Weird Science

Glowing Purple – Chemiluminescence of a Common Manganese Compound

Polish ver­sion is here

The fol­lo­wing article was ori­gi­nally publi­shed in the jour­nal for edu­ca­tors Che­mia w Szkole (eng. Che­mi­stry in School) (6/2018):

Ilustracja

Ples M., Fio­let świeci - che­mi­lu­mi­ne­scen­cja pow­szech­nie dostęp­nego związku man­ganu (eng. Glo­wing Pur­ple – Che­mi­lu­mi­ne­scence of a Com­mon Man­ga­nese Com­po­und), Che­mia w Szkole (eng. Che­mi­stry in School), 6 (2018), Agen­cja AS Józef Szew­czyk, pp. 16-19

I believe I don’t need to convince my Este­e­med Rea­ders that che­mi­stry, and the natu­ral scien­ces in gene­ral, are fasci­na­ting fields. Howe­ver, how can we per­su­ade others who find scien­ce­—both in terms of know­ledge and edu­ca­tio­n—chal­len­ging, com­plex, and, at the same time, unin­te­re­sting?

Intri­gu­ing che­mi­cal phe­no­mena can be par­ti­cu­larly help­ful in this regard. Their role is to cap­ture atten­tion, hol­ding the vie­wer’s inte­rest just long eno­ugh to spark curio­sity about the mecha­ni­sms behind the obse­rved phe­no­me­non. In some case­s—ho­pe­fully, as many as pos­si­ble­—this ini­tial spark will lead to a desire for dee­per under­stan­ding, fur­ther explo­ra­tion, and a jour­ney into dee­per know­ledge…

Among the most stri­king and intri­gu­ing che­mi­cal phe­no­mena are che­mi­lu­mi­ne­scent reac­tions, in which light is emit­ted without a ther­mal cause. There are nume­rous reac­tions of this type, but it may come as a sur­prise that even a widely ava­i­la­ble and rela­ti­vely non-toxic sub­stance like potas­sium per­man­ga­nate, when used under appro­priate con­di­tions, ena­bles the obse­rva­tion of che­mi­lu­mi­ne­scence.

Of course, we won’t stop at the­o­re­ti­cal con­si­de­ra­tions, as this expe­ri­ment is rela­ti­vely sim­ple to con­duct.

Ilustracja
ani­ma­tion: sup­ple­men­tary mate­rial

Expe­ri­ment

To pre­pare for this expe­ri­ment, we need to gather the fol­lo­wing sub­stan­ces:

All descri­bed solu­tions sho­uld be pre­pa­red using distil­led water.

Potas­sium per­man­ga­nate has a wide range of appli­ca­tions in indu­stry, che­mi­cal ana­ly­sis, and even medi­cine, where it is used as a disin­fec­tant [1]. It is rea­dily ava­i­la­ble in phar­ma­cies and is some­ti­mes cal­led “pur­ple pow­der” due to its distinc­tive color. Under nor­mal con­di­tions, this com­po­und appe­ars as deep vio­let cry­stals, nearly black in con­cen­tra­ted form (Photo 1).

Ilustracja:

Kliknij, aby powiększyć

Photo 1 – Cry­stals of potas­sium per­man­ga­nate

This salt dis­so­lves rela­ti­vely well in water. Even tiny cry­stals of potas­sium per­man­ga­nate drop­ped into water pro­duce distinc­tive stre­aks tra­i­ling dis­so­lving cry­stals (Photo 2A), and even at very low con­cen­tra­tions, the resul­ting solu­tion exhi­bits a reco­gni­za­ble vio­let color (Photo 2B).

Ilustracja:

Kliknij, aby powiększyć

Photo 2 – Potas­sium per­man­ga­nate; A – color­ful stre­aks tra­i­ling dis­so­lving cry­stals, B – solu­tion

Altho­ugh it has medi­ci­nal appli­ca­tion­s—al­beit less fre­qu­en­tly nowa­day­s—po­tas­sium per­man­ga­nate can be harm­ful to the body, espe­cially with pro­lon­ged expo­sure. When han­dling this sub­stance, it is cru­cial to remem­ber its strong oxi­di­zing pro­per­ties, as mixtu­res with redu­cing agents can be unsta­ble.

Sodium boro­hy­dride is an inor­ga­nic che­mi­cal com­po­und belon­ging to the group of hydri­des. It is a white solid, often ava­i­la­ble in gra­nu­lar form (Photo 3). It is pri­ma­rily used as a redu­cing agent. Sodium boro­hy­dride reacts slowly with water but more rea­dily in aci­dic or alka­line solu­tions. The sim­pli­fied reac­tion can be repre­sen­ted as:

NaBH4 + 2H2O → NaBO2 + 4H2↑

During this pro­cess, flam­ma­ble hydro­gen gas is rele­a­sed. For this rea­son, sodium boro­hy­dride must be sto­red in air­ti­ght con­ta­i­ners to pre­vent con­tact with moi­sture. This com­po­und is also hazar­dous to human tis­sues and is toxic, requ­i­ring care­ful han­dling.

Ilustracja:

Kliknij, aby powiększyć

Photo 3 – Sodium boro­hy­dride

Sodium hexa­me­ta­pho­sphate is an inor­ga­nic com­po­und, spe­ci­fi­cally a salt of meta­pho­spho­ric acid and sodium. Its inte­re­sting struc­tu­ral for­mula is shown in Figure 1.

Ilustracja
Figure 1 – Struc­tu­ral for­mula of sodium hexa­me­ta­pho­sphate

This com­po­und is widely used as a water softe­ning agent in indu­stries such as lea­ther, texti­les, glass, and dye pro­duc­tion. It is also a com­mon ingre­dient in many deter­gents [2]. Clas­si­fied as a food addi­tive under E452i, it belongs to a cate­gory of emul­si­fiers, lea­ve­ning agents, and gel­ling agents (E400–499). Sodium hexa­me­ta­pho­sphate is not toxic and does not pose any signi­fi­cant health hazards.

Under nor­mal con­di­tions, it appe­ars as a white cry­stal­line pow­der (Photo 4).

Ilustracja:

Kliknij, aby powiększyć

Photo 4 – Sodium hexa­me­ta­pho­sphate

Solu­tions of sul­fu­ric acid and sodium hydro­xide must be han­dled with extreme cau­tion, as both sub­stan­ces are highly cau­stic and can cause severe burns.

All che­mi­cal mani­pu­la­tions sho­uld be per­for­med using per­so­nal pro­tec­tive equ­ip­ment and with great atten­tion to safety. I empha­size this because it’s easy to let your guard down. Howe­ver, safety sho­uld always come first.

To per­form the expe­ri­ment, we must pre­pare two solu­tions:

Solu­tion A has such a deep pur­ple color due to the high con­cen­tra­tion of per­man­ga­nate that it may appear nearly black (Photo 5). It is rela­ti­vely sta­ble and, espe­cially when sto­red at a lower tem­pe­ra­ture and away from light, can be pre­se­rved for some time.

Ilustracja:

Kliknij, aby powiększyć

Photo 5 – Solu­tion A

Solu­tion B, on the other hand, is color­less (Photo 6). As pre­vio­u­sly men­tio­ned, boro­hy­dride decom­po­ses upon con­tact with water, so this solu­tion sho­uld be pre­pa­red imme­dia­tely before the expe­ri­ment.

Ilustracja:

Kliknij, aby powiększyć

Photo 6 – Solu­tion B

Once both solu­tions are pre­pa­red, the expe­ri­ment can pro­ceed. In a tall bea­ker with a capa­city of at least 100cm3 (3.38 fl oz), place solu­tion A and begin vigo­rous stir­ring (Photo 7A). A magne­tic stir­rer is par­ti­cu­larly use­ful for this step. While con­ti­nu­o­u­sly stir­ring, add 2–3cm3 (0.07–0.10 fl oz) of solu­tion B to the bea­ker. This results in intense foa­ming (Photo 7B), and after a few seconds, the solu­tion beco­mes com­ple­tely color­less (Photo 7C).

Ilustracja:

Kliknij, aby powiększyć

Photo 7 – The expe­ri­ment; A – vigo­ro­u­sly stir­red solu­tion A, B – just after adding solu­tion B, C – after a few seconds (color disap­pe­a­rance)

If the expe­ri­ment is con­duc­ted in dark­ness, in addi­tion to the foa­ming and deco­lo­ri­za­tion, there is a brief but intense emis­sion of red-orange light (Photo 8).

Ilustracja:

Kliknij, aby powiększyć

Photo 8 – Che­mi­lu­mi­ne­scence of a man­ga­nese com­po­und

The obse­rved light is the result of che­mi­lu­mi­ne­scen­ce­—me­a­ning it is not ther­mally indu­ced but instead ori­gi­na­tes from the energy rele­a­sed during the che­mi­cal reac­tion.

Mecha­nism of Che­mi­lu­mi­ne­scence

The pre­sen­ted che­mi­lu­mi­ne­scent reac­tion pro­ce­eds accor­ding to the fol­lo­wing equ­a­tion:

8 MnO4− + 5 BH4− + 29 H+ → 8 [Mn2+]* + 5 H3BO3 + 17 H2O

As seen above, the reac­tants inc­lude per­man­ga­nate anions MnO4−, boro­hy­dride anions BH4−, and hydro­gen cations H+. Sodium hexa­me­ta­pho­sphate plays an auxi­liary role in the reac­tion. In addi­tion to water H2O and boric acid H3BO3, man­ga­nese(II) ions Mn2+ are for­med as reac­tion pro­ducts. These ions ini­tially exist in an exci­ted state with high energy. The exci­ted man­ga­nese(II) ions quic­kly return to their gro­und state, emit­ting pho­tons in the visi­ble spec­trum:

8 [Mn2+]* → 8 Mn2+ + hν

Accor­ding to the law of con­se­rva­tion, the energy dif­fe­rence between the exci­ted and gro­und sta­tes must be rele­a­sed into the envi­ron­ment. In most cases, this occurs as heat, but in che­mi­lu­mi­ne­scence, a por­tion of the energy is conver­ted into elec­tro­ma­gne­tic radia­tion within the visi­ble light range. The energy of the emit­ted pho­tons is given by the pro­duct of Planck's con­stant h and the fre­qu­ency ν. In this case, the emis­sion peak is aro­und 690 nano­me­ters, which cor­re­sponds to red light.

The reac­tion scheme X → [Y]* → Y + hν, where the sub­strate(s) X trans­form into an exci­ted inter­me­diate pro­duct [Y]*, which then spon­ta­ne­o­u­sly rele­a­ses energy hν and ulti­ma­tely assu­mes the form of Y, is cha­rac­te­ri­stic of almost all che­mi­lu­mi­ne­scent reac­tions. Many well-known che­mi­lu­mi­no­pho­res are orga­nic com­po­unds, such as lumi­nol C8H7N3O2 (Photo 9A), lophine C21H16N2 (Photo 9B), bis(2,4-dini­tro­phe­nyl) oxa­late with a sen­si­ti­zer (Photo 9C), luci­ge­nin C28H22N4O6, luci­fe­rin C11H8N2O3S2, and even poly­phe­nols pre­sent in green tea (Photo 9D). Che­mi­lu­mi­ne­scence of inor­ga­nic com­po­unds is less com­mon, but seve­ral exam­ples of this phe­no­me­non are known. In a pre­vious issue of "Che­mi­stry in School," che­mi­lu­mi­ne­scence of sin­glet oxy­gen 1O2 (Photo 9E), metal­lic sodium Na—whose mecha­nism appe­ars simi­lar to the lumi­ne­scence of white allo­trope pho­spho­rus P—and a very inte­re­sting sili­con com­po­und, Wöh­ler’s silo­xene Si6O3H6 (Photo 9F) have been descri­bed [4] [5] [6].

Ilustracja:

Kliknij, aby powiększyć

Photo 9 – Che­mi­lu­mi­ne­scence; A – lumi­nol, B – lophine, C – sen­si­ti­zed bis(2,4-dini­tro­phe­nyl) oxa­late, D – poly­phe­nols from green tea, E – sin­glet oxy­gen, F – Wöh­ler’s silo­xene

As we can see, potas­sium per­man­ga­nate is capa­ble of che­mi­lu­mi­ne­scence. The expe­ri­ment can be modi­fied by using various man­ga­nese com­po­unds. It quic­kly beco­mes appa­rent that there is a spe­ci­fic pat­tern in their beha­vior: com­po­unds in which man­ga­nese is in the +7 oxi­da­tion state pro­duce the bri­gh­test che­mi­lu­mi­ne­scence in this reac­tion. If we use man­ga­nese com­po­unds with lower oxi­da­tion sta­tes (+4, +3), we obse­rve that the inten­sity of che­mi­lu­mi­ne­scence decre­a­ses as the oxi­da­tion state drops. If a man­ga­nese(II) com­po­und is used, no light emis­sion is obse­rved at all [7]. This is logi­cal when we con­si­der that the exci­ted Mn2+ ion is gene­ra­ted thro­ugh the reduc­tion of man­ga­nese com­po­unds in higher oxi­da­tion sta­tes.

The descri­bed expe­ri­ment pro­vi­des an excel­lent oppor­tu­nity to demon­strate che­mi­lu­mi­ne­scence in a sim­ple yet visu­ally enga­ging way. Fur­ther­more, it intro­du­ces fun­da­men­tal con­cepts of redox che­mi­stry, reac­tion kine­tics, and energy trans­fer. It also serves as a remin­der that scien­ti­fic explo­ra­tion often leads to une­xpec­ted and fasci­na­ting disco­ve­ries. Even see­min­gly ordi­nary sub­stan­ces, such as potas­sium per­man­ga­nate, can reveal sur­pri­sing pro­per­ties when used under the right expe­ri­men­tal con­di­tions.

Finally, I enco­u­rage rea­ders to explore more about che­mi­lu­mi­ne­scence, whe­ther in a labo­ra­tory set­ting or thro­ugh fur­ther rea­ding. The study of light-emit­ting reac­tions not only cap­ti­va­tes the ima­gi­na­tion but also finds prac­ti­cal appli­ca­tions in fields ran­ging from foren­sic science to medi­cal dia­gno­stics and advan­ced mate­rials rese­arch.

Refe­ren­ces:

All pho­to­gra­phs and illu­stra­tions were cre­a­ted by the author.

This text has under­gone sli­ght edi­to­rial modi­fi­ca­tions com­pa­red to the ver­sion publi­shed in the jour­nal to bet­ter suit online pre­sen­ta­tion.

Marek Ples

Aa