Re: Why do phenolphthalein agar cubes turn white on the edges in NaOH?

Danni,

I think you are right: the phenophthalein is diffusing into the solution.

If this is true, then it may be true that this may happen with other indicators. Try this again using something like alizarin yellow or phenol red (which have "acidic" colors other than colorless) and see if the same effect occurs.

You can also try thymolphthalein cubes as well. As you probably know, thymolphthalein is very similar to phenolphthalein but is blue which may be easier to see diffuse. Try to put the cube in a very small amount of acid (less than 10 mL). Perhaps the blue will be visible upon comparison to the original solution.

Lastly, although I doubt this, it is possible that long exposures to an extremely basic solution may alter the molecule within the gel so that the stereochemistry is changed. Sounds like a Ph.D. thesis investigation and not one I'd consider doing! Good luck!

Tim Oborn
Chemistry Teacher-Calgary, Canada