Question

What is a self-indicator? Give an example.

Answer 1

We know that indicators are substances that exhibit changes in colour because of change in pH. They are known as acid-base indicators. They are generally acid (or) bases but their form of conjugate base (or) acids exhibit different colours due to difference in their absorption spectra.

Complete step by step solution:
We can say indicators are substances which are employed to obtain the chemical nature (acidic (or) basic) of solution/substance. When we add them to a solution involving an acidic (or) basic nature, the indicators change their colour.
For example: The natural indicator litmus is obtained from lichens. Blue litmus changes red in acids and in base, red becomes blue.
We can define self-indicator as a chemical substance which could spot the endpoint of a titration or any other reaction in the company of self-participation in the reaction.
Let us take the example of acidified Potassium dichromate paper which changes from orange to green in presence of sulphur dioxide. We can write the chemical equation as,
${K_2}C{r_2}{O_7} + {H_2}S{O_4} + 3S{O_2} \to {K_2}S{O_4} + C{r_2}{\left( {S{O_4}} \right)_3} + {H_2}O$
Here, the colour of $C{r_2}{\left( {S{O_4}} \right)_3}$ is green because of which, the total colour becomes bright green. But certain indicators such as litmus do not change its own chemical composition to specify change in colour.

Note: We can easily remember a self-indicator as a reactant in any reaction which itself shows the end point of reaction by altering the color of reaction mixture. There is no need to add any indicator. A best example of self-indicator is potassium permanganate which plays as self-indicator in oxidation reduction titration by disappearing or again showing its pink color while the reaction is finished.