What are chemical indicators

Introduction 
Substance which undergo some easily detectable change (such as change of colour, precipitation, etc.) during titration and which thereby indicate the equivalence point are called Chemical indicators.

Example:  Phenolphthalein, Methyl orange, Methyl red, Starch, etc.

Main Characteristics Of Chemical Indicators

Chemical indicators possess one color in the presence of an excess of the substance to be estimated, and another in the presence of an excess of the standard solution of the reagent used and thus these substances indicate the exact end-point.  A familiar example of an indicator is litmus, which is blue in the presence of an excess of alkali and red in the presence of excess of acid.
A good chemical indicator must possess the following two essential characteristics.

1. The color change of the indicator must be clear and sharp, i.e. it must be sensitive.  Thus, it would be useless if 2 or 3 c.c. of the reagent is sufficient to bring out the color change.
2. The Ph-range over which the color change takes place must be such as to indicate when the reaction is complete.

Classification Of Chemical Indicators

All well-known indicators can be classified in two ways; either on the environment in which they are used or on the basis of the types of titrations in which they are used.  In general, an indicator may be internal or external.

1. Internal indicator: If the indicator is added to the liquid which is being titrated, it is called an Internal indicator viz. litmus solution, Phenolphthalein, potassium chromate, etc.
2. External indicator: If the indicator is used outside the vessel in which the reaction is taking place, and drops of the liquid are taken out of the reaction vessel from time to time and mixed with the indicator, it is called external indicator.

Potassium ferricyanide, the most familiar example of external indicator, is used during the titration of ferrous ions as it gives blue color with ferrous ions and a brown coloration with ferric ions and thus tells us when a ferrous solution has been completely oxidized to the ferric state.  The indicator is used as external since if it is added to the ferrous solution to start with, it would have been reacted with it.