pH stands for ‘potential of hydrogen’ and measures how acidic or basic a solution is. The pH scale ranges from 0 to 14, with 7 being neutral. This scale was first introduced in 1909 by Danish chemist Søren Peter Lauritz Sørensen, and helps us understand the acidity or basicity of substances based on their hydrogen ion concentration.
There are three ranges of pH:
- Acidic: 0 – 6.9
- Neutral: 7
- Basic/Alkaline: 7.1 – 14
The higher the concentration of hydrogen ions, the more acidic the solution will be. On the contrary, chemical solutions with a low hydrogen ion concentration are more basic or alkaline. Strong acids or bases are known as corrosives or caustics, meaning they can cause severe skin irritation or burns.
Base vs. Alkali
Substances and solutions with a high pH are known as either a base or an alkali. However, these two terms have slightly different meanings. The simple definition of a base is a solution with a pH greater than 7.0, and can neutralize acids. All alkalis are bases, but not all bases are alkalis. An alkali is water-soluble, whereas a base is not water-soluble. However, just like a base, alkalis also have a pH value ranging from 7.0 – 14 and can neutralize acids. Since the only difference between the two is water solubility, alkalis are sometimes referred to as water-soluble bases.
pH of Skin
On average, the pH of the skin ranges from 4.1 – 5.8. A pH of 5.5 is considered optimal, but a healthy pH will differ depending on the person’s skin type and even the specific area of their body. Human skin is covered by an ultra thin film known as the ‘acid mantle’. The acid mantle is an antimicrobial protective barrier primarily composed of an oily substance called sebum and sweat. This layer’s pH ranges from 4.4 – 6. The acidity of the acid mantle supports the growth of normal flora and serves as the body’s first line of defense against external pathogens, bacteria, and other potential threats. It is also vital for locking in moisture and maintaining the skin’s smoothness and suppleness.
pH of Hair
The natural pH of human hair is even more acidic than that of the skin. The pH of the hair shaft itself is about 3.67, while the pH of the scalp is slightly higher, ranging from 4.5 – 5.5. Just like the skin, the scalp is also coated in an acid mantle film that protects the hair follicles from pathogens and locks in moisture. However, the hair shaft doesn’t have this special layer of protection, instead, it is protected by a sheath of scaly, layered cells known as the cuticle.