Can you use extracts of autumn leaves as a pH indicator?

Probably - but let’s experiment anyway.

In the Autumn (Fall in another part of the world), the leaves on the trees change colour from green to not-as-green-perhaps-red. Red organic compounds are somewhat pH sensitive - you’ve probably seen the experiment where the extract from red cabbage is made to change colour with the addition of vinegar or baking soda. Can we do the same with leaves?

Extracting the colour from leaves

I know from years of experience that alcohol will extract the chlorophyll from green leaves, so I’ll try it to exact the red stuff.

Chopping the leaves increases the surface area and helps the solvent penetrate the cut edges instead of the waxy, flat surfaces. I ended up stirring the leaves in a warm 50% ethanol solution (25ml of methylated spirit and 25ml of water).

The extract is quite yellow (#TeamYellowChemistry). The red components don’t dissolve as well but have faded in the leaves. The dilution in the alcohol solution may reduce the strength of the colour.

After filtering to remove the leaf bits, the solution is a slightly cloudy yellow. I could have tried to spin this down in a centrifuge to clean up the cloudiness, but this was just a quick experiment to kill time on a dull afternoon when I should have been doing something more useful.

Adding some dilute 0.1M hydrochloric acid to the extract causes a colour change toward red / orange

I also added a dilute sodium hydroxide solution to some of the extract - this caused the extract to turn dark

While not very quantitative and not something you could use in a lab setting, I did at least prove to myself that the colours in autumn leaves are pH-sensitive.

One thing I didn’t try and will try when I get back to the lab is to measure the pH of crushed leaves with minimal solvent - I wonder what the native pH is and if some of the colour of the particularly deeply red leaves is because they are full of organic acids.