Acid-base indicator

What is an acid-base indicator?

An acid-base indicator is an organic substance that changes color when the pH of the medium in which it is found changes. They are generally weak acids or bases whose dissociation equilibrium is shifted to the left or to the right depending on the [H+] and that present different color in basic and acid form.

InH ↔ In + H+

Acidic form (color1) ↔ Basic form (color2)

The color change is called indicator shift and occurs in a pH range of about two pH units (transition range).

Table 1: Colors of the acidic and basic forms of various pH indicators
 
Indicator
Color of the form
 acid basic
Tornasolredblue
Phenolphthaleincolorlesspurplish-red
Methyl orangeredyellow

The universal indicator is a mixture of several indicators with a pH range from 1-14 with a pH scale.