The nature of the solution (acidic, alkaline (or) neutral) can be represented in terms of either hydrogen ion concentration (or) hydroxyl ion concentration. This naturally explains the pH concept.
In 1909, Sorenson used a logarithmic scale for expressing the H+ concentration. This scale was called “pH”, Where “P” stands for ‘Power’ and ‘H’ for hydrogen ion concentration.
Sorenson defines pH (pH concept) of a solution as
PH = log ————- = – log [H+]
The symbol “P” denotes “negative logarithm of ”.
For a precisely neutral solution at 250C in which the concentration of hydrogen ions is 1.0 X 10-7 M, the pH can be calculated as follows:
PH =log ————— = log (1.0 X 107)
1.0 X 10-7
=log 1.0 + log 107
= 0 + 7.0
The value of 7.0 for the pH of a precisely neutral solution is not an arbitrarily chosen figure. It is derived from the absolute value of the ion product of water at 250 C, which by convenient coincidence is a round number.
Importance of pH
- Soils of specific pH are required for optimum group growth and better yields of crops.
- Specific pH values are to be maintained for the biological process and industrial process to occur.
- Specific pH is also to be maintained by the blood.
- PH plays an important role in chemical analysis.
Development reason of pH
- The pH scale was developed taking water as the standard.
- It is an experimental fact that only 1 mole in 5,50,000,000 moles of water ionizes into a H+ and OH–.
- This is the same proportion as one-gram hydrogen ion in 10,000,000 litres of water.
- Hence, one litre of water contains 1/10,000,000 (or 1/107 of a gram of H+.
- For everyday use, only the ‘Power’ figure was used and the symbol pH placed before it.