Contents
Acid-base indicators
An acid-base indicator is a weak organic acid or base that indicates the end point of acid base titration by a visual change in colour. They are weak acid or base having different colour in basic and acidic solution.
Phenolphthalein and methyl orange are two common examples of acid base indicator.
Mechanism of acid-base indicator action – Ostwald’s theory
According to Ostwald’s theory,
- Acid-base indicators are regarded as organic weak acid or weak base.
- Acid-base indicators undergo partial ionization in water to furnish H+ ions and OH– ions.
- Undissociated form of indicator has different colour than the dissociated form of indicator.
The colour produced by indicator depends on the pH of the solution.
- Phenolphthalein is a weak organic acid. It is colourless at its unionized state while it has pink colour in its ionized state.
In acidic medium, equilibrium shifts to the left and solution is colourless. In alkaline medium, equilibrium shifts to the right forming pink coloured solution.
- Similarly, methyl orange is a weak organic base. Its unionized molecule is light yellow in colour but its ions are red in colour.
In alkaline medium, equilibrium shifts to the left and solution becomes light yellow. In acidic medium, equilibrium shifts to the right and solution becomes red.
Selection of indicator in acid-base titration
Indicators are the chemical species that indicates the end point by changing its own colour. Each pH indicator has its own pH range for its colour change. In order to determine the accurate end point of acid-base titration, the indicator should be selected in such a way that the pH range for the colour change of the indicator must coincide with the pH change (jump) at the end point of reaction.
Mainly two types of indicators i.e. methyl orange and phenolphthalein are used during acid-base titration. Methyl orange has pH range 3.1-4.4 and phenolphthalein has 8.2-10.
There are four types of acid base titrations.
1.Strong acid vs strong base titration:
The indicators pH range of both methyl orange (i.e. 3.1-4.4) and phenolphthalein (i.e. 8.2-10) coincide with the pH jump (i.e. 3-11) at end point. Hence, either methyl orange or phenolphthalein can be used as indicator during strong acid vs strong base titration.
2. Strong acid vs weak base titration:
The indicators pH range of only methyl orange (i.e. 3.1-4.4) coincides with the pH jump (i.e. 3-8). Hence, only methyl orange can be used as indicator during strong acid vs weak base titration.
3. Weak acid vs strong base titration:
The indicators pH range of phenolphthalein (i.e. 8.2-10) only coincides with the pH jump (i.e. 6-11). Hence, only phenolphthalein can be used as indicator during weak acid vs strong base titration.
4.Weak acid vs weak base titration:
The indicators pH range of neither methyl orange (3.1-4.4) nor phenolphthalein (i.e. 8.2-10) coincides with the pH jump ( 6-8). Hence, weak acid vs weak base cannot be usually titrated due to lack of suitable indicators.
Note : Some acid and base examples
Strong acid – HCl, HNO3, H2SO4, etc.
Strong base – NaOH, KOH, Ca(OH)2, etc.
Weak acid – formic acid, acetic acid, oxalic acid, etc.
Weak base – NH4OH, Cu(OH)2, Fe(OH)3, etc.
References
- Negi, A.S., Anand, S.C., A Text Book of Physical Chemistry, Seventh Edition, New Age International Pvt. Ltd. Publishers, 1999.
- Atkins, Peter, Paula, de Julio, Atkin’s Physical Chemistry, Seventh Edition, Oxford University Press, (Printed in India, 2002).
- Gurtu, J.N., Snehi, H., Advanced Physical Chemistry, Seventh Edition, Pragati Prakashan India, 2000.
- Madan, R.L., tuli, G.D., Physical Chemistry, S. Chand and company, New Delhi, 2012.
