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

1 comment:

Science It said...

please double check your indicator colors