Colour changing poinsettia

We have previously had fun exploring how we can use red cabbage as a pH indicator. The anthocyanins that give red cabbage a deep purple colour change to pink in acidic solution and blue in alkaline solution. The small boy desperately wanted to do 'red cabbage science', as he calls it, over Christmas, but we didn't have a red cabbage and I'd used up all the frozen portions.

My parents in law, who we visited after Christmas, had a poinsettia plant which was losing a few red bracts (these are the bits that look like flowers, but are actually just a type of leaf). I remembered that poinsettia bracts get their colour from anthocyanins too, and thought we could try using them instead of red cabbage when we got home.

Poinsettia plant

The small boy was a bit grumpy after a long car journey home, so I thought we could test out the colour changing properties of the poinsettia.  I've never done this before, so I wasn't quite sure how good the colour change would be. I chopped and microwaved five bracts that had fallen off the plant in the last couple of days (so they weren't yet very dried out) in about 100ml water, and then used half of the dark pink liquid, once it had cooled, for each experiment.  We did this in our lab set, using pipettes to transfer liquid, but if you don't have these then clear or white yoghurt pots (or similar) would also work.

He added acid to the first portion (spirit vinegar diluted about one part vinegar to one part water) and watched it turn a lovely bright pink, almost red. He quite liked this, and then added bicarbonate of soda solution (about half a teaspoon in 50ml water) to see what happened. The liquid turned a dark browny-pink.  He was a bit unimpressed so I thought we should try making the solution alkaline first.

Testing the poinsettia colour change

The second time around he added the bicarbonate of soda solution first, and he got a more browny-yellow colour.  It still wasn't as good as the blue that you can get with red cabbage, and he told me as much quite bluntly.  He added vinegar to make it turn bright pink, then declared that he was done with science and would rather watch TV. 

Unfortunately, this wasn't a huge success, and we spent less than 10 minutes on the whole thing.  It certainly wasn't as good as the red cabbage version of the experiment for getting a vivid colour change.  However, if you only had an ageing post-Christmas poinsettia to hand rather than a red cabbage, it was definitely worth doing (particularly if your child hasn't done the red cabbage experiment before).  I've tried to show the colour change below for reference - the liquid with just a little extra water added is in the centre.

Colour change - acid on left and alkaline on right