the bromate-bromide oscillator was still at work ( 6 ) even though there was not enough change in the redox potential to be detected at the platinum electrode. The fast Fourier transform application to oscillations will not be elaborated any further, since a report on the subject has just appeared (7). In summary, oscillating chemical reactions are very attractive experiments, during which the student can follow variations in chemical concentrations on the computer screen. In addition, they represent simplest practical demonstration of how the deterministic view of chemistry fails when far-from-equilibrium processes are involved.
Figure 3. Oscillations in the potential of bramide-ion-selective electrode vs. SCE with cerium(ll1) (catalyst). malonic acid, and variable cancentratmns of bromate.
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the system can be deduced, eaeh system is a separate case; eaeh set of chemical reactions must be investigated and may well produce a qualitatively different behavior. Nevertheless, one general result has been obtained, namely a necessary condition for the chemical instability: in a chain of chemical reactions accuring in the system, the only reaction stages that under certain conditions and circumstances may jeopardize the stability of the stationary state are precisely the catalytic loops, stages in which the product of a chemical reaction is involved in its own synthesis (3).
A Colorimetric Titration Experiment with Laser Excitation and Computer-Interfaced Endpoint Detection Manlsh A. Mehta and Richard F. Dalllnger Wabash College Crawfordsvllle, IN 47933 The subject of instrumentally oriented colorimetric titrations has received a eood deal of attention in this Journal (8-12). Colorimetric titration experiments are often performed in undereraduate laboratories because a varietv of instrumental components (light sources, detectors, etc.)can be studied in a low-cost, hands-on format. We have devised a colorimetric titration experiment that combines several attractive and instructive features, most notably the use of a laser light source and computer-interfaced detection. This experiment is assembled from items commonly found in most chemistry departments or inexpensively purchased. I t is suitable for an instrumental analysis lab or a special proiect in instrumental analvsis. " . and i t makes a colorfnl.~impressive lecture demonstration. The experimental apparatus is shown in Figure 4. The light source for the colorimetric titration was a simple helium-neon (HeNe) alignment laser. (We used a Spectra-Physics Model 155, which costs approximately $325, although any HeNe with any type of polarization may he used.) This laser provides a safe (Class 11) output of approximately one milliwatt a t 632.8nm. The laser output was directed through the sample cuvette and was focused such that the diverging beam exiting the sample filled the face of the photocell detector. This experiment can provide a student's first experience with the properties, use, and alignment of laser beams with virtuallv no safetv Students should. of . problems. . course, he instructed to use proper laser safety procedur& in this experiment despite the minimal hazard. The~hotocelldetector was constructed from a cadmium sulfide photocell (Radio Shack, $1.29), which exhibits a large resistance in the dark (-1 MQ) and a much smaller resistance when illuminated (-100 0). The detector responds to light transmitted by the sample solution, and thus the photocell resistance changes as the solution transmittance at 632.8 nm changes. The photocell was incorporated into a circuit, shown in Figure 5, containing a 9-V dry-cell battery and a 3.3-kQ current-limitinc resistor. This circuit orovides an,output voltage, measure; across the photoce~l,~ that is, pro~ortionalto the ohotocell resistance. This circuit is neck s ~ & since ~ the combuter interface card responds to changes in voltage, not resistance. The computer system was based around an Apple IIe
microrompt~ter.The i n t d a c r was the IMI ADALAH parkage (Inleractive Yicruware, Inc.. PO. Hox 771. State CoIlige, PA 16801; cost = $4'95) "sing the VID~SAMPLER software (13). This interface. which reauires no oroeramming to actuate save the VID~SAMPLERcommands;samples the analogue voltage output of the ohotocell detector circuit a t a user-adjustable rate of 0.018 to 20 times per second, performs an analosue-to-digital conversion. and sends the digital informationto one offour data buffers. The IMI VIDICHART graphics software permits interactive display of the data in theinterface buffers and includes several handy numerical functions for data massage. One of the attractive features of the VIDICHART graphics for this experiment is the capability to generate first-derivative plots of the titration curves. This allows for easy location of the titration endpoint. The data can also he stored on floppy disk for later manipulation, such as plotting with the IMI SCIPLOT package. We used this apparatus to examine some straiehtforward acid-base titrations. The analyte solution was placed in a 25mm absorption cuvette (total volume of a ~ ~ r o x i m a t e20 lv mL) and was stirred with a magnetic stirrkr. The titrant solution was added using a syringe pump (Sage Instruments model 341; cost = $750) that could deliver from 0.092 to 3.6 mL/min with a 10-mL syringe. The endpoint was detected by using a colored acidibase indicator in the sample cuvette. The only requirement for an indicator in this experiment was that i t show a change in transmission a t the red HeNe laser wavelength (632.8 nm) between the acidic and basic forms. In order to maximize this change. we chose indicators that have a yellow acidic form (red-transparent) and a hlue basic form (red-absorbing). There is a wide selection of these types of indicators covering virtually the entire range of pH transitions, including methyl violet, hromcresol green, bromcresol purple, hromthymol blue, n-naptholphthalein, cresol purple, thymol hlue, and thymolphthalein. Using a test soldtion ronralnmg several drups of hromthvmol hlue, tor example, we ohirrved the voltage outpur of our photocell detector to rhanee from 260 mV for the hlue form to 135 mV for the yellow form. This voltage change can he easily monitored hv ADALAB interface.1 A c o k n e t r i c titration was performed hy adding an aliquot of theanalvesolurion and 5-7 droosof indicator to the iuvette and delkering standardized titrant from the syringe. The start of the syringe drive and the start of the data acquisition routine were manually synchronized. Each titration curve took only 1-3 minutes to collect. Plots were generated both for the traditional sigmoidally shaped titration curve as well as the first derivative. We calculated the endpoint titrant volume from the VIDICHART-determined ~
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Our exoerlments were run in a darkened maximize the - - roam -. to ~. change in dark11ghl response. The experiment can oe run in a lighted room with some loss of s gnal-to-noise ratio, or the pholoceil can oe shieloed if tne experiment need be run in a I ghtea room. ~
Figure 4. Experimental apparatus. HeNe is helium-neon laser, L is biconvex lens. SD is syringe drive, MS is magnetic stirrer, SC is sample cwene. PD is photocell detector. IB is interface box and All is Apple lie computer.me laser beam oath is hatched.
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Figure 5. Photocell detector circuit. Lioht strikesthe ohotaceil. which functions pnoloresmor me c rc.d luncwms as a vo tage awder wllh me ompn valtagc V., proponmonal to the photocell resnslance A 9-V ary-cell battery pro" ~estneclrcblluol1aqe.andtherwllch Smay oelurnedon loconservetne banery when the detector is not in use. as a
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Figure 6. Titration curve of NaOH vr. HCi. me abscissa is volume of HCI and the wdlnate is the voltage output of the photocell detector. [NaOH] = 0.1054 M, [HCI] = 1.319 M. CurveA isthe plat ofdetectw signalvs. HCi volume. and CWB B is lh$ first derivative plot of curve A. The curves are generated by pioning every other data point. endpoint, in units of seconds, and the calibrated syringe drive delivery rate in milliliters per second. Alternatively, the titrant could be standardized against a primary standard using this apparatus. Figure 6 shows the results of a titration of 2.00 mL of 0.1054 M NaOH with 1.319 M HCl. usine a svrinee drive flow rate of 0.13 mlfmin and a data acquisityon ;ate of 10 pointsf s. The signal-to-noise ratio is very high, except in the region just before the endpoint. This is a result of the finite mixing time of the solutions in the cuvette, the 10:l acidbase ratio and the steepness of the curve near the endpoint. This noise is reduced when the acid and base concentrations are more nearly equal or when the titrant delivery rate is decreased. Repetitive runs of the NaOH/HCl titration indicated very high precision, comparable to a standard buret titration, once the operator learns to synchronize the start of titrant delivery and the start of the computer data acquisition program accurately. The data collection rate and syringe drive speed can be slowed to increase the precision. A very interesting example of the utility of our system is shown in Figure 7, which displays the titration curve of 1.00 (cuvette) with 0.1054 M NaOH (symL of 0.146 M ringe). is a triprotic acid (pKal = 2.15, pKu = 7.20, pK.3 = 12.35) that shows two distinct endpoints when titrated with strong base. We simply added two yellowblue indicators, bromcresol green and thymol blue, whose pH transition ranges corresponded to the first two titration breaks of HaPo6 As the NaOH was added, the two endpoints were optically detected as the bromcresol green turned from yellow to blue a t the first endpoint and the thymol blue turned from yellow to blue a t the second endpoint. The absorbance a t 632.8nm simwlv adds for the twodifferent indicators. The number of drops of each indicator was empirically adjusted, with final values of 2 drops of bromcresol ereen t o 3 drons of thymol blue, to give r o ~ & h lequal ~ ahsoFbance changes a t each of the two endpoints. The first-derivative plot shows
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Journal of Chemical Education
Yol NaOH Flgura 7 . Tiration Curve of YPO, vs. NaOH. Curve A is the plot of detector signal vs. NaOH volume, and curve B is the firstderivative plot of curve A. [H3POI]= 0.146 M. [NaOHl = 0.1054 M. that the two endpoints can be easily determined. This experiment ties together many aspects of instrumental analysis in one simple package.2The student experiences the use and handling of laser beams. the construction of a simple and inexpensive ~hotodetector.and the use of a comme&ally available computer interface system all within the framework of the familiar titration curve. The nhotocell ~~~~detector was especially interesting to build and use; i t exhibits surprisingly high quality characteristics and should be quite useful in a variety of analytical experiments. This experiment also has a great deal of flexibilitv and adawtibility, as i t is possible touse this apparatus to do experiments such as very low volume titrations, redox or acidbase titrations with colored species, and acidbase mixture titrations, to name a few. Student reaction t o working with the laser, constructing the inexpensive photocell detector and using the computer interface system was extremely enthusiastic. ~
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Literature Cited 1. Emtein, I. R, Kustin. K.:DeKcpper, P.; Orbao. M. Sei. Am. 1983,248,112. 2. Harria. D. C. Qvontitoriw ChemicolAwlya&: Plenum: New York. 1982;p 337. 3. Pdgagine.1.: Stengem, I. Order out 01Choos;Batam: New York. 1984 4. De Kepper, P.; Bar-Eli,K. J. Phys. Chem. 19S3.67,480. 5. Field, R.J.; Koros, E.:Noyea, R.M.J. Am. Chem. Soc. 1912.94.8849.
6. Bat-Eli. K.; Geiaeler, W. J.Phya, Chom. 1983.67.3769.
7. Eastman. M. P.; Kastel, G.:Mayhew, T.J. Cham. Edue. 1986,63,453. 8. Lopez, E.; Vassos, B. H.J. Cham. Educ. 1984,61,1025. 9. Byme. J. E.J. Cham. Edue. 1984.61.829.
10. Underuwd.A. L. J. Chem.Educ. 1954.31.394. 11. Beilby, A. L.: Laodowki, C. A. J Chom. Edue. lw0,47,288. 12. Olsen, E. D.: Foreback C. C. J. Chm. Edue. 1972.49.2ffi. 13. Thia interfaeelsoftware package haa been used to d-t mulometric tifration endpoint% Greensmn, P. D.; BurchReld, D. E.; Veening, H. J. Chem. Edue. 1986. 62, 688.
The authors would be very pleased to provide more detailed assembly information to anyone interested in using this experiment.
