Weird Science

Colorful Contrasts: The Briggs–Rauscher Oscillating Reaction

Polish ver­sion is here

The fol­lo­wing article was ori­gi­nally publi­shed in the jour­nal for edu­ca­tors Che­mia w Szkole (eng. Che­mi­stry in School) (3/2022):

Ilustracja

Ples M., Barwne kon­tra­sty - oscy­la­cyjna reak­cja Briggsa-Rau­schera (eng. Color­ful Con­tra­sts: The Briggs–Rau­scher Oscil­la­ting Reac­tion), Che­mia w Szkole (eng. Che­mi­stry in School), 3 (2022), Agen­cja AS Józef Szew­czyk, pp. 44-47

Ano­ther recipe for the Briggs–Rau­scher reac­tion — also tested by me — is ava­i­la­ble here.

Chan­ges, chan­ges!

We expe­rience chan­ges at every turn. The motto of today's article is a quo­ta­tion from a book by Austra­lian author Trudi Cana­van:

Nothing stays the same. All you can be sure of in life is change.

Cana­van T., Prie­stess of the White

A spe­cial class of often extre­mely spec­ta­cu­lar and utterly fasci­na­ting che­mi­cal reac­tions are those in which self-orga­ni­za­tion occurs. This term refers to the spon­ta­ne­ous emer­gence of orde­red struc­tu­res or time cor­re­la­tions in a sys­tem. The pro­ces­ses lea­ding to this can be che­mi­cal, phy­si­cal, or mixed in nature [1].

One of the ear­liest stu­died exam­ples of self-orga­ni­za­tion is the for­ma­tion of cha­rac­te­ri­stic rings, first tho­ro­u­ghly descri­bed in 1896 by Raphael Edu­ard Lie­se­gang [2]. Phe­no­mena rela­ted to self-orga­ni­za­tion are wide­spread in nature and, as we now see more cle­arly, likely play a signi­fi­cant role in many bio­lo­gi­cal pro­ces­ses. The scale of events influ­en­ced by self-orga­ni­za­tion phe­no­mena is very broad, star­ting at the micro­sco­pic level (when we con­si­der cohe­ren­tly acting ensem­bles of che­mi­cal par­tic­les or even sin­gle mole­cu­les) and exten­ding up to the cosmic scale — for instance, it appe­ars that the ban­ded struc­ture of Saturn’s rings may be attri­bu­ted to this very phe­no­me­non.

One mani­fe­sta­tion of self-orga­ni­za­tion is the exi­stence of so-cal­led oscil­la­ting reac­tions. They dif­fe­red so gre­a­tly from the pre­vio­u­sly known, mono­to­ni­cally pro­ce­e­ding reac­tions that, at first, the scien­ti­fic com­mu­nity was reluc­tant to ack­now­ledge their exi­stence. A nota­ble exam­ple con­fir­ming this is the story of the Soviet che­mist Boris Pavlo­vich Belo­u­sov. While attemp­ting to describe the oscil­la­tions he obse­rved in a sys­tem con­ta­i­ning orga­nic acids and cata­ly­zed by cerium com­po­unds (Ce), he enco­un­te­red such intense skep­ti­cism and dif­fi­cul­ties publi­shing his results that it had a cata­stro­phic effect on his men­tal health, ulti­ma­tely requ­i­ring him to seek medi­cal advice [3].

Oscil­la­ting reac­tions can be clas­si­fied in various ways, often divi­ded into those occur­ring in homo­ge­ne­ous and hete­ro­ge­ne­ous sys­tems. The oscil­la­tions them­se­lves may be tem­po­ral, spa­tial, or spa­tio­tem­po­ral. Intro­du­cing expe­ri­ments rela­ted to such exo­tic reac­tions into tea­ching might seem some­what risky. Howe­ver, it is worth con­si­de­ring that, despite their often very com­pli­ca­ted mecha­ni­sms, they spark inte­rest in the field of che­mi­cal kine­tics. This is a great edu­ca­tio­nal aid, espe­cially because this branch of che­mi­stry is often dee­med by stu­dent­s — and some­ti­mes by edu­ca­tors — as both chal­len­ging and some­what une­xci­ting.

Ilustracja
Cyc­lic color chan­ges of the solu­tion during the B-R reac­tion
Ani­ma­tion: sup­ple­men­tary mate­rial

An oscil­la­ting reac­tion that is par­ti­cu­larly well sui­ted for tea­ching and demon­stra­tion pur­po­ses is the Briggs-Rau­scher (B-R) reac­tion. During this reac­tion, very con­tra­sting color chan­ges occur [4].

Expe­ri­ment

To per­form the expe­ri­ment, we need:

Potas­sium iodate under nor­mal con­di­tions is a white solid, mode­ra­tely solu­ble in water, and odor­less. At its mel­ting point, it decom­po­ses with the rele­ase of iodine. It is quite a good oxi­di­zing agent — it may react with flam­ma­ble mate­rials, some­ti­mes even explo­si­vely. Potas­sium iodate is com­monly used to iodize table salt, where its con­cen­tra­tion is very low. It is also used to treat cases of radio­ac­tive iodine absorp­tion by displa­cing it from the thy­roid.

Malo­nic acid — or, using sys­te­ma­tic nomenc­la­ture, pro­pa­ne­dioic acid — is an orga­nic com­po­und from the group of dicar­bo­xy­lic acids (Fig.1). Its salts and esters are cal­led malo­na­tes, and the name deri­ves from the Latin word malum mea­ning “ap­ple.” It stron­gly irri­ta­tes skin and mucous mem­bra­nes. Its aci­dity is simi­lar to that of ace­tic acid. Under nor­mal con­di­tions, it is a white cry­stal­line solid.

Ilustracja
Fig.1 – Struc­tu­ral for­mula of malo­nic acid

Man­ga­nese(II) sul­fate mono­hy­drate, under nor­mal con­di­tions, is a cry­stal­line solid with a faint pink color. It dis­so­lves well in water and has hygro­sco­pic pro­per­ties. Its mel­ting point ran­ges from 57–117°C (134.6–242.6°F) due to gra­dual loss of cry­stal­li­za­tion water. The anhy­drous salt, howe­ver, melts at 700°C (1292°F).

Keep in mind that sul­fu­ric acid and hydro­gen pero­xide are highly cor­ro­sive and cause severe burns. Potas­sium iodate and man­ga­nese(II) sul­fate are toxic, and malo­nic acid is irri­ta­ting. You must exer­cise cau­tion and use per­so­nal pro­tec­tive equ­ip­ment, as always when wor­king in a labo­ra­tory!

The last sub­stance we need is starch. It is most conve­nient to use so-cal­led solu­ble starch, but in its absence, ordi­nary potato starch can be sub­sti­tu­ted (Photo.1).

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Photo.1 – Potato starch

We pre­pare the potato starch solu­tion by disper­sing a pinch of starch in about a dozen cubic cen­ti­me­ters (~0.41 fl oz) of cold distil­led water, then pou­ring this into 200 cm3 (about 6.76 fl oz) of water that has just been bro­u­ght to a boil. After coo­ling, the solu­tion sho­uld be fil­te­red to col­lect a clear or sli­gh­tly clo­udy liquid (Photo.2).

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Photo.2 – Starch solu­tion

We can verify that the solu­tion is pre­pa­red cor­rec­tly by adding a few drops of iodine tinc­ture (an alco­ho­lic solu­tion of iodine, or iodine in an aqu­e­ous solu­tion of potas­sium iodide KI) – the liquid sho­uld then turn a cha­rac­te­ri­stic dark navy-blue, almost black (Photo.3).

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Photo.3 – Starch solu­tion with the addi­tion of iodine

To pro­ceed with the main expe­ri­ment, we must pre­pare three solu­tions:

Distil­led or demi­ne­ra­li­zed water must be used to pre­pare the solu­tions, as it can­not con­tain chlo­ride ions Cl–, which would inhi­bit the reac­tion. The pro­cess is sca­la­ble: you can adjust the reac­tion volume by pro­por­tio­nally decre­a­sing or incre­a­sing the quan­ti­ties of rea­gents. With the amo­unts given, you sho­uld obse­rve at least seve­ral cyc­les of spon­ta­ne­ous color chan­ges — if you need to pro­long the pro­cess, you can scale up accor­din­gly [5].

At the start of the demon­stra­tion, all solu­tions sho­uld be at or sli­gh­tly below room tem­pe­ra­ture (aro­und 20°C or 68°F). Ini­tially, all solu­tions are color­less; only the one con­ta­i­ning starch may appear sli­gh­tly clo­udy.

During the reac­tion, ensure tho­ro­ugh mixing — a magne­tic stir­rer is very help­ful here. All solu­tions must be added in the descri­bed order!

First, com­bine solu­tions A and B — their mixture is also color­less (Photo.4).

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Photo.4 – Mixture of solu­tions A and B

After a brief moment, during which these solu­tions sho­uld mix tho­ro­u­ghly, add solu­tion C — this almost imme­dia­tely chan­ges the color to a distinct yel­low (Photo.5).

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Photo.5 – Mixture of solu­tions A, B, and C as the ready reac­tion sys­tem

Howe­ver, do not get used to that par­ti­cu­lar color, because after a moment, the entire volume of the solu­tion will instan­tly become dark navy-blue, almost black (Photo.6A).

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Photo.6 – Che­mi­cal color oscil­la­tions; A – the navy-blue phase, B – the yel­lo­wish phase, C – the color­less phase

After some time, the solu­tion deco­lo­ri­zes (Photo.6C), then turns yel­lo­wish (Photo.6B), and then the navy-blue color returns, com­ple­ting the cycle.

The period of color chan­ges usu­ally lasts seve­ral dozen seconds, depen­ding on the tem­pe­ra­ture of the solu­tions.

After some time, the oscil­la­tions wea­ken and stop — the solu­tion rema­ins in the navy-blue phase and gives off the cha­rac­te­ri­stic, irri­ta­ting odor of ele­men­tal iodine.

Expla­na­tion

The sub­ti­tle of this sec­tion is some­what iro­nic because, while the reac­tions pre­sen­ted here are stra­i­ght­for­ward to carry out, expla­i­ning their exact mecha­nism is not so sim­ple. Indeed, in some respects, it goes well bey­ond the scope of a typi­cal school or even uni­ver­sity cur­ri­cu­lum. Suf­fice it to say that many models of such pro­ces­ses assume that dozens of inter­re­la­ted che­mi­cal reac­tions occur simul­ta­ne­o­u­sly. For that rea­son, I will only pre­sent the most impor­tant ones below.

During the reac­tion, iodate(V) ions are redu­ced to iodous acid HIO by hydro­gen pero­xide, rele­a­sing free oxy­gen accor­ding to the fol­lo­wing equ­a­tion:

IO3– + 2H2O2 + H+ → HIO + 2H2O + 2O2

This iodous acid then reacts fur­ther with hydro­gen pero­xide to pro­duce iodide ions:

HIO + H2O2 → I– + H+ + H2O + O2

Inte­re­stin­gly, iodide ions then react with iodous acid as fol­lows:

HIO + I– + H+ → I2 + H2O

This yields ele­men­tal iodine. These pro­ces­ses cause the solu­tion to take on the obse­rved yel­low color. The free iodine then reacts with iodide ions, for­ming triio­dide:

I2 + I– ⇆ I3–

It is pre­ci­sely the triio­di­des that form a navy-blue com­plex with starch.

Mean­while, the reac­tion of iodine with malo­nic acid is also ongo­ing:

I2 + CH2(COOH)2 → ICH(COOH)2 + H+ + I–

Ano­ther reac­tion con­su­ming ele­men­tal iodine is:

5H2O2 + I2 → 2HIO3 + 4H2O

Both reac­tions con­sume ele­men­tal iodine, cau­sing the navy-blue com­plex to disap­pear and simul­ta­ne­o­u­sly rege­ne­ra­ting iodate(I) ions.

These reac­tions explain the color chan­ges during the pro­cess but not why oscil­la­tions occur instead of a sim­ple, mono­to­nic change in rea­gent con­cen­tra­tions. A more deta­i­led expla­na­tion goes bey­ond the scope of this article, but it is worth noting that cata­ly­tic and auto­ca­ta­ly­tic pro­ces­ses play a major role.

The above recipe for the Briggs-Rau­scher reac­tion has been veri­fied and repe­a­te­dly tested in per­son. Of course, there are other variants that you can try with equal suc­cess, inc­lu­ding those in which sul­fu­ric acid is repla­ced, for exam­ple, by per­ch­lo­ric acid HClO4 [6].


Refe­ren­ces:

All pho­to­gra­phs and illu­stra­tions were cre­a­ted by the author.

The above text inc­lu­des minor edi­to­rial modi­fi­ca­tions com­pa­red to the ver­sion publi­shed in the jour­nal, aimed at sup­ple­men­ting and adap­ting it for online pre­sen­ta­tion.

Marek Ples

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