Indicators are substances which indicate the acidic or basic nature of the solution by the colour change.
- Types of Indicator: There are many types of indicators. Some common types of indicators are:
1. Natural Indicators:
Indicators obtained from natural sources are called Natural Indicators. Litmus, turmeric, red cabbage, China rose, etc., are some common natural indicators used widely to show the acidic or basic character of substances.
- Litmus: Litmus is obtained from lichens. The solution of litmus is purple in colour. Litmus paper comes in two colours- blue and red. An acid turns blue litmus paper red. A base turns red litmus paper blue.
- Turmeric: Turmeric is another natural indicator. Turmeric is yellow in colour. Turmeric solution or paper turns reddish brown with base. Turmeric does not change colour with acid.
- Red Cabbage: The juice of red cabbage is originally purple in colour. Juice of red cabbage turns reddish with acid and turns greenish with base.
2. Olfactory Indicator:
Substances which change their smell when mixed with acid or base are known as Olfactory Indicators. For example; Onion, vanilla etc.
- Onion: Paste or juice of onion loses its smell when added with base. It does not change its smell with acid.
- Vanilla: The smell of vanilla vanishes with base, but its smell does not vanish with an acid.
Olfactory Indicators are used to ensure the participation of visually impaired students in the laboratory.
3. Synthetic Indicator:
Indicators that are synthesized in the laboratory are known as Synthetic Indicators. For example; Phenolphthalein, methyl orange, etc.
Phenolphthalein is a colourless liquid. It remains colourless with acid but turns into pink with a base.
Methyl orange is originally orange in colour. It turns into the red with acid and turns into yellow with base.
Indicator | Original Colour | Acid | Base |
Red litmus | Red | No Change | Blue |
Blue litmus | Blue | Red | No change |
Turmeric | Yellow | No Change | Reddish brown |
Red cabbage juice | Purple | Reddish | Greenish yellow |
Phenolphthalein | Colourless | Colourless | Pink |
Methyl Orange | Orange | Red | Yellow |
Onion | n/a | No change | Smell vanishes |
Vanilla | n/a | No change | Smell vanishes |
pH value shown by different colours role of pH everyday life:
- pH in our digestive system: Dilute HCl (Hydrochloric acid) helps in digestion of food (proteins) in our stomach. Excess acid in stomach causes acidity (indigestion). Antacids like magnesium hydroxide [Mg(OH)2] also known as milk of magnesia and sodium hydrogen carbonate (baking soda) are used to neutralize excess acid.
- Tooth decay caused by acids: The bacteria present in our mouth converts the sugar into acids. When the pH of acid formed in the mouth falls below 5.5, tooth-decaying starts. The excess acid has to be removed by cleaning the teeth with a good quality toothpaste because these kinds of toothpaste are alkaline in nature.
- Soil of pH and plant growth: Most of the plants have a healthy growth when the soil has a specific pH (close to 7) range which should be neither alkaline nor highly acidic.
