why is the ph level of each shampoo is same or very close to each other?

Answer 1

The surface of a strand of hair is covered with overlapping sheets, somewhat like the scales on a fish, or the shingles on a house. This surface is called the cuticle. Alkaline solutions raise these scales, so they stand up. This makes the hair rougher, makes it look dull, and makes the hair shafts stick together due to the rough texture. Most shampoos are made slightly acidic, to keep the cuticle smooth and lying flat on the hair shaft. Ingredients like citric acid are added to acidify the shampoo. As the shampoo mixes with the water in the shower or bath, or mixes with dirt on the hair, it can become less acidic as the acids mix with alkaline water or dirt. A compound that releases more acidifying ions when the acidity gets low, or absorbs acid when the acidity gets too high, is called a buffer. A typical buffering agent used in shampoo is sodium citrate. Since the goal is to keep the shampoo slightly acid, the term "pH balanced" is actually a misnomer. We want the balance to be tipped slightly to the acidic side.

Answer 2

I suppose because a. most shampoos are made of the same basic substances, b. they all try to make the PH similar to that of the skin and hair