Acid-Base Indicators

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

An indicator is used to measure pH instead of using a pH meter. The indicator is a chemical substance added to an acid or base, which changes colour according to the pH. Indicators are much cheaper than pH meters.

Litmus

The most common indicator is found on "litmus" paper. It is red below pH 4.5 and blue (or bluish-purple) above pH 8.2. A near neutral solution will either not change its colour, or the litmus paper will look purple.

pH	Colour of litmus
< 4.5	Blue
4.5 – 8.2	No colour change, or purple.
> 8.2	Red

Damp litmus paper can be used to test for ammonia gas, which dissolves in the water and turns the litmus blue.

Litmus paper can be used to test for chlorine gas, which bleaches the litmus paper white. Unlike the normal red-blue colour change, this one is not reversible.

Litmus has been used since about AD 1300. It's a water-soluble dye extracted from some species of lichen.

Methyl orange

Methyl orange changes colour as a mid-strength acid, and is red below pH 3.1 and yellow-orange above pH 4.4.

Phenolphthalein

Phenolphthalein is used by magicians to turn water into wine, because it is colourless below pH 8.2, and reddish purple above pH 10.0. It's actually a little more complicated than that - see table below. Phenolphthalein is insoluble in water, so is usually dissolved in alcohol. It is not the same as phenol red, which is yellow below pH 6.6 and fushia pink above pH 8.0.

pH	Colour of phenolphthalein
< 0	Orange
0 – 8.2	Colourless
8.2 – 10	Pink
> 10	Reddish purple
	In strongly basic solutions, phenolphthalein's pink colour undergoes a rather slow fading reaction and becomes colourless again.

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.)

Acid

Base

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.

For more information, see the Red Cabbage Juice page.

Thymolphthalein

Disappearing ink can be made from phenolphthalein (see above) or another acid-base indicator called thymolphthalein. It is colourless below pH 9.3 and blue above pH 10.5.

Wikipedia: Dissolve the thymolphthalein in alcohol solution and add sodium hydroxide (NaOH) dropwise until it turns blue. It can then be sprayed onto cloth. After several minutes, it will turn colourless. (Carbon dioxide from the air will react with the basic solution. When the pH drops below 9.3, the thymolphthalein turns colourless.) Such inscription can be turned visible again by submerging it in liquid of pH higher than 10.5.

Turmeric

Turmeric, the spice which gives curry its yellow colour, is yellow below pH 7.4 and red above pH 8.6.

Experiment suggestion: Can it be used to detect a sodium bicarbonate solution (a weak base)?

Universal indicator

A universal indicator is an indicator that has multiple colour changes across a wide range of pH values. They are made from a combination of other indicators chosen so that their colours combine to give distinctly different colours from pH 1 to pH 14.

Universal indicators are available as liquid and paper test strips. The test strips are particularly useful with solutions which are not colourless, while liquid universal indicator has the edge testing colourless solutions.

Hydrangea flowers

Hydrangea flowers are pink or blue depending on soil pH. In acidic soils the flowers are blue; in alkaline soils the flowers are pink. Around the transition pH the flowers are purple.

Experiment idea: Can hydrangea flowers be used as a pH indicator for household substances, in the same way as red cabbage juice? Is it possible to work out what the pH of the transition is?

Others

There are many other indicators that give a wide choice of colour changes at lots of different pH values.