Quick chemistry lesson. Hair has a natural pH of 5.5.
Regular store shampoos range anywhere from a 4 to a 9 on the pH scale.
Why does that matter? I'll tell you.
Neutral solution: 7
Alkaline solution: 7.1-14
Acidic solution: 0-6.9
Shampoos that are more alkaline are more harsh and can make hair porous, brittle, and dried-out. A shampoo with a high pH causes color-treated hair to fade.