Red Cabbage Juice – universal pH indicator

Acids and Bases pages: Acids and Bases | Indicators | Red Cabbage Juice

Red cabbage juice is called a universal indicator because it shows a range of colours depending on the pH of the solution it is added to. Red cabbage leaves contain a chemical called anthocyanin, which changes colour with pH. This makes it useful for testing, but unfortunately it doesn't have a long life and it can smell bad. (Leaving it as red cabbage until needed is less smelly.)

The swatches in this table show the approximate colours the juice turns in solutions of differing pH. Note that the green colour slowly loses its vividness, and green slowly fades through greenish yellow into yellow. Take the colour just a few seconds after adding the red cabbage juice, not several minutes later.

Easy to do at home

Making red cabbage juice is an easy and inexpensive way to test acids and bases at home. No special equipment is needed and the many colour changes make it popular with children of all ages.

Red cabbage juice is prepared by chopping red cabbage leaves (the pieces don't have to be very small), putting them in a saucepan with water, and boiling for a short time. The resulting juice is a strong violet colour.

Tip it into a jug using the saucepan lid to hold back the pieces of cabbage. If the jug has a lid it'll help keep the smell in until you use it.

The juice can simply be washed down the drain when you've finished with it.

The green and greenish-yellow had faded by the time this photo was taken.

Some suggestions of substances to test

These substances are commonly found in kitchens, or can be bought at most supermarkets. From comparing the colours that known acids turn the red cabbage juice, you should be able to figure out which of the others are acids and which ones are bases.

• Apple juice.

• Antacid tablet.

• Ammonia-based window cleaner. This often has its own blue tint, but window cleaners containing ammonia are not as popular these days and so can be hard to find.

• Baking soda (sodium bicarbonate).

• Carbonated water (soda water).

• Citric acid (food additive E330).

• Kitchen or bathroom surface cleaner.

• Lemon juice.

• Lemonade. Testing a colour free soft drink means the colour produced is just the indicator.

• Orange juice.

• Tartaric acid (food additive E334).

• Vinegar (acetic acid, also called ethanoic acid).

• Vitamin C tablet (ascorbic acid, food additive E300). These often have an orange colouring added.

Some other common substances should only be tested with close adult supervision, and preferrably with gloves worn.

• Bleach.

• Washing soda (sodium carbonate, can be made by heating baking soda in a saucepan on a stove – try making some and testing the pH of baking soda and the new sodium bicarbonate, and comparing the colours they give).

• Dish washing powder.

• Caustic soda (sodium hydroxide, also called lye or drain unblocker – some of which are just sodium hydroxide, some are sodium hydroxide with some aluminium added).

After you've found the strong acids and bases, are you able to get a different colour by diluting the acid or base with water before adding the red cabbage juice?

See www.aqion.de/site/191 for the pH values of some common acids and bases at various concentrations.