Today, though, I wanted to cover pH. Many products claim to personalize your shade based on your individual pH, and I thought I'd address that today.
In chemistry, pH = -log ([H+]). (Stay with me, guys). This simply means it's a measurement of the H+ (or proton) concentration of a solution - a measurement of how acidic or basic a solution is. What that means in practical terms, is shown below. pH runs on a scale from 0 - 14, with acidic solutions falling in the lower end of the scale, and basic solutions at the high end of the scale.
You'll see familiar items on the graphic: vinegar is acidic at pH ~2, water is neutral at 7, bleach is basic at ~13. Solutions at the extreme ends of the scale usually aren't great for humans to touch or consume (i.e., oven cleaner, pH ~13 or battery acid at pH 0) and biological functions generally flourish at near-neutral pH (i.e., ~7; for example, the pH of human blood is ~7.4).
Dikstein reports that the pH of skin ranges from 4-6 (thus called the "acidic mantle of skin" - also helps us sidestep the problem of skin being a solid rather than a solution...#chemistproblems); it differs between different areas of the body, and seems to be related to race and genetic background.
What this means is, the skin pH of most people is going to fall within that range (healthy, alive people, that is). The pH of my skin? Probably from 4-6. The pH of your skin? Probably from 4-6. The idea of a "personalized product for your pH!" is therefore rather a stretch. (Incidentally, this is why straight up lemon juice as a facial treatment is considered damaging. It's pH ~2. Products that contain lemon juice as an ingredient probably have other items to balance out the pH - I am hoping! - so I don't write those off without further research).
Bonus: This great post at Of Faces and Fingers explains how the "color-changing" ingredients work; it's a worthwhile read and really informative.
1. S. Dikstein and A. Zlotogorski, Measurement of skin pH, Acta Dermato-Vereologica (Stockh) Suppl., 185, 18-20 (1994).
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