Chemistry Topics cover a wide range of subjects, including atomic structure, chemical reactions, and the periodic table.
List of Indicators Used for Testing Acids and Bases
Acids, bases and salts are the three important groups of chemical compounds (or chemical substances) which are useful to us in many ways. For example, our stomach makes an acid (hydrochloric acid) which is necessary for the digestion of food ; baking soda (sodium hydrogencarbonate) used in baking bread is a base ; whereas common salt (sodium chloride) used in cooking food is a salt. Some of the acids, bases and salts occur in nature whereas many acids, bases and salts are made artificially in factories.
There are, however, many other chemical compounds (such as water, glucose, canesugar, urea, etc.) which are neither acids nor bases nor salts. In this Chapter we will study only acids, bases and salts. In order to find out whether a substance is an acid or base, we should first know the meaning of the term ‘acid-base indicator’ or just ‘indicator’.
Indicators are special type of chemicals which are used to test whether a given substance is an acid or base. Indicators change their colour when added to a solution containing an acid or a base. We will now discuss the various types of indicators in detail.
Indicator for Testing Acids and Bases
An indicator is a ‘dye’ that changes colour when it is put into an acid or a base. An indicator gives different colours in acid and base. Thus, an indicator tells us whether the substance we are testing is an acid or a base by change in its colour.
A substance which contains an acid is said to be acidic whereas the substance which contains a base is said to be basic. So, we can also say that an indicator tells us whether the substance we are testing is acidic or basic by change in its colour. Acid-base indicators are of two types : Natural indicators and Synthetic indicators.
- Litmus, China rose and Turmeric are naturally occurring indicators.
- Phenolphthalein is a synthetic indicator.
We will now describe how these indicators are prepared and how they help substance is an acid or a base (acidic or basic).
Litmus is a natural indicator. Litmus solution is a purple coloured dye which is extracted from a type of plant called ‘lichen’ (see Figure). Litmus has a purple colour (mauve colour) in water. In other words, when litmus solution is neither acidic nor basic (it is neutral), then its colour is purple.
When litmus is added to an acidic solution, it turns red. And when litmus is added to a basic solution, it turns blue. Though the natural colour of litmus is purple, it is made into blue litmus and red litmus for the sake of convenience in detecting colour change when an acid or base is added to it.
Thus, litmus is made into two types : Blue litmus and Red litmus. Litmus can be used in the form of litmus solution (like blue litmus solution and red litmus solution) or in the form of strips of litmus paper (blue litmus paper and red litmus paper). The blue litmus paper strip and red litmus paper strip are shown in Figure.
Litmus is the most common indicator for testing acids and bases in the laboratory.
- Acids turn blue litmus to red.
- Bases turn red litmus to blue.
So, a convenient way to find out whether a solution is acidic or basic is to test it with blue litmus paper and red litmus paper, turn by turn, and observe the change in colour which takes place.
(a) If a drop of the given solution turns blue litmus paper to red, then the given solution will be acidic in nature (or it will be an acid).
For example, a drop of lemon juice turns blue litmus paper to red, so lemon juice is acidic in nature. That is, lemon juice contains an acid.
(b) If a drop of the given solution turns red litmus to blue, then the given solution will be basic in nature (or it will be a base).
For example, a drop of baking soda solution turns red litmus paper to blue, so baking soda is basic in nature. That is, baking soda is a base.
Please note that in order to test a solid substance with dry litmus paper, it is necessary to make a solution of the solid substance in water (otherwise the colour change will not take place). For example, solid baking soda will not turn dry red litmus paper blue. We have to first make a solution of baking soda in water and then use it for testing with red litmus paper. A drop of baking soda solution will turn red litmus paper to blue.
2. China Rose Indicator
China rose is also a natural indicator. China rose is called ‘GudhaV in Hindi. China rose indicator is a light pink coloured solution which is extracted from the red flowers of China rose plant with water (see Figure).
We can make the China rose indicator ourselves as follows: Collect some petals of China rose flower and place them in a beaker. Add some warm water. Keep the China rose petals immersed in water for some time till water in the beaker acquires a light pink colour. Remove the petals by filtration. The light pink solution thus obtained is the China rose indicator. This China rose indicator can now be used for testing acids and bases.
- Acids turn China rose indicator to magenta (deep pink).
- Bases turn China rose indicator to green.
So, another way to find out whether a solution is acidic or basic is to test it with China rose indicator and observe the change in colour which takes place.
(a) If a drop of the given solution turns China rose indicator from light pink to magenta (deep pink), then the given solution will be acidic in nature (or it will be an acid) (see Figure).
For example, lemon juice turns China rose indicator to magenta (deep pink), so lemon juice is acidic in nature. That is, lemon juice contains an acid.
(b) If a drop of the given solution turns China rose indicator from light pink to green, then the given solution will be basic in nature (or it will be a base) (see Figure).
For example, baking soda solution turns China rose indicator to green, therefore, baking soda is a base.
3. Turmeric as Indicator
Turmeric is another natural indicator. Turmeric is a bright yellow powder obtained from a plant. Turmeric is called ‘haldi’ in Hindi. Turmeric contains a yellow dye. Turmeric turns red in basic solution. Turmeric is used as indicator in the form of turmeric paper.
Turmeric paper can be prepared as follows : Take a tablespoonful of turmeric powder, add a little water and make a paste. Deposit the turmeric paste on a blotting paper (or filter paper) and dry it.The yellow paper thus obtained is the turmeric paper. Cut thin strips of turmeric paper. Use these strips of turmeric paper as indicator.
Turmeric paper is used as indicator. Turmeric paper is yellow in colour.
- Turmeric paper is yellow in acid solution.
- Bases turn the yellow turmeric paper to red.
If a drop of the given solution turns the yellow turmeric paper to red, then the given solution will be basic in nature (or it will be a base). For example, if a drop of baking soda solution is put on the strip of a turmeric paper, the yellow turmeric paper turns red. This shows that baking soda solution is basic in nature. That is, baking soda is a base. The yellow turmeric paper, however, remains yellow on adding an acid solution.
We have just learnt that a base turns yellow turmeric paper to red. Now, if we put a drop of an acid on this turmeric paper (which has been turned red by a base), then the turmeric paper will change from red to yellow. This is because acid cancels the effect of base and restores the original yellow colour of turmeric paper.
Many times we have noticed that a yellow stain of curry on a white shirt (which is due to the presence of turmeric in curry), turns red when soap is scrubbed on it. This is due to the fact that soap solution is basic in nature which changes the colour of turmeric in the curry stain to red.
4. Phenolphthalein Indicator
Phenolphthalein is a synthetic (man-made) acid-base indicator.
- Phenolphthalein indicator is colourless in acid solution.
- Phenolphthalein indicator gives pink colour in basic solution.
We will be using phenolphthalein indicator when we carry out the neutralisation reaction of an acid and a base. Please note that if we add acid in the solution of a base (which has been turned pink by phenolphthalein indicator), then the solution will change from pink to colourless. This is because acid cancels the effect of base.
There are some substances which have no effect on any indicator. Such substances are neither acidic nor basic. The substances whose solutions do not change the colour of any indicator in any way are called neutral substances.
For example, the substances whose solutions do not change the colour of either red litmus or blue litmus are known as neutral substances (they are neither acidic nor basic). Pure water (distilled water), common salt and sugar are neutral substances. So, pure water, common salt solution and sugar solution do not change the colour of any indicator.