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 [H_{3}O^{+}] [In aqueous solution, H^{+} ion remain as H_{3}O^{+} 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, [H
_{3}O^{+}] = [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 [H_{3}0^{+}]: 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.