Acid-Base Indicators (College Board AP Chemistry)

• Acid-base indicators change colour depending on the pH of the solution they are in 
• They are weak acids or weak bases whose respective conjugate bases and conjugate acids have different colours
• For example, the weak acid, Hln dissociates as follows:

HIn (aq)  + H₂O (l)  ⇌ H₃O⁺ (aq) + In^– (aq)

colour 1                   ⇌                    colour 2

• Hln (weak acid) and In^– (its conjugate base) are different colours 
• The relative concentrations of the two species determines the colour of the solution
• If the solution is alkaline:
  • The equilibrium shifts to the right 
  • More In^– are present 
  • Colour 2 will be observed
• If the solution is acidic
  • The equilibrium will shift to the left 
  • More HIn are present
  • Colour 1 will be observed 
• The colour depends on the pH of the solution 
• The colour will not change suddenly but gradually over a pH range
• The K_a value of the indicator determines the pH at which the transition occurs

• The point at which the concentrations of [HIn] and [In^–] are equal results in the end point of the reaction i.e when neutralisation has occurred 

• Taking the negative log of both side: 

pK_a  = pH

• This means that the pK_a of the an indicator is the same as the pH of its endpoint 
• The shift in colour for the majority of indicators occurs within a pH range of pK_a  ± 1

Choosing a suitable indicator 

• The pH changes very rapidly around the equivalence point of a titration
• Indicators change colour over a narrow pH range, approximately centred around the pK_a of the indicator
• An indicator will be appropriate for a titration if the pH range of the indicator falls within the rapid pH change for that titration

Indicator	Colour in acid	Colour in alkali	pKa	pH range of colour change
Thymol blue	red	yellow	1.7	1.2 - 2.8
Methyl orange	red	yellow	3.7	3.1 - 4.4
Bromophenol blue	yellow	blue	4.1	3.4 - 4.6
Methyl red	red	yellow	5.1	4.4 - 6.2
Phenolphthalein	colourless	pink	9.3	8.3 - 10.0

Strong Acid - Weak Base:

• The pH transition occurs from 4 to 7 at the endpoint
  • Methyl red is the most suitable, although methyl orange is often utilized due to its significant color change at the endpoint and broader availability.

Weak Acid - Strong Base:

• The pH shifts from 7 to 10 at the endpoint
  •  Phenolphthalein is the only widely available indicator suitable for these titrations

Strong Acid - Strong Base:

• In titrations involving strong acid and strong base, the pH transition spans from 4 to 10 at the endpoint. so a suitable indicator must exhibit a color change within this pH range
  • Methyl red and phenolphthalein serve as appropriate indicators for these titrations although methyl orange is not ideal, its significant color change at the endpoint makes it widely employed used

Weak Acid - Weak Base:

• There is no abrupt pH change at the endpoint so there are no suitable indicators for these titrations, and determining their endpoints is challenging

Titration curve for a weak acid- weak base 

The overlay on the graph shows that both phenolphthalein and methyl orange would change colour outside the point of inflection in a weak acid-weak base titration so they would not be able to show the equivalence point of the titration