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You can make several pH indicators using different coloured plants - try making a few of these indicators and testing different household items to find out their pH. Which indicator do you think is the best? (Not promising that they will all work very well!) Are there any other foods you think you could make indicators from?

Ingredients

  • Some of the following:
    Red cabbage, beetroot, turmeric and rubbing alcohol, blueberry jam, grape or cranberry juice.
  • Different substances to test- some suggestions:
    lemon juice, water, baking soda dissolved in water, detergents, milk, various drinks, vinegar

Recipe

Cabbage/Beetroot indicator

Add 1/4 cup grated red cabbage or beetroot to 2 cups water and boil for around 10 minutes or until the water turns purple. You could also chop up the vegetables with the water in a food blender. Strain the liquid and remove the vegetables (you can eat these now if you're a bit weird and want to).

Tea indicator

Make some VERY strong tea and allow to cool.

Turmeric indicator

Mix 1 teaspoon turmeric in 1/4 cup rubbing alcohol (this is quite nasty stuff so be careful and get some help if you're not confident). The alcohol will separate when you add the indicator to whatever you're testing so pour in slowly down the side of the glass.

Blueberry jam indicator

Mix a tablespoon of jam with 1/4 cup of water (more or less water depending on how well your jam mixes in). Strain to get rid of the bits.

Grape juice/ Cranberry juice indicator

Just use the juice as it comes out the carton.

Using the indicators:

Add a bit of indicator to the substance you want to test. (You may need to dilute the substance in some water if its thick or dark in colour !). Record the colour change and try to decide whether it is acidic or basic. You can use the indicator in solution or soak some blotting or filter paper in the liquid and then allow to dry to make pH strips (don't do this with the turmeric indicator though). You can use this to test stuff by dipping it in to the substance - you can even measure the pH of your own mouth by putting it on your tongue! You can also write invisible messages with vinegar or lemon juice, allow them to dry and reveal them by spraying lightly with indicator solution!

You know when leaves go red and purple in the autumn? Well this is due to the presence of some substances in the leaves called anthocyanins, members of the flavonoid group. If leaves contain acidic sap, the anthocyanins will appear red and if the sap is less acidic, they will go purple. This occurs because in an acidic solution there are lots of hydrogen ions and the addition of hydrogen ions to the anthocyanins alters the wavelength of light that is reflected by the molecules. White light is made up of the whole spectrum of colors in the rainbow, all of them have different wavelengths - the colour of something depends on which wavelengths lights are absorbed and which wavelengths are reflected and detected by our eyes. In a high pH solution there are few hydrogen ions and this means the anthocyanin loses hydrogen ions.

Red cabbage and beetroot contain anthocyanins and this is why they work as indicators.

Red cabbage indicator pH indicator (rough) colors:

pH 2.............4.............6.............8.............10.............12

.....Red......purple..... .violet........blue.... .blue/green... ..green

Turmeric will turn red in a basic solution and yellow in an acidic solution.

Red cabbage indicator. V. Belkhir, site de l'Académie de Nantes http://commons.wikimedia.org/wiki/File:Indicateur_acnantes_ind114.jpg?uselang=en-gb

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This work is licensed under a Creative Commons Licence.