Indicators are most useful in titrations. In a titration approaching the equivalence point, the pH changes sharply as the last drop of acid or alkali is added.  A suitable indicator has to change colour at the point where,

the number of moles of acid = number of moles base [INSERT IND_CI_02_ima]

Hence, the pH range of the indicator’s colour change must be within the range of the sudden pH change at the equivalence point. Consider the range of colour change in the following examples:

INSERT [IND_CI_03_ima  table with colour]

Which of these indicators might you choose for the titration of a strong acid with a strong base?

Either phenolpthalien or methyl orange is suitable. Look at the titration curves and notice that each of these indicators changes colour at the equivalence point. INSERT [IND_CI_04_ima]

Which of these indicators would be suitable for the titration of a weak acid with a strong base?

Phenolpthalien is suitable because its range of colour change is within the pH change at the end point. The pH at the equivalence point  'greater than sign' 7.

 INSERT [IND_CI_05] 

Which of these indicators would be suitable for the titration of a strong acid with a weak base?

Methyl orange is suitable because its range of colour change is within the pH change at the end point.  The pH at the equivalence point  'less than sign' 7. INSERT [IND_CI_06_ima]

Which indicator is suitable for the titration of a weak acid and a weak base?

This depends upon the pH at the equivalence point, which depends upon the relative strengths of the specific acid and base. Another indicator with a different pH range of colour change may be required. This is considered further in the Exploring Understanding section.

INSERT [IND_CI_07_ima]

It is worth remembering that Universal Indicator is a mixture of indicators, showing many colours at various pH values. This is explored further with a practical investigation in the Developing understanding section.