By Andres Tretiakov and Tom Kuntzleman
A classic experiment involves use of the permanganate ion (MnO4–) to test for the presence of oxalic acid (C2H2O4) in rhubarb stems. Oxalic acid reduces the colored permanganate ion, which contains manganese (VII), to the colorless manganese (II) ion. In the process, oxalic acid is oxidized to carbon dioxide:
2 MnO4–(aq) + 5 C2H2O4(aq) + 6 H3O+(aq) à 2 Mn2+(aq) + 10 CO2(g) + 14 H2O(l)
A video of this experiment can be seen below:
However, the common explanation1 that oxalic acid is responsiblefor the color change may not be correct. While oxalic acid is certainly present in rhubarb, it tends to be concentrated in the leaf, and not the stem.2,3 In fact, over twice as much malic acid (C4H6O5) than oxalic acid is present in rhubarb stems.2,3 Furthermore, malic acid is also oxidized by permanganate ion4 to form oxaloacetic acid (C4H4O5):
2 MnO4–(aq) + 5 C4H6O5(aq) + 6 H+(aq) à 2 Mn2+(aq) + 5 C4H4O5(aq) + 8 H2O(l)
Thus, it is likely the case that malic acid is the predominant cause of the reduction of permanganate ion when rhubarb is placed in solutions of permanganate. To test this idea, we decided to place some super sour candies5 in solutions of permanganate. These candies contain relatively large amounts of malic acid, which is the component that provides the extreme tartness. Check out the results of these investigations in the video:
Realizing that malic acid can reduce permanganate, we decided to test this idea further using apples. The tart flavor of apples comes from malic acid, which makes up 98% of the acid content of apples.6 As you might expect, the sour-tasting Granny Smith apple variety contains roughly 2.5 times more malic acid than the sweeter tasting Red Delicious apple.6
It is known that the malic acid content of apples decreases as they ripen. This is probably why unripe apples taste so sour. We think it would be fun to use this permanganate test to qualitatively compare the amount of malic acid in apples during the ripening process. Do you have suggestions for other investigations based on the experiments presented here? If you have any ideas – or better yet, try some experiments on your own – be sure to let us know!
In closing, we note that our investigations and experiments do not prove that malic acid is the main contributor to the loss of purple color when rhubarb is placed in permanganate solutions. However, they do strongly indicate that this is the case. What kinds of tests do you think we could conduct to strengthen the case that it is the malic acid in rhubarb which decolorizes permanganate in this classic experiment? We look forward to hearing from you!
Tom’s note: Andres Tretiakov, is a science technician at St. Paul’s School in London. He is a chemist and science enthusiast who enjoys making pyrotechnics and other energetic materials. Andres has shared with me several experimental ideas to share on ChemEdX. This blog post represents the results of investigating one such experiment. Hopefully, Andres and I will be able to share additional collaborations in the future. You can follow Andres on Twitter @Andrestrujado, and be sure to check out his YouTube channel.
NOTE: we used Jungle Brand Clear Water aquarium treatment (purchased at a local pet store) as a source of permanganate ion.
References:
1. http://www.sserc.org.uk/national-5/1131-chemical-changes-and-structure-n5/3065-rhubarb-rhubarb
2. http://www.compoundchem.com/wp-content/uploads/2015/04/The-Chemistry-of-Rhubarb.png
3. http://www.ct.gov/caes/lib/caes/documents/publications/bulletins/b424.pdf
5. See, for example, http://www.impactconfections.com/warheads/
6. http://www.sciencedirect.com/science/article/pii/S0308814606006157