pH Indicator: Chemical Reaction?

Name: Bob
Status: educator
Grade: 9-12
Location: IL

Question: When a pH indicator such as phenol red is added to vinegar
or ammonia, is the color change indicative of a chemical reaction in
the phenol red? Does changing the color back indicate yet another
chemical reaction with the indicator?
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Not usually.  Well, OK, it IS a reaction, but just a simple acid-base
reaction.  Let's take simple phenol as an example.  When phenol is
deprotonated, the resulting phenoxide anion is such that its negative
charge can be delocalized to several positions on the phenyl ring. This
essentially increases the conjugation length of the chromophore, letting
it absorb longer-wavgelength light.  Indicators have chromophores that
are the right size so that when they are protonated or deprotonated,
their absorbance change is in the range that is easily seen by our eyes.

Richard Barrans, Ph.D., M.Ed.
Former organic chemist
Department of Physics and Astronomy
University of Wyoming
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This is tough to answer in general, because there are always exceptions. 
When a pH indicator is added to a solution and the pH is changed, a chemical 
reaction occurs. Exactly what that reaction is depends upon the indicator, 
and the pH range. "Usually" this change is reversible, but one could envision 
some combination of indicator(s) and reagent(s) where this might not happen. 
The color change occurs when the absorption of visible light by the indicator 
changes due to some electronic or structural change in the indicator. I think 
one would have to research each combination of reagents and indicators to 
determine if the reaction is reversible at the same pH. One could envision 
systems where going from high to low pH would result at a different value 
than going in the other direction. A difficult question to give a universal 
answer.

Vince Calder
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