I’m making pH indicator paper with some of my classes this week, using the coloured leaves of red poinsettia plants, which set me thinking about the chemistry behind why these plants can be used as indicators.
Poinsettias 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. Not being poisonous obviously isn’t quite the same as being edible, and eating poinsettia leaves can potentially cause stomach pain and vomiting – but there have been no recorded deaths as a result of the plant. As its leaves also have a reportedly ‘indescribably awful’ taste, few could probably bear more than a nibble.
As any secondary school science teacher will know, it’s not just poinsettias that can be used to fashion rudimentary indicator solutions. Red cabbage, beetroot and blueberries can all be used as alternatives. The reason they can all be used as indicators is due to the presence of a particular chemical compound – anthocyanins.
Anthocyanins are water soluble pigments, derived from another class of compounds called anthocyanidins, which in turn belong to a parent class of molecules called flavanoids. The structure of the anthocyanins is similar to the anthocyanidins, but with sugar molecules also attached to the structure at various points. The general structure of the anthocyanidins is shown below; different anthocyanidins will have varying combinations of —H, —OCH3, —OH, groups attached at the R positions.
Anthocyanin pigments are responsible for the red, purple and blue colours of many different flowers, fruits and vegetables, as well as being the pigments that give autumn leaves their colour as plants and trees cease chlorophyll production. They are also responsive to changes in pH – making them useful as an indicator if extracted.
At a pH of 3 or lower, the anthocyanin is orange or red, and exists as a flavylium cation. They then tend to appear colourless just below neutral pH, as the structure changes due to hydration and proton transfer reactions, whilst at higher pHs deprotonation and ring-opening reactions lead to the formation of molecules that give a green, blue or purple colouration.
It’s worth pointing out that this is at best a very basic overview of the chemical compounds and reactions involved in the pH sensitivity of the anthocyanins. The majority of this has been gleaned from online research – for a more in depth analysis, the journal articles referenced below offer a detailed appraisal.
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.
References & Further Reading: