9.3.B - Acid/Base Identification


An indicator is a chemical compound that is added in small amounts to a solution so that the pH (acidity or basicity) of the solution can be determined visually. Indicators are chemical detectors for hydrogen (hydronium) ions (H+/H3O+). Normally, the indicator causes the colour of the solution to change depending on the pH.

Indicators have a variety of uses in chemistry, such as titrations and pH identification. They also have widespread use in domestic applications to determine the pH of soil and to measure the pH of water in fish tanks and swimming pools.

In this unit you will learn:

  • how to use indicators to determine the pH or pH range of a solution
  • how indicators are made
  • about some of the common uses of indicators


How Indicators Work

The substances in many plant products such as tea, beetroot, red cabbage or grapes react with acids (or bases), resulting in changes at the molecular level. This causes their colours to be different at different pH levels.

All pH indicators, such as litmus paper, change colours depending upon whether they donate or accept protons, (acids are proton donors and bases are proton acceptors). Therefore, pH indicators are themselves acids or bases. Chemists pick selective natural dyes to indicate specific pH levels based on their colour changes. For example, a commonly used acid-base indicator, named bromothymol blue, is yellow in acid and blue in base and intermediate in between at pH 7. Not all indicators have their colour change around a pH 7. Phenolphthalein is colourless at pH less than 8 and pink at pH greater than 10. Generally, chemists would use a variety of indicators in conjunction with one another to work out a more accurate pH value for a substance. Universal indicator is a mixture of many indicators and can give a more precise pH value.



The table below shows some pH ranges for some common indicators.


Consider the following results obtained from an experiment where a solution of unknown pH was tested with four different indicators.

Indicator Alizarin Yellow Phenolphthalein Bromothymol Blue Methyl Orange
Colour yellow colourless yellow yellow-orange


If we compare this information to the information in the table above showing the colours of different indicators, we see that the solution has a pH greater than 4.5 but less than 6.5. This represents a range of 2 pH units. If we had only used one indicator, say bromothymol blue, then the only information we could obtain was that the pH of the unknown solution was somewhere less than 6. Using a combination of indicators ensures a more precise value for pH. This could be even further narrowed by using more indicators, such as chlorophenol red. Of course the most precise pH could be obtained by using a pH meter or a data logger with a pH probe.

While it is not necessary that you remember the pH range of the colour change for all indicators, you should ensure that you can recall the details of at least one indicator changing colour in the neutral, acidic and basic range. Examples of these are shown in the table below.

Indicator Colour Change Range pH Range Colour Change
Methyl orange Acidic 3.1 - 4.4 red to yellow
Bromothymol blue Neutral 6.0 - 7.6 yellow to blue
Phenolphthalein Basic 8.5 - 9.5 colourless to pink

 

Where Indicators Come From

Flower and leaf pigments are often used as acid-base indicators. For example, take rose petals and crush them with alcohol and you have an acid-base indicator solution. Stew some red cabbage and pour off the juice and you have an acid-base indicator solution. Many indicators can be extracted from plants and others like phenolphthalein and methyl orange are synthetic. Here are a few 'natural' acid-base indicators:

  • Alizarin is an orange dye present in the root of the madder plant. It was used to dye wool in ancient Egypt, Persia, and India. In an 0.5% alcohol solution, alizarin is yellow at pH 5.5 and red at pH 6.8. Several synthetic modifications of alizarin are also used as acid-base indicators.

  • Cochineal is an acid-base indicator made from the bodies of dried female cochineal insects, found in Mexico and Central America. You'll have to grind about 70,000 insects to make one pound of dry indicator. The powder is about 10% carminic acid, which is yellow in acidic solution, and deep violet in alkaline solution. Cochineal solutions are not used much as acid-base indicators these days.

  • Curcumin, or turmeric yellow, is a natural dye found in curry powder. It turns from yellow at pH 7.4 to red at pH 8.6.

  • Esculin is a fluorescent dye that can be extracted from the leaves and bark of the horse chestnut tree. You'll need to shine a black (ultraviolet) light on the indicator to get the full effect. Esculin changes from colourless at pH 1.5 to fluorescent blue at pH 2.

  • Anthocyanin is probably the most readily available acid/base indicator. It is the plant pigment that makes red cabbage purple, cornflowers blue, and poppies red. It changes colour from red in acid solution to purplish to green in mildly alkaline solution to yellow in very alkaline solution. The colour changes for red cabbage juice are shown to the right.

  • Litmus is a blue dye extracted from various species of lichens. Although these lichens grow in many parts of the world, almost all litmus is extracted and packaged in Holland. Litmus is red at pH 4.5 and blue around pH 8.3. While most litmus is used to make litmus papers, some is used as a colouring for beverages.

  • Logwood is a dye obtained from the heartwood of a tree that grows in Central America and the West Indies. The extract contains hematoxylin and hematein, which turn bright red in alkaline solution.

  • Beetroot changes from red to yellow in very basic solution.

 

Making Indicators

Colour changes for red cabbage indicator.

Nature provides us with pH indicators in the form of plant pigments known as anthocyanins, which change colour over different pH ranges, depending on the source.

You will do an experiment to prepare a natural indicator and one simple plant source is red cabbage. Red cabbage juice will function over a wide pH range, from as low as pH 1 up to pH 12. The indicator solution is prepared by chopping up the red cabbage into small pieces, preferably by running them through a blender. Then the juice is strained off from the resulting mush and the mush can also be extracted with water to yield even more of the pigment. The solution is filtered to remove any remaining plant matter and can then be used as is or diluted if the colour is too intense. Solutions of NaOH or HCl in water with pH's of 0 through 13 tested the indicators yield the results shown in the picture on the right. Outside the shown ranges, the indicators will not change colour.

 

Uses of Indicators

Indicators are used as solutions or can be adsorbed onto the surface of paper such as filter paper. Acid-base indicators and pH testing kits are used in a range of everyday situations, including in:

  • the soil testing part of a nursery or hardware store
  • a swimming pool store or the pool chemicals in a large supermarket
  • the aquarium supplies section of a pet shop.

In schools, it is recommended that indicators are used to determine if chemical solutions are neutral before disposal down drains. Domestic waste water and waste water from light industries is often tested with indicators to ensure that waste water is not acidic so that it will not corrode sinks, drains and sewerage pipes.