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 Saint Louis University St. Louis. MO 63103

CKCKEO BY:

H. Slotnik Central ConneCtiC~t State College New Britain. CT 06050

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 nu- cleation 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 oscilla- tory 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 e d several 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:~ted HNO, 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 re- action 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 r color chanzcs from faint sellou, to blue berin to appear. ~ u r i n ~ the two i o three minutes that these czor oscillations persist, there is a marked evolution of gases (principally COz and Nos) each time the blue color disap- pears. Finallv, the blue color persists, but if the solution is stirred vigorously, this color disappears with considerable evolul.ton of CO, and N02. I f 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 con- centrated 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 ex- periment. We have prepared a "Superd" home movie of the color changes and have found this to be quite satisfactory for demonstration purposes.

Acknowledgment The authors are grateful for a summer NSF Undergraduate

Research Participation grant to one of them (R.E.T.).

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.

A Demonstration of an Autocatalytic Reaction Sueulrxo BY:

Dubravko Marit Rudjar BoSkoviC lnstilute BijeniEka 54 YU-41001 Zagreb and Franjo Strajnar Oepartment of Physical Chemistry Faculfy of 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 The redox reaction

C103 + HSOs- + 2SOa2- +

pHv= 7 pH << 7

using bromophenol blue as indicator is used for demonstrating autocatalysis. This reaction proceeds in acidic medium only. The products of the reaction are acidic, thus catalyzing the reaction. When a reaction is catalyzed by its own products, it is said to be autocatalytic.

In all experiments 33% H2S04 is used. I t is prepared by adding 2 volume equivalents of commercial HzS04 into 7 volume equivalents of HpO.

Two solutions are prepared: one containing 12.60 g (100 mmol) Na2SO:+, 4.10 g (33.4 mmol) KC103, and 5 mg hromo- phenol blue in 50 ml of water, and the second one containing 4.00 ml (16.7 mmol) H2SOl in 50 ml of water. Slowly, with stirring, the second solution is added to the first. KC102 dis- solves to completion only after the two solutions have been mixed. The solution prepared as described has a violet color and 6.5 < pH < 7.0, i.e. it is buffered by the HSO:$-1SO:j- ratio.

Experlment I A 25-ml gduated cylinder is fi1lt.d with thevhrion, and the re.

avtwn is started off hy wrei'ully putting a couplr: of dropsof H2SO4 mtu the surtnce. In its acid~c form l~romophcnd IAE has :i yrllow color, so as the reaction progresses, the yellow-hlue interface moves downward. Experiment 2

The following experiment is performed in order to prove that the

994 Journal of Chemical Education

change in color of the indicator in the previous experiment is not a quantity of liquid, the color of their contentsare different: thesolution consequence of mere diffusion of acid. Two tesr tubes are marked A in test tube A is yellow, while that in test tube B is blue. This results and B. A drop of H a S 0 4 is introduced in each. The solution is added because in test tube B the added drop of H2S04 was buffered before dropwise into test tube A while it is shaken continuously. Test tube there was time for the reaction to take place. This experiment proves B is filled with the solution instantaneously, and then shaken. Al- that the added drop of H2SO4 can start off the reaction but does not though after the experiment both test tubes may contain the same suffice to change the color of the indicator in the whole volume of the

solution.

Volume 60 Number 11 November 1983 995