Indicators: Definition, Characteristics and Uses!
Definition:
Certain dyes which can determine the hydrogen ions in a solution are said to be Indicators.
Characteristics:
1. The dyes are weak acids (or, less commonly, weak bases).
2. They have the property of dissociating in solution to yield ions of different colours from the un-dissociated molecules.
3. The pH at which dissociation will occur depends upon the strength of the acid; the stronger the acid the lower the pH at which it dissociates.
4. The free acid of methyl orange, which is red, can be represented as MH.
It is dissociated as below on dissolving in water:
M− being a yellow ion, the colour of the solution will depend upon the extent of ionisation, so that in very dilute solution it will be yellow.
5. It may be orange in stronger solutions with incomplete dissociation, or red with no dissociation.
6. If acid (i.e. H+) is added, the ionisation will be depressed and MH will be reformed and the colour will be red. But if alkali (i.e. OH−) is added, the OH– will combine with H+ to form water and MH will ionize to great extent. Hence, the colour will be yellow.
Uses:
1. Indicators are used to determine the titratable acidity or alkalinity.
2. When an indicator gives its intermediate colour (i.e. it is partially dissociated), it can be easily assumed that the pH of the solution is at about the middle of the range.
3. Solutions of mixed indicators which give a number of colour changes over a wide range of pH are called “universal” indicators. They are used for indicating quickly in what part of the pH range the pH of a given solution lies. For accurate determination of pH, the “pH meter” is used.
4. The following table gives a list of some common indicators which are used for the biological pH range:
