pH variation in Thionin: The Two-Faced Solution

Integrated-Science students Erin Smyers and Nicole Loughner from Homer-Center High School in Homer City, PA, worked with their science team leader, Mark Shopland, to investigate the pH changes in an oxidation-reduction reaction. The students chose to work with Thionin based on the demonstration “Thionin – The Two-Faced Solution” published by Flinn Scientific Inc. (Publication No. 0815.00). According to the Flinn Chem-Fax, Thionin is an organic compound that in the oxidized form is purple and in the reduced form is colorless. When a reducing agent such as iron(II) ion (Fe2+) is added to an acidic thionin solution, the thionin (Thio+) molecule accepts two hydrogen atoms and is reduced to its colorless form. This reaction only takes place in the presence of an intense light source.

Thio+ + 2Fe2+ + 2H+ + Light ThioH2+ + 2Fe3+
[purple] [colorless]

The graph shows the relatively constant pH under normal lighting conditions and the varying pH levels of the Thionin solution while being exposed to an intermittent light source. The graph supports the change in hydrogen ion concentration predicted by the equation. Nevertheless, there is certainly more going on than meets the eye, as shown in the pH data. A secondary reaction is decreasing the pH over time. A hint as to the source of the increase in hydrogen concentration comes from investigating the change in pH during the “light on” cycles. This could be a great follow-up project for Erin and Nicole as they continue their study of chemistry.


View the video of the project or download the Logger Pro file that includes a video clip synced to data collection.

For additional reference on this reaction see:
Heidt, L. J.Chem.Ed., 26, 525 (1940)