Acid - Base Indicators

Acid - base indicators (also known as pH indicators) are substances which change color with pH. They are usually weak acids or bases.

Consider an indicator which is a weak acid, with the formula HIn.  At equilibrium, the following chemical equation is established.
 

HIn(aq) + H2O(l) In-(aq) + H3O+(aq)
acid base
color A color B

The acid and its conjugate base have different colors.  At low pH, the concentration of H3O+ is high and so the equilibrium position lies to the left.  The equilibrium solution has the color A.  At high pH, the concentration of H3O+ is low and so the equilibrium position thus lies to the right and the equilibrium solution has color B.

Phenolphthalein is an example of an indicator which establishes this type of equilibrium in aqueous solution:
 

colorless (acid) magenta (base)

Phenolphthalein is a colorless, weak acid which dissociates in water forming magenta anions. Under acidic conditions, the equilibrium is to the left,and the concentration of the anions is too low for the magenta color to be observed.  However, under alkaline conditions, the equilibrium is to the right, and the concentration of the anion becomes sufficient for the magenta color to be observed.

We can apply equilibrium law to indicator equilibria - in general for a weak acid indicator:

Kln is known as the indicator dissociation constant.  The color of the indicator turns from color A to color B or vice versa at its turning point. At this point:

So from the equilibrium expression:

The pH of the solution at its turning point is pKln and is the pH at which half of the indicator is in its acid form and the other half in the form of its conjugate base.

Indicator Range

At a low pH, a weak acid indicator is almost entirely in the HIn form, the color of which predominates. As the pH increases, the intensity of the color of HIn decreases and the equilibrium is pushed to the right.  Therefore, the intensity of the color of In- increases.  An indicator is most effective if the color change is distinct and over a small pH range.  For most indicators the range is within ±1 of the pKln value.

The table gives examples of common indicators.
 

Indicator Color   pKIn pH range
  Acid Base    
Thymol Blue - 1st change red yellow 1.5 1.2 - 2.8
Methyl Orange red yellow 3.7 3.2 - 4.4
Bromocresol Green yellow blue 4.7 3.8 - 5.4
Methyl Red yellow red 5.1 4.8 - 6.0
Bromothymol Blue yellow blue 7.0 6.0 - 7.6
Phenol Red yellow red 7.9 6.8 - 8.4
Thymol Blue - 2nd change yellow blue 8.9 8.0 - 9.6
Phenolphthalein colorless magenta 9.4 8.2 - 10.0

Indicators are used in titration solutions to signal the completion of the acid-base reaction.

A universal indicator is a mixture of indicators which give a gradual change in color over a wide pH range.  The pH of a solution can be approximately identified when a few drops of universal indicator are mixed with the solution.