Difference between External Indicators and Internal Indicators.

Hint: Indicators are used to show a colour change when they come in contact with a specific chemical species. Indicators are classified based on how they behave when they react with acids and bases. They are classified into three categories: (i)Self indicator (ii) Internal indicator (iii) External indicator.

Complete answer:
We have to differentiate between Internal Indicators and External Indicators. The name itself gives an idea about how they participate in a chemical reaction.
Let’s first see the definition of Internal Indicators and External Indicators.
Internal Indicators: The indicator which acts as a reactant in the reaction is known as Internal Indicator. This indicator reacts with other reactants to show the presence or absence of the desired chemical substance that reaches the endpoint. Internal indicators are generally used in Redox titrations.
An example of an internal indicator is Potassium permanganate.
External indicators: The indicators which are added externally to the chemical reaction and do not act as reactants are termed as external indicators. However, the use of external indicators is avoided because it decreases the volume of the reaction mixture. They are used as indicators in acid-base titration to detect the alterations in pH.
Examples of external indicators are Methyl orange, Phenolphthalein, and diphenylamine. Potassium ferrocyanide acts as an external indicator when ammonium sulphate reacts with Potassium dichromate.
The indicators are generally colourless, but develop colour as the pH changes. The maximum colour change is seen as the solution reaches the endpoint.

Note:
Remember that the indicators are weak acids. External indicators are mixed with some substance to form a solution and are then used to titrate the product. Colourless alcoholic solution of Phenolphthalein changes to pink colour in a basic medium. Methyl orange is mixed in hot water and then added to acids to give red colour and to bases to give a yellow colour.