Selection of indicators in acid-base titration and PH curve

Selection of indicators in acid-base titration

The amount of an acid (or a base), which is exactly equivalent chemically to the amount of some standard base (or an acid), is determined by an acid-base titration. The point of equivalence is called end point. The solution of a strong acid and strong base will be neutral at the end point and have a pH of 7 as they are strong electrolytes. However, if either the acid or the base is a weak electrolyte, the solution at the equivalence point will be either slightly alkaline (pH>7) or slightly acidic (pH<7).>⁺ ion concentration (i.e. pH), which depends upon the nature of the acid and the base and the concentration in the solution.

A large number of acid-base indicators are available which possess different colours according to H⁺ion concentration of the solution. These indicators do not change from predominantly 'acid' colour to predominantly 'base' suddenly and abruptly, but within a small interval of pH (generally about 2 pH units). This is called colour-change interval of the indicator. The position of the colour-change interval in the pH scale varies widely with different indicators. Therefore, for acid base titrations, an indicator is chosen which shows a distinct colour change at a pH close to that of equivalent point.

Colour changes of indicators with pH

Indicator	pH range	Colour Acid solution	Colour base solution
Cresol blue	1.2 — 1.8	Red	Yellow
Thymol blue	1.2 — 2.8	Red	Yellow
Methyl yellow	2.9 — 4.0	Red	Yellow
Methyl orange	3.1 — 4.4	Pink	Yellow
Methyl red	4.2 — 6.3	Red	Yellow
Litmus	5.0 — 8.0	Red	Blue
Bromothymol blue	6.0 — 7.6	Yellow	Blue
Phenol red	6.4 —8.2	Yellow	Red
Thymol blue (base)	8.1 — 9.6	Yellow	Blue
Phenolphthalein	8.3 — 10.0	Colourless	Blue
Thymolphthalein	8.3 — 10.5	Colourless	Blue
Alizarin yellow R	10.1 — 12.0	Blue	Yellow
Nitrarnine	10.0 — 13.0	Colourless	Orange/brown