Weird Science

Sensitive Liquids: Reversible Dye-Based Redox Reactions

Polish ver­sion is here

The fol­lo­wing article was ori­gi­nally publi­shed in the jour­nal for edu­ca­tors Che­mia w Szkole (eng. Che­mi­stry in School) (1/2015):

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Ples M., Wrażl­iwe cie­cze - odw­ra­calne reak­cje redoks z udzia­łem barw­ni­ków (eng. Sen­si­tive Liqu­ids: Rever­si­ble Dye-Based Redox Reac­tions), Che­mia w Szkole (eng. Che­mi­stry in School), 1 (2015), Agen­cja AS Józef Szew­czyk, pp. 27-28

Che­mi­cal color-change reac­tions inva­ria­bly cap­ture the inte­rest of che­mi­stry enthu­sia­sts. Unde­nia­bly spec­ta­cu­lar, they can pro­vide a cer­tain aesthe­tic ple­a­sure. Howe­ver, it is impor­tant to remem­ber that without dee­per reflec­tion, these displays have limi­ted edu­ca­tio­nal value. In such cases, they func­tion merely as a form of adver­ti­se­ment, inten­ded to enco­u­rage vie­wers to ask que­stions [1].

The expe­ri­ment pre­sen­ted here will undo­ub­te­dly prompt every obse­rver to won­der about the nature of these so-cal­led sen­si­tive liqu­ids. They pos­sess a fasci­na­ting pro­perty: when left undi­stur­bed, they become color­less, yet with each shake, they reveal fle­e­ting — yet unde­nia­bly beau­ti­ful — colors.

[…]

Pre­pa­ra­tion

The fol­lo­wing che­mi­cal sub­stan­ces are requ­i­red for the expe­ri­ment:

When pre­pa­ring the expe­ri­ment, remem­ber that sodium hydro­xide is highly cor­ro­sive — con­tact with skin or eyes can result in severe che­mi­cal burns. The dyes used in this expe­ri­ment (their struc­tu­ral for­mu­las are shown in Fig. 1) can be harm­ful, so avoid direct skin con­tact and do not inhale any dust. Keep in mind that methy­lene blue and resa­zu­rin can leave par­ti­cu­larly stub­born sta­ins on skin or fabrics.

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Fig. 1 – Struc­tu­ral for­mu­las of the dyes used in the expe­ri­ment

[…]

First, it is a good idea to pre­pare the dye solu­tions:

These solu­tions are rela­ti­vely sta­ble and can be sto­red for an exten­ded period.

The next step of the expe­ri­ment is to pre­pare the base solu­tion. In 100 cm3 (3.38 fl oz) of water, dis­so­lve 2 g (0.07 oz) of sodium hydro­xide and about 3 g (0.11 oz) of one of the pre­vio­u­sly men­tio­ned sugars, such as glu­cose (Photo 1). This solu­tion is unsta­ble and sho­uld be used as soon as pos­si­ble!

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Photo 1 – Alka­line glu­cose solu­tion

At the start of the demon­stra­tion, pour appro­xi­ma­tely 10 cm3 (0.34 fl oz) of the base solu­tion into each of two small bot­tles, and then add a few drops of one of the dye solu­tions to each. […]

If you sim­ply leave the liqu­ids undi­stur­bed, after some time (ran­ging from a few seconds to seve­ral minu­tes) their color begins to fade. Even­tu­ally, both solu­tions become com­ple­tely color­less and clear (Photo 2).

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Photo 2 – Deco­lo­ri­zed solu­tions

Howe­ver, upon sha­king the bot­tles, the solu­tions regain their intense, cha­rac­te­ri­stic colors (Photo 3). When left undi­stur­bed, they spon­ta­ne­o­u­sly become color­less again. Sha­king the bot­tles re-oxi­di­zes the dye back to its blue form, and with con­ti­nued sha­king, this color-change cycle can be repe­a­ted many times. […]

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Photo 3 – Color­ful solu­tions after sha­king; on the left, the solu­tion con­ta­i­ning resa­zu­rin, on the right, methy­lene blue

Expla­na­tion

The color chan­ges obse­rved in this expe­ri­ment are the result of two oppo­sing reac­tions taking place in the solu­tion: the reduc­tion and oxi­da­tion of the respec­tive dyes — in this case, methy­lene blue and resa­zu­rin [2].

[…]

Glu­cose (or ano­ther redu­cing sugar) is oxi­di­zed to glu­co­nate at the expense of the dye (methy­lene blue or resa­zu­rin). As a result, the dyes them­se­lves are redu­ced to their color­less leuco forms, which we obse­rve as deco­lo­ri­za­tion [3].

Sha­king intro­du­ces oxy­gen from the air into the solu­tion, rapi­dly oxi­di­zing the leuco dyes back to their colo­red forms, while the oxy­gen is redu­ced to hydro­gen pero­xide (H2O2) or water (H2O) [4]. Howe­ver, this state is not per­ma­nent because they are once again redu­ced by glu­cose.

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

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