What is pH?

The symbol pH is derived from the Danish word ‘potenz’ meaning power. Thus it’s means power of hydrogen ions.

In 1909, Sorensen suggested a new term of expressing the concentration of hydrogen ions known as pH. It is defined as the negative logarithm of H⁺ molar concentration in a solution.

Mathematically, it is expressed as,

pH = -log [H⁺]

or, pH = -log [H₃O⁺] [In aqueous solution, H⁺ ion remain as H₃O⁺ ion] Thus we have a scale in which the degree of acidity is expressed in power of [H⁺] values.

Advantages:

Avoiding large fractional number: For neutral solution, at 298K, [H₃O⁺] = [OH^–] = 10^-7 mol L^-1 So, we get, pH = -log 10^-7 = 7. This is how, we can avoid a large number by replacing an easy figure to indicate the acidity of the solution.
Determining [H⁺] or [H₃0⁺]: If the power of [H⁺] value is known, then we can also determine the [H⁺]. For example, the pH of Johnson’s soap is 5.5.

We know, pH = -log [H⁺]

or, [H⁺] = log^-1 (-5.5)

= 3.162 x 10^-6 mol L^-1

Avoiding negative value: The power of [H⁺] in a solution appears as negative e.g. 10^-6, 10^-5, 10^-4, etc. In order to remove such negative power which are difficult to remember, the pH is used.