Define pH or pH scale.

Sorensen in 1909 discovered a new tern for expressing the concentration of hydrogen ions known as pH or pH scale. The pH of a solution is defined as:
The negative logarithm of the H₃O⁺ ion concentration in moles per litre.
Mathematically, it may be expressed as:
pH = – log [H₃O⁺]
For neutral solution at 298 K,
[H₃O⁺] = [OH^–] = 1 x 10^-7 mol L^–
So that,
pH = – log [H₃O⁺] = – log (1.0 x 10^-7) = 7
similarly, by substituting different values of [H₃O⁺] in the above relation, it can be seen that
For acidic solution pH < 7 For basic solution pH > 7
For neutral solution pH = 7