Q.10: What is meant by pH? Explain.
The acidity or basicity of a solution is expressed in terms of pH.
Introduction:
Soren Peter Lauritz Sorenson in 1909 suggested a simple number scale based on H⁺ ion
concentration to express the acidic or basic nature of the solution, which is called the pH
scale. The values on this scale range from 0 to 14.
Definition:
The logarithm of the reciprocal of the molar concentration of H⁺ ions is called pH.
Mathematically:
pH = log(1 / [H⁺])
⇒ pH = log1 - log[H⁺] (log m/n = log m - log n)
⇒ pH = 0 - log[H⁺] (log 1 = 0)
⇒ pH = -log[H⁺]
Therefore, pH can also be defined as: The negative logarithm of the molar concentration of
H⁺ ions is called pH.
Meaning of p in pH:
p stands for potenz (potential to be). Thus, pH means potential of H⁺ ions, while it denotes
the negative logarithm, i.e., p = -log.
Comparison of pH values:
A solution having a pH value of 7 is neutral, less than 7 is acidic, while a solution having a
pH value more than 7 is basic.
Each whole number value of pH below 7 is ten times more acidic than the next higher value.
For example:
A pH of 2 is ten times more acidic than a pH of 3 and 100 times more acidic than a pH value
of 4.
Experimental determination of pH:
Experimentally, pH is determined by two methods:
i) By pH paper:
A rough value of pH is determined by using pH paper.
ii) By pH meter:
A pH meter is an instrument which determines the accurate value of pH.
