Molecule of the Day

Here’s a quickie, phenolphthalein:

Phenolphthalein is known as an acid-base indicator because it gives a colorimetric change depending on the pH. Above a certain critical pH, called its pKa, phenolphthalein adopts mainly the lower structure shown above . Below this pH, it is mainly the upper structure. The lower structure is pinkish-purple, the upper is colorless.

As you can imagine, molecules like this have all sorts of uses. You can figure out the concentration of an acid or base, using the color as an indicator of whether your solution is acidic or basic (8.2 is sufficiently close to 7, neutral pH, that you can get away with this). It apparently is used to test for the presence of blood. It used to be used as a laxative, but its carcinogenicity has not been completely ruled out. It is also used in Barbie Hollywood Hair (scroll down to “uses”).

The broad class of dyes are used in “pH papers” for lab use; papers soaked with one or more dyes will change color depending on the pH; a series of dyes can give resolution below even 1 pH unit, which is pretty good. “Litmus” paper is the best known of these, containing a dye extracted from lichen.

You can extract your own pH indicator from cabbage. If you never did this, you had a deprived childhood and should try it over the weekend. If you are lazy, you can just use red wine or grape juice; there is a similar pigment with red acidic and blue basic forms. This broad class of pigments is known as anthocyanins. Many of the deep reds and blues in nature are due to this class of pigments, and they make great pH indicators.