We all know examples of everyday substances that can be classified as acids or alkalis: lemon juice is acidic, bleach is alkaline, and so on. Another substance that can be found in your kitchen can be used to test other substances to determine whether they are acidic or alkaline. The chemicals that give red cabbage its colour also allow it to be used as a pH indicator – this post looks at how!
You probably remember the pH scale from school chemistry lessons, but in case you don’t here’s a brief reminder. In short, substances are deemed acidic if they have a pH lower than 7, and alkaline if they have a pH higher than 7. Indicators are chemicals which change colour at different acidities or alkalinities, allowing us to determine whether a substance is acidic or alkaline.
Red cabbage gets its colour from compounds called anthocyanins in its leaves. These anthocyanins are peculiar in that they’re pH-sensitive, and this allows them to be used as pH indicators. If they are extracted from the red cabbage leaves by boiling the cabbage in water, the resulting solution can be added to different substances to test them. The pH of the solution they are added to can affect the structure of the anthocyanin molecules, subtly changing them in a way that causes them to appear a different colour, as shown in the graphic above.
Red cabbage is just one example of something you might find at home that can be used in this manner. The red leaves of poinsettia plants, common around Christmas time, can also be used!
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3 replies on “Making a Red Cabbage pH Indicator: The Method and the Chemistry”
I did this as a child. One caveat: The indicator liquid has a tendency to “go off” after a time. I suspect a little yeast gets in from the air, causing it to ferment, or perhaps some other bacteria causes it to decay and become ineffective. I am not sure how litmus is stabilized. I wonder if a similar technique could be used.
It’s true it doesn’t keep for long if you just leave it out – it seems to last a lot longer if you refrigerate it though. I made a batch over a month ago, stuck it in the fridge, and used it with no issues this week.
Better to freeze it and make “ice cubes” of it