What is pH?
pH is a measure of the acidity or alkalinity of a solution. It is measured and assigned a value from 1 to 14. 1 is the most acidic and 14 is the most alkaline or basic. The scale is divided in the middle with a value of 7 as the neutral value.
pH of less than 7 are acidic and pH of more than 7 is alkaline.
pH is an abbreviation for power of Hydrogen and is used to indicate the measure of hydrogen ion concentration in a solution.
Solution in this context implies it is a water based solution. If there is no water there is no pH to put it simply. To state it differently a product that is a mixture of oils or alcohols, pH value is not applicable.
Water = 7 -> neutral
Lime juice = 2 -> acidic
Distilled apple cider vinegar = 3
Undiluted raw apple cider vinegar = between 4.25 to 5
Bentonite clay with 25% water = between 8 and 9.7
Baking soda with 25% water = 9 -> alkaline
Lye = 14 -> alkaline or basic
pH scale is on logarithmic scale therefore the difference between each number is a factor of 10. For example, baking soda with a pH of 9 is 100 times more alkaline than water. If the skin pH is 5.5, putting on lime juice or baking soda would irritate and inflame the skin as lime juice is acidic and baking soda is alkaline.
pH of skin
Skin has a pH of 5.5 and has a thin protective layer of sebum. Sebum when it gets mixed with lactic and amino acids from sweat creates the acid mantle and contributes to the skin's pH of 5.5
Many factors disrupt the skin's acid mantle and pH value examples being stress, pollution, age, dietary habits, smoking, etc. As we age, skin becomes more acidic thereby reducing its ability to protect itself.
Why pH is important in skincare?
So when we use skincare products we should be aware of the product's pH value as it will be higher or lower than the skin's pH. As time goes by, skin which is our largest organ is bombarded relentlessly by several external and internal factors such as pollution, smoking, diet, stress etc. and therefore protection and defense become more important when we age.
The acid mantle would need to be maintained in order to provide protection to skin. If the pH is too acidic or too alkaline, the mantle cannot survive and skin condition such as dermatitis, rosacea and other skin ailments would start to show up.
Source: pH table https://www.sciencebuddies.org/science-fair-projects/references/acids-bases-the-ph-scale