Weird Science

Blue Bottle

Polish ver­sion is here

Che­mi­stry and Color

By defi­ni­tion, dyes are sub­stan­ces capa­ble of selec­ti­vely absor­bing light radia­tion at spe­ci­fic wave­leng­ths. Based on their che­mi­cal struc­ture, dyes can be cate­go­ri­zed into seve­ral gro­ups, inc­lu­ding nitro, nitroso, thia­zine, azo, xan­thene, and indi­goid com­po­unds, among others. Various types of dyes are among the most widely used che­mi­cal sub­stan­ces.

Methy­lene Blue

Methy­lene blue is an orga­nic com­po­und belon­ging to the thia­zine dye group. Its sys­te­ma­tic name is 3,7-bis(dime­thy­la­mino)phe­no­thia­zin-5-ium chlo­ride (C16H18ClN3S). The mole­cu­lar struc­ture of this com­po­und exhi­bits distinct sym­me­try:

Ilustracja

Under nor­mal con­di­tions, methy­lene blue appe­ars as a dark green pow­der. It dis­so­lves in water, pro­du­cing an inten­sely blue solu­tion even at low con­cen­tra­tions.

Methy­lene blue is widely used in bio­logy and medi­cine because it selec­ti­vely sta­ins cer­tain bac­te­rial cells. It is also fre­qu­en­tly employed as an indi­ca­tor in che­mi­stry. One of its key pro­per­ties is its abi­lity to tran­s­i­tion into a color­less leuco form, which we will uti­lize in this expe­ri­ment.

What Do We Need?

The list of requ­i­red mate­rials is short:

Glu­cose is the easiest to obtain and can be pur­cha­sed in gro­cery sto­res. Sodium hydro­xide and methy­lene blue are best acqu­i­red from a che­mi­cal sup­ply store. Below is an image sho­wing all the neces­sary sub­stan­ces for the expe­ri­ment.

Ilustracja:

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War­ning: Sodium hydro­xide is a strong base with highly cor­ro­sive pro­per­ties. Avoid any con­tact with skin and eyes! Methy­lene blue is not toxic, but cau­tion sho­uld still be exer­ci­sed. The author assu­mes no respon­si­bi­lity for any poten­tial dama­ges. Pro­ceed at your own risk!

The Expe­ri­ment

Pour appro­xi­ma­tely 75 cm3 (2.5 fl oz) of water (pre­fe­ra­bly distil­led) into a flask. Dis­so­lve about 1 g (0.035 oz) of NaOH and 4 g (0.14 oz) of glu­cose in it. Then, slowly add a few drops of methy­lene blue solu­tion. The solu­tion ini­tially takes on a cha­rac­te­ri­stic blue color, which fades after a short while. Now comes the most exci­ting part: sim­ply sha­king the flask will turn the liquid blue again! A simi­lar effect can be achie­ved by stir­ring or pou­ring the liquid. Howe­ver, the color is not per­ma­nent; after a short time, the solu­tion will become color­less again. The cycle of color chan­ges can be repe­a­ted mul­ti­ple times. Below is a video I recor­ded demon­stra­ting the expe­ri­ment:

Expla­na­tion

The obse­rved color chan­ges in the solu­tion result from two oppo­sing reac­tions occur­ring within the mixture: reduc­tion and oxi­da­tion of the dye. Glu­cose is a redu­cing sugar, and the addi­tion of sodium hydro­xide cre­a­tes an alka­line envi­ron­ment. Such con­di­tions are essen­tial for glu­cose to exhi­bit its redu­cing pro­per­ties. Glu­cose redu­ces methy­lene blue (while itself oxi­di­zing to glu­co­nate), conver­ting it into its leuco form. The leuco form is color­less, which expla­ins the disap­pe­a­rance of color. Sha­king the con­ta­i­ner intro­du­ces oxy­gen from the air into the solu­tion, allo­wing it to oxi­dize the leuco form back into methy­lene blue. Howe­ver, the methy­lene blue will be redu­ced again, mea­ning the color cycle can be repe­a­ted until the reac­tants are deple­ted.

A simi­lar expe­ri­ment can be found here: Che­mi­cal Traf­fic Lights.

Have fun expe­ri­men­ting!

Fur­ther rea­dings:

Marek Ples

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