Find out what pH means and how it affects hair products.

“pH” is an abbreviation for “potential hydrogen” and is a scale used for ranking the relative acidity or alkalinity of a liquid solution. The precise mathematical definition of pH is the negative logarithmic value of hydrogen ion (H+) concentration in the solution.


pH = - log [H+]

 

 

pH

Example

 

 

0

battery acid
 

1

stomach acid
 

2

orange juice
 

3

vinegar
 

4

acid rain
 

5

human hair (4.5-5)
 

6

wine, beer, milk, magnesium sulfate
 

7

purified water
 

8

toothpaste
 

9

baking soda
 

10

milk of magnesia
 

11

ammonia
 

12

oven cleaner
 

13

bleach
 

14

drain cleaner
     
     
 

Scanning electron micrograph of a human hair, showing the overlapping scales of the cuticle layer.

   

A scale of zero to fourteen is used for pH, with 7.0 being a neutral solution (water). A number below 7.0 is considered to be acidic, with a lower number being more acidic, and anything above 7.0 is considered to be alkaline or basic, with 14 being the strongest alkaline value. Due to the logarithmic nature of the pH scale, a solution with a pH of 2.0 is ten times more acidic than one with a pH of 3.0. Human hair is a solid material, a composite of protein molecules with three distinct layers. The central portion called the medulla is not present in every hair and is usually just made up of air. The surrounding layer, known as the cortex, is composed of bundles of fibrous coils made of keratin protein molecules that supply the hair strand with its strength and elasticity. The cortex also contains particles of melanin, which impart color to the hair strand. The outer layer is comprised of multiple layers of overlapping, keratinized scales and is called the cuticle, which acts to protect the cortex and medulla. Hair and skin are both covered by a very thin fluid layer comprised of oil, salt and water, called the mantle, which is slightly acidic (pH = 4.5 – 5.0). This acid mantle is very important in maintaining the proper moisture balance in our hair and skin. It is also instrumental in making the cuticle scales lie flatter against the surface of the hair shaft, which makes hair smoother and shinier as the flat scales reflect light more coherently. Scales that lie more snugly against the hair shaft also prevent moisture loss more efficiently, which helps hair to be stronger and healthier. With the normal exposure to the environment as well as washing and styling, this acid mantle can become contaminated or removed and must be restored with the use of properly pH-balanced products.

Mildly acidic products can be applied to the hair to harden the outer layer, flatten the cuticles, and shrink the diameter of the hair. This serves to make the hair glossy, shiny and less prone to tangling and snagging on adjacent hair strands. Hair that is close to its ideal pH of 4.5 – 5.0 is also at its peak strength. Shampoos and conditioners that are mildly acidic also have been noted to provide longer life to the color of hair that has been dyed.


Alkaline products cause the hair to swell, the cuticle to lift and remove oils from the air. This results in frizzy, dull, brittle hair that is prone to breakage and tangling. Extremely alkaline solutions cause the disulfide bonds between keratin protein molecules to break down and can eventually dissolve the protein completely. These types of solutions are the ones used to perm or relax the hair and can be extremely damaging, especially to hair that is already fragile.


Most modern shampoos and conditioners are formulated to be slightly acidic, having a pH around that of the hair’s acid mantle (4.0-5.0). For this reason, you may see chemicals such as citric acid or sodium hydroxide or triethanolamine towards the end of the ingredient list of products you use. These are added in very tiny amounts in order to adjust the pH of the product to the proper level.