Shampoo and hair should be in the pH range of 4.5-5.5
I posted this comment earlier and got the info from trichologist Lisa Akbari's book "The Black Woman's Guide to Beautiful Hair". What I am posting now is directly from her book:
p.21-22
"
The Right Shampoo
Always use the proper shampoo, one that is the pH of your hair and scalp. Many may think that all the shampoos are the same, but that is simply not true. Some shampoos can cause a negative buildup or cause your scalp to dry out. And many shampoos on the market today have a very high pH.
Your scalp and hair have a pH of 4.5 to 5.5. In order to have a healthy scalp and encourage hair growth, stay as close to the natural pH as possible. Never go past pH 6.
Never use all-in-one shampoos. Some shampoos claim to clean and condition hair at the same time. These shampoos never get the scalp clean and cause a flaky buildup on the hair and scalp, which at some point could lead to low- and high-level scalp damage."