What is the color of
phenolphthalein in an acidic medium?
Phenolphthalein is a pH indicator,
meaning that it has different colors, depending on the pH. In the lab, phenolphthalein’s most
common application is to detect the end-point for acid-based titrations. When you titrate base against acid,
the phenolphthalein will dramatically turn pink at the end-point. This is because at the end-point,
the pH of the solution rapidly shifts from being acidic to basic. And phenolphthalein is pink in
basic conditions. But below this critical pH,
phenolphthalein has no color whatsoever. The critical pH value is 8.2. So that’s already into the basic
end of the spectrum.
The question is asking what is the
color of phenolphthalein in an acidic medium. An acidic medium will have a pH of
less than seven. A solution with a pH of seven will
definitely have a pH below 8.2. So phenolphthalein in an acidic
medium will be colorless. Interestingly, if you do take the
pH the absolute extremes, you can get different colors out of phenolphthalein. It’s only between pH 8.2 and pH 10
that you get the pink color constantly. Above pH 10, phenolphthalein isn’t
Above pH 10, phenolphthalein will
start degrade and lose its color again. But this takes time. So it’s not all that important for
most titration applications. At the other end of the spectrum
between pH zero and 8.2, phenolphthalein is colorless as we said. But in really acidic conditions
where the pH is so low, it’s actually negative, phenolphthalein turns red.
This rare case isn’t all that
important because phenolphthalein is rarely going to be exposed to such extreme
pHs. But the gradual decay of the color
of phenolphthalein at highly basic pHs is of interest for kinetic studies. Nonetheless, we have a relatively
simple question here. And we’re going to assume that the
pH of the acidic medium is not going to go below zero.
So the color of phenolphthalein in
an acidic medium is colorless.