What are the indicators?

Indicators are substances that change colour when they are added to acidic or alkaline solutions. Litmus, phenolphthalein, and methyl orange are all indicators that are commonly used in the laboratory.

Natural indicators

  • The indicators that are obtained from plants are known as natural indicators. A few examples of natural indicators are turmeric, china rosa, red cabbage and grape juice.

  • Red cabbage is useful as a pH indicator because of the leaves contain a pigment molecule from the family of anthocyanins, known as Flavins, which are responsible for many of the red/purple/blue colours observed in fruits and vegetables. 

  • In neutral solutions, the colour observed is purple. As the solution becomes more acidic a red colour results. Increasing the pH in basic solutions changes the colour from blue to green and at high pH, the solution is observed as greenish-yellow. 

  • Grape juice indicator- Purple grape juice changes its colour depending on the pH of the solution. An acidic solution like Vinegar turns grape juice pink/red. An alkaline solution like Baking Soda turns grape juice dark green.

Olfactory Indicators

  • Substances which change their smell when mixed with acid or base are known as olfactory indicators. For example onion, vanilla, clove, etc.

  • Onion: Paste or juice of onion loses its smell when added with base. It does not change its smell with acid.

  • Vanilla essence is also an olfactory indicator. Vanilla extract has a characteristic pleasant smell. If a basic solution like sodium hydroxide solution is added to vanilla extract then we cannot detect the characteristic smell of vanilla extract.

  • An acidic solution like hydrochloric acid, however, does not destroy the smell of vanilla extract.

 Synthetic indicators-

  • Methyl orange, Phenolphthalein, Methyl red, Congo red etc. 

  • Methyl orange is a synthetic, pH indicator used in titration because of its clear and distinct colour variance at different pH values. Methyl orange shows a red colour in acidic medium and yellow colour in basic medium.

Universal Indicator

  • The universal indicator is used to know the strength of acid or base. 

  • Universal indicator is a mixture of several indicators. 

  • It is a pH indicator made of a solution of several compounds that exhibits several smooth colour changes over a wide range pH value to indicate the acidity or alkalinity of a solution.

  • The universal indicator shows different colours at different concentrations of hydrogen ions in a solution.

  • Example: litmus paper.

Updated on: 10-Oct-2022


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