Do color-changing lipsticks actually react to pH?

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There are a number of color-changing lipsticks on the market from the 1980s MOODMatcher lipsticks to the vegan/cruelty-free Winky Lux Flower Balm. The marketing pitch claims they create a custom shade based on your mood and/or skin pH. Is that actually possible? What kinds of dyes are used? What kinds of dyes react with pH, and are they food-safe? Does skin pH vary enough to create different shades?

In: Chemistry

Yes, you can have dyes that react to pH. It wouldn’t have much to do with mood, and probably wouldn’t vary much between people, but it does react.

The first thing to understand is how things are colored. There are two main types of pigments. Inorganic pigments have metals in them, and the energy gap between the metal and the carbons and stuff attached to them is about right to absorb in the visible range (or UV, like zinc oxide for sunscreen). However, most metal ions are not safe for humans, so we try to avoid using those these days! Instead, we use long chains of carbons and nitrogens and hydrogens that are set up in such a way that they have extra electrons that float above the whole system. That’s called “aromaticity,” and it lets you have electron gaps that correspond to colors. That’s how carrots get their color.

pH reactivity in molecules has to do with hydrogens coming on and off, generally. These molecules have lots of hydrogens with various pKa’s (pHs at which they come off). So in a really basic solution, you might lose three or four hydrogens, and then as you move up to more and more acidic you get all those hydrogens reattached, or even extra ones attached to make the overall charge positive. This changes the energy of the aromaticity because you’re changing where the electrons are, and thus it changes colors visually. These transitions don’t happen all at once, so you’ll get mixed colors.

Your skin is a lower pH than lipstick, as most skincare products tend to be a bit basic and skin is slightly acidic. So putting the lipstick on makes it gain hydrogens, changing the color. It probably doesn’t vary as much from person to person as they pretend it does, but probably they are slightly different as to which exact combination of colors it has. Of course, there’s no guarantee that will be flattering.

Lastly, I am not sure exactly what dyes they usually use, but purple cabbage actually has pH reactive color in it. You can boil purple cabbage in water and then add vinegar to some and baking soda to another to see how the color changes.