What exactly is pH and why do we care when it comes to skin care products? pH is a measure of acidity or alkalinity. If you see a pH of 0 to 6.9, this means the thing is acidic. The closer to 0 a thing is, the more acidic it is. A pH of 7 is considered neutral, like water. And a pH of 8 to 14 is considered basic or alkaline.
There seems to be some debate about the pH of our skin. I have seen values from 4 to 7. I have taken this definition from PubMed.
Today, the term "normal skin pH" is understood to be the pH value of the surface of the skin of the lower arm of a healthy adult male Caucasian. Its mean value lies in the range 5.4-5.9. In most cases, it is determined by means of a flat glass electrode. The parameter "skin pH" depends mainly on the area of skin and on age, but it also depends to a lesser extent on sex, race and the time of day when values are determined.
Our skin has an acid mantle, which is a fine, slightly acidic film on the skin that acts as a barrier to bacteria, viruses, and other icky things that might penetrate our skin. This acid mantle is a result of production of amino acids and lactic acid that settles on our skin. It is a good environment for the good bacteria and other tiny creatures that live on our skin, and they can protect us from chemical and natural attack by other tiny creatures or skin unfriendly chemicals.
We don't want to mess with the acid mantle, so we need to formulate our products within the skin's pH range - usually 5 to 8 is good. We don't want to use harsh surfactants or very alkaline soaps as they can destroy the acid mantle, which leads to dry skin, reduction in the stratum corneum lipids, and lowered resistance to microbial or chemical assaults.
How do we change the pH of our products? For surfactants, most are in the 5.5 to 6.5 range, so you don't need to adjust the pH for your body or facial washes. Plantapon 2000 (but not decyl glucoside) has a very high pH (11.5 to 12.5), so you will need to adjust the pH by adding citric acid or another acid (citric is the easiest and cheapest for homecrafters). You will have to experiment to find the right amount of citric acid to add to your products.
For a lotion or cream, you generally don't need to worry about pH. If you are including things like AHA in your products, you shouldn't be using enough to alter the pH radically, so we don't need to worry about increasing the pH. If you really want to increase the pH in your lotion, you can include a titch of sodium lactate (the conjugate base of lactic acid, which creates a buffer) or you could add some lye (very basic, around 12.4). Again, I wouldn't really worry about it - if you're using so much AHA or other acid in your lotion that it goes really acidic, you're using too much!
As a note, the pH of your hair is one of the reasons most people can't use cold processed soap as a shampoo bar. It is generally an alkaline product, which means it messes with the pH of your hair (we'll be getting into hair in the future...), which leaves the cuticles all sticky-outy and sharp, which leads to tangles and dryness.
You can test the pH of your products with a pH meter or pH strips. Or you can make your own pH tester solution with red cabbage. Or you can use strawberry extract! They contain anthocyanins and anthocyanidins. The colour of these flavonoids is dependent upon pH. When the solution is below pH 3 (very acidic), the colour is red. At neutral pH (7), they show violet, and above pH 11 (very basic) they show blue. (Although you'll only really known that it is below 3, between 3 and 11, and above 11. But it is kinda cool!)
Join me tomorrow for the first posts on skin types!