Tuesday, 13 December 2016


The classic way to make your own indicator whether it is as a liquid or paper is to use red cabbage which is readily available at this time of year in the UK.

I think the easiest way to do it is to chop up the cabbage - depending on how much indicator you are making you will probably only need a quarter -  and boil in water until it is soft. 

Discard the cabbage and keep the now purple liquid. Once cool you have your indicator - one small tip red cabbage juice smells  and it gets worse the longer you keep it!

BTW I know that many high schools and chemists extract the indicator using solvents but I really think it works just as well using water and boiling up plus primary schools often won't have access to chemicals but use whatever method you want to.

If you are making indicator paper I have found the best  to use is cheap watercolour paper in a pad. You could use coffee filter paper or even school filters but art paper is by far the cheapest and  available in large sheets.
Use a large tray and immerse the paper then let it soak up the colour. Let it dry and cut into strips - there you have it.

You can use larger pieces for pH painting where you dip either cheap paintbrushes or cotton buds into solutions of different pH such as clear vinegar, bicarbonate of soda and then when you paint you will get different colours depending on the pH of the liquid.

You will get reds and pinks in acids, purple in neutral and greens and blues in alkali.

If you don't want the smell of cabbage or you want to do this is summer then you can use pretty much any plant that has a purple colour - squish them up and use the juice. Blackberries work well.

some children testing household solutions with indicator paper we have made

At this time of year poinsettia plants are all over the place and the red bracts chopped up and gently cooked ( or extracted with solvent) give a reddish juice which again is an indicator.

This brilliant poster is part of CompoundChemical's advent calendar which if you are not following it you need to do!
At the time of writing this is the live link

The posters are so good that I am sure they will be archived

Many red and purple plants contain anthocyanins—you may have read about a colourful diet being healthy due to this group. You can also see it in roses and most easily hydrangea which can be turned red or blue by changing the pH of the soil they are growing in.

There are other natural indicators—Turmeric is one, going bright red in alkaline solutions—you don’t get a range of colours but it is quite a startling colour change and great for pH painting.

the change from yellow to bright red 

Fascinating Fact: did you know that litmus paper was made from lichen??