Video: Giving the Color of Phenolphthalein in an Acidic Medium

What is the color of phenolphthalein in an acidic medium? [A] Blue [B] Red [C] Pink/Purple [D] Colorless [E] Yellow


Video Transcript

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 stable.

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.

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