Compound Interest: The Chemistry of Poinsettia Plants

Following on from the start of the Chemistry Advent Calendar yesterday, here’s another festive post, this time looking at the chemistry of the poinsettia plant. The red leaves of the poinsettia plant can be used to make a pH indicator, due to their chemical composition; this is actually something of an upgrade on one of the oldest posts on the site, now complete with a explanatory graphic!

You’ll have almost certainly learnt about pH in chemistry lessons, be that recently or otherwise. In case you’re in need of a reminder, pH is a scale that allows us to compare the acidity or basicity of solutions. More specifically, it’s determined by the concentration of hydrogen ions in solution; the greater the concentration, the lower the pH and the more acidic the solution is. A number of different indicators are available to test for the pH of a solution (a number of which were summarised in a previous graphic), but you can also make a pH indicator from the leaves of the poinsettia plant.

This is possible due to their chemical composition. The red colouration of the leaves is caused by pigments called anthocyanins. These compounds are responsible for colouration of a huge variety plant tissues, including red cabbage, blueberries and raspberries, as well as the red leaves of the poinsettia. They also happen to be pH sensitive, meaning that variations in pH can lead to subtle changes in their structure, and in turn influence the colour that they impart.

At a pH of 3 or lower, the anthocyanin is orange or red, and exists as a cation. They then tend to appear colourless just below neutral pH, as the structure changes due to hydration and proton transfer reactions, whereas at higher pHs deprotonation and ring-opening reactions lead to the formation of molecules that give a green, blue or purple colouration.

The steps involved in making an indicator solution from the plant’s leaves are actually pretty simple. You can make your own poinsettia indicator by simply removing the red leaves from the plant and then soaking them in boiling water for up to ten minutes. After the leaves are strained out, the remaining deep-red liquid can be used as an indicator solution as is, or you can soak strips of filter paper in it then allow them to dry in order to produce strips of indicator paper.

Whilst we’re discussing them, poinsettias also have a reputation for being poisonous – a claim that is in fact entirely unfounded. A quick google search will reveal that the myth of poisonous poinsettias potentially originates from a ingestion of poinsettia leaves being mistakenly attributed as the cause of poisoning of an American child in 1919. Despite its reputation, there have been no recorded deaths as a result of poinsettia exposure, and a toxicology test that aimed to find a lethal dose in rats was unable to do so, even at dosages approaching the equivalent to 600 poinsettia leaves. The milky sap of the plant can cause minor skin irritation in some individuals, but it too is otherwise harmless.

That said, not being poisonous obviously isn’t quite the same as being edible, and eating poinsettia leaves can potentially cause stomach pain and vomiting if enough are eaten. However, as its leaves also have a reportedly ‘indescribably awful’ taste, few could probably bear more than a nibble!