Indicators are the chemical substances which are used to determine the end points of titrations. Methyl orange, phenolphthalein, starch solution, erichrome black-T are some examples of indicators.
Indicators are used in acid-base titration and have different colours in acidic and basic mediums. For example, methyl orange has red colour in acidic medium and yellow colour in alkaline medium. Similarly, phenolphthalein has pink colour in alkaline medium and it has no colour in acidic medium.
Indicators used in acid-base titration are weak organic acids or bases. For example, methyl orange is a weak base and it exists in molecular form in alkaline medium and ionic form in acidic medium.
Indicators change their colour in certain pH ranges. The pH range of methyl orange and phenolphthalein are 3.1-4.4 and 8.2-10.0 respectively.
Choice of Indicator
Let us consider that a small amount of acid is taken into conical flask and alkali is added dropwise into it with the help of burette. On addition of each drop of alkali, it neutralizes H+ ion of solution resulting to the increase in pH of solution. The curve obtained by plotting pH of a solution against the volume of alkali added is called titration curve. At the neighbourhood of end point, addition of a drop of alkali neutralizes all acid solution and turns the solution into alkaline solution. Thus, there must be sudden increase of pH of solution near the end point. The shape of titration curve depends upon the nature of titration.
As mentioned above, titration curve is the curve obtained by plotting pH of a solution against the volume of alkali added to it. On the basis of the acids and bases used in titration, the nature of titration curves can be categorized into four types. They are mentioned and explained below:
- Titration between strong acid and strong base (HCl versus NaOH): During the titration between strong acid and strong base, the sudden increase in pH of solution near end point ranges from 3 to 11. Thus, only indicators whose pH range lies in this pH interval can be used in such titration. Since pH ranges of both methyl orange (3.1-4.4) and phenolphthalein (8.2-10.0) lie above pH interval, both of them can be used in such titration.
- Titration between strong acid and weak base (HCl versus NH4OH): In titration between strong acid and weak base, the sharp increase in pH ranges between 3 to 8. Since the pH range of methyl orange lies above the pH interval, only methyl orange can be used as indicator in such titration.
- Titration between weak acid and strong base (CH3COOH versus NaOH): In titration between weak acid and strong base, the sudden increase of pH near end point ranges between 6 to 11. The pH range of only phenolphthalein lies above pH interval. Hence, only phenolphthalein can be used as indicator in such titration.
- Titration between weak acid and weak base: In titration between weak acid and weak base, the sudden increase in pH ranges between 6 to 8. Since no any indicator can change its colour in such a small pH interval, the titration between weak acid and weak base cannot be carried out by using indicators.