How do indicators work?

2 Answers
May 27, 2017

It is done by putting the indicator in the Acid or Base.


pH indicators detect the presence of #H^+ and OH^-#
. They do this by reacting with #H^+ and OH^-#
: they are themselves weak acids and bases. If an indicator is a weak acid and is colored and its conjugate base has a different
color, deprotonation causes a color change.

May 27, 2017

Well, see this old answer.


An indicator is a large, weak organic acid whose acid and base forms have distinctive colours. And we can represent this species by #HIn#. As with any aqueous acid we may assess the acid base equilibrium:

#HIn(aq) + H_2O(l) rightleftharpoonsH_3O^+ + In^(-)#

The point is that #HIn# and #In^-# have quite DISTINCTIVE colours, and depending on the choice of indicator, you can signal the stoichiometric endpoint of a reaction by a macroscopic colour change.

The graph shows (poorly) the titration curve of a strong acid, when titrated by a strong base. Because the rise in #pH# is so precipitous, i.e. so steep, just a drop of titrant (approx. #0.01*mL#) will effect a dramatic change in #pH#, and in these scenarios is does not really matter which indicator you use.