Turmeric: a natural indicator

You will need:

• A glass or disposable plastic cup – turmeric can stain
• A teaspoon
• A suitable work surface – such as a stainless steel sink or a glass surface protector
• Water – tap water is fine
• Turmeric
• Kitchen cleaning spray – you want fairly strong stuff that is soapy, rather than a vinegar-based cleaner (both Fairy Power Spray and Mr Muscle Kitchen Cleaner worked for us)
• Vinegar

Safety

The ingredients for this experiment are common household items, but it is sensible to remind ourselves of a few safety tips before we begin.

Kitchen cleaning sprays are usually a mixture of compounds that can irritate eyes and skin. Because they are sprays, they can also irritate airways. Have a quick look at the warnings on the bottle. If contact does occur, wash thoroughly with water.

Workspace and activity

Turmeric can leave yellow stains, so try to find a work surface that is unlikely to become stained – a stainless steel kitchen sink or glass surface protector would work.

Avoid working over a carpeted area or on unfinished wood, have a cloth or kitchen roll to hand. If your kitchen worktop ends up with a bit of a stain, the kitchen cleaning spray may well be useful to get this off. Avoid spraying kitchen cleaner at people and ensure good ventilation.

And finally, it can be helpful to have some thick gloves and newspaper ready to help you collect and wrap up any glass breakages that happen.

Procedure

• Step 1: Put some water into your glass or cup. You don’t need a lot; a half-full shot glass is sufficient.
• Step 2: Use a teaspoon to add a small amount of turmeric to the water and give it a good stir. Little will dissolve, but it should colour the water yellow.
• Step 3: Spray 1-2 squirts of your kitchen cleaner into the turmeric solution in the glass. You should now see a colour change to dark red.
• Step 4: Once you have observed a colour change, add a little vinegar to the solution and see if you can return it to the original colour (yellow).
• Step 5: Wash your solution down the sink with lots of water and clean up any spillages.

Brief explanation

Turmeric contains a compound called curcumin, which is a naturally occurring indicator. Indicators are molecules that change colour depending on the pH of a solution.

For curcumin, this means it is yellow in acidic and neutral solutions (up to pH 7.5), and red in more basic solutions (to about pH 8.4).

The kitchen cleaning spray is basic and so it has a high pH (the older word for basic is alkaline). Spraying it into the water changes the pH of the solution; the solution becomes more basic, causing the colour change to the red form of curcumin. Adding vinegar, which is a weak acid, makes the solution acidic, so the yellow form of curcumin dominates again. And you can even observe a gradual colour change as it takes a bit of time for the acids and bases to mix and react.

Figure 1: The 2 forms of curcumin – yellow (left) below pH 7.5, red (right) above