A natural pigment in plants is Anthocyanin which changes colour based upon pH. This is commonly used in ripening fruit but is also present in other parts of plants like leaves. A vegetable containing high quantities of Anthocyanin is the red cabbage. By extracting the Anthocyanin we can use it as a pH indicator and also a natural dye. The Anthocyanin has a colour range of red, pink, purple/blue, green and yellow which makes it an entertaining experiment for kids.
Red cabbage pH indicator activity
What you need:
- Red cabbage
- Boiling water
- Container for cabbage
- Smaller containers for tea
- Sieve with optional strainer cloth
- Citric acid powder
- Calcium Oxide or Bicarbonate of Soda
What you need to do:
- First you must chop or blend some of the red cabbage, perhaps a quarter or half, then steep it in boiling water to make a tea. With the chopped cabbage in a container pour water from the kettle over the cabbage until it is all submerged them leave to cool.
- Second you must strain the cabbage, through a sieve should be fine but using a muslin cloth or coffee filter would be better to get a clearer liquid. We strained some paper kitchen towel.
- Lastly you need to portion the cabbage tea out into containers and add the pH adjusters to change the colour.
To make an acid solution and change the colour more towards the pink and red we recommend adding Citric Acid powder, a little at a time to slowly shift the pH down the scale.
To make an alkali solution and change the colour more towards green and yellow we recommend adding Calcium Oxide (builders lime,) a little at a time to slowly shift the pH up the scale. Careful not to add too much else you make white wash.
Both these chemicals sit quite far on either end of the pH scale of 0 to 14. Water is neutral at 7, Citric Acid is acidic at 2 and Calcium Hydroxide is alkaline at 12. Any chemical that is highly alkaline or acidic is an irritant and can even result in burns upon over exposure, SO DON’T LET YOUR KIDS PLAY WITH IT! That said, use gloves, supervise your children, wear goggles if you have them and use common sense. The chemicals you buy will likely have similar warnings and advice on the packaging.
Citric Acid powder is commonly used in baking and can be quite easily acquired. If you don’t know where to get Calcium Oxide and would prefer a less extreme alkaline alternative then Bicarbonate of Soda can be used which is an 8 on the pH scale. (That’s “Baking Soda” not “Baking powder” which contains the acidifying agent “Cream of Tartar” and makes it useless for this.)
Note: Mixing the Citric Acid and Bicarbonate of Soda in one glass to adjust the pH might seem like a clever idea but will make a multi colour bubbling mess. It produces carbon dioxide also, so open a window if you are doing a big reaction in a confined space.
If the colours are too dark or too cloudy for you, dilute the solution with water. Water being pH neutral the colour shouldn’t change.
When you are done, all chemicals are safe to rinse down the sink.
This activity was a lot of fun and highly addictive. We used a pre washed 100% cotton pillow case and some yarn that was 100% cotton and the results were great.
What you need:
- Red cabbage
- White vinegar
- Bicarbonate of soda
What to do:
- Chop the cabbage up into small pieces and put it into a large saucepan with enough water to cover and then boil for half an hour
- Using a sieve/muslin strain the cabbage juice and divide into three separate bowls
- In one bowl add 1tbsp White vinegar, in another bowl and 1 tbsp bicarbonate soda and leave the last bowl with just the red cabbage juice.
- Wearing gloves, fully submerge the cloth into the bowls and leave for an hour, then remove, wring it out and leave to air dry.
- Once mostly dry wash it in cold salted water after to remove acidity/alkalinity and dispose of water down sink. The salt supposedly helps fasten the colour to the cloth though this might be an old wives tale as I’ve never found an explanation for it. You might find it best to wash in the sink of even the bath so there is plenty of water to wash out the remaining acid or alkali chemicals.
We hope you enjoy these activities and we would love for you to share with us on social media any of your own naturally dyed fabric.