edited by GEORGE L. GILBER; Oeniron University Granville. Ohio 43023
Color Oscillations in the Formic Acid-Nitric Acid-Sulturic Acid System SUBMITTED BY:
C. J. G. Raw, J. P. Kublk, and R. E. Tecklenburg
Literature Cited (1) Morgan, J. S.,J.Chem. Soc. (London). lW.274 (19161. (2) Bowem,P. G., and Rawji, G.. J. Phys. Chpm.,81,1549 (19771. (3) Showalter, K..and Noyes.R.M.. J. A m m Chem. Sw.. 100.1M2(19781. (4) Noye. R. M. Pemnal eurnrnunicalion (19811. (5) Raw. C. J. G., Rierdich, J.. Perrino, F.,and Jex, G., J. Phys Ch~m..82.1952(19781.
Saint Louis University St. Louis. MO 63103 CKCKEO BY:
H. Slotnik
Central ConneCtiC~tState College New Britain. CT 06050
A Demonstration of an Autocatalytic Reaction
A number of studies of oscillatory gas evolution from the formic acid-concentrated sulfuric acid system (the Morgan reaction) have been published (1,2,3). In this acid-catalyzed dehydration of formic acid, it seems that homogeneous nucleation from supersaturated solution plays an important part in the mechanism of oscillatory carbon monoxide evolution (4). In a previous publication (5). we reported on the oscillatory gas evolution when concentrated nitric acid was also present: CO2 and NO2, as well as CO, are produced in this case. In his original article, Morgan (1) had noted the presence of a blue color which ameared and d i s a ~ ~ e a r several ed times when nitric acid was added to the formic acid-concentrated sulfuric acid mixture. >lor=sn found these color oscillations difficult to reproduce, bucwe have devised a procedure for their reliable reproduction. The experimental procedure for producing color oscillations is as follows: Formic acid (2 ml) is added initially to 20 ml of concentrated HzSOl in a large test tube a t 50°C, followed by a further addition of 4 ml of fomic acid after 5 mio. When CO gas oscillations arr well estal~lished(after about 5 min more), 4 ml uf concentr:~tedHNO, is added 18, the reactinn mixture. After another 60-70 min,-a further 4 ml of formic acid is added. The entire procedure is carried out without stirring, or with only very minimal agitation of the solution. If the reaction is carried out with vigorous stirring, no color changes appear. After the final 4 ml of formic acid has been added, a faint yellow layer is noticed gradually moving down the test tube with a hlue color iust beneath it. Gas evolution from the botton~of the test tubecauses mixingofthe twocolored layem, and at this ~ o i n color r chanzcs from faint sellou, to blue berin to appear. ~ u r i the n ~two i o three minutes that these c z o r oscillations persist, there is a marked evolution of gases (principally COz and Nos) each time the blue color disappears. Finallv, the blue color persists, but if the solution is stirred vigorously, this color disappears with considerable evolul.ton of CO, and N02. If the resulting ale yellow sniutim is left tostand, the hlue color will once again appear. We have observed that CO is not oxidized to COs by a mixture concentrated nitric and sulfuric acid. The production of C02 and NO2 when nitric acid is added to the Morgan reaction is therefore probably superimposed on the CO oscillations which proceed independently. Morgan ( I ) had originally suggested that the hlue color may be due to N20:,(N20:%=NO NOz) and this seems to be the most probable explanation since the blue color has been observed to disappear as the burst of gas evolution occurs. Great caution should he exercised in carrying out this experiment. We have prepared a "Superd" home movie of the color changes and have found this to be quite satisfactory for demonstration purposes.
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994
Journal of Chemical Education
Acknowledgment The authors are grateful for a summer NSF Undergraduate Research Participation grant to one of them (R.E.T.).
Sueulrxo BY: Dubravko Marit Rudjar BoSkoviC lnstilute BijeniEka 54 YU-41001Zagreb and Franjo Strajnar Oepartment of Physical Chemistry Faculfyof Sciences, University of Zagreb Marulieev trg 19 YU-41000 Zagreb Checked by: Melvyn W. Mosher Missouri Southern State College Jopiin, MO 84801 Chemicals Used NazSO. anhydrous, analytical grade KC103 analytical grade hromophenol blue indicator concentrated HpSO1 (96%) analytical grade. The Reaction T h e redox reaction C103 HSOs- + 2SOa2pHv= 7 pH
