Indicator : An indicator is a chemical substance that undergoes a colour change at the end point. The end point of an acid-base titration can be determined using acid-base indicators. Acid Base indicators are either weak organic acids or weak organic bases. They change their colour within a certain pH range. Two theories have been proposed to explain the change of colour of acid-base indicators with change in pH.
Ostwald Theory of Indicators:
According to this theory :
(a) The colour change is due to ionisation of the acid-base indicator. The unionised form has different colour than the ionised form.
(b) The ionisation of the indrcation is largely affected in acids and bases as it is either a weak acid or a weak base. In case, the indicator is a weak acid, its ionisation is very much low in acids due to common H+ ions while it is fairly ionised in alkalies. Similarly if the indicator is a weak base, its ionisation is large in acids and low in alkalies due to common OH– ions.
Considering two important indicators, Phenolphthalein (a weak acid) and Methyl orange (a weak base). Ostwald theory can be illustrated as follows:
Phenolphtalein : It can be represented as HPh. It ionises in solution to a small extent as:
The undissociated molecules of phenolphthalein are colourless while Ph– ions are pink in colour. In presence of an acid, the ionisation of HPh is practically negligible as the equilibrium shifts to left hand side due to high concentration of H+ ions. Thus, the solution would remain colourless. On addition of alkali, hydrogen ions are removed by OH– ions in the form of water molecules and the equilibrium shifts to right hand side. Thus, the concentration of Ph– ions increases in solution and they impart pink colour to the solution.
Titration between HNO3 (strong acid) and KOH (Strong bose)
For this titration, base is taken in a burette and acid is taken in a beaker. The pH of strong acid is very less.
Initially when base is added pH changes gradually but at end point, pH changes rapidly from 3 to 10. The curve obtained when volume of base used and pH change are plotted is known as titration curve. The range of strong acid and strong base is very large. Many indicators come in this range.
The diagram shows the pH curve for adding a strong acid to a strong base. Superimposed on it are the pH ranges for methyl orange and phenolphthalein
You can see that indicator does not change colour at the equivalence point.
However, the graph is so steep at that point that there will be virtually no difference in the volume of acid added whichever indicator you choose. However, it would make sense to titrate to the best possible colour with each indicator.
If you use phenolphthalein, you would titrate until it just becomes colourless (at pH 8.3) because that is as close as you can get to the equivalence point.
