Hydrogen Ion Concentration – pH scale
Sorensen, a Danish biochemist developed a scale to measure the acidity in terms of concentrations of 
in a solution. As defined by him, “pH of a solution is the negative logarithm to the base 10 of the concentration of H+ ions which it contains.”
or 
Just as pH indicates the hydrogen ion concentration, the pOH represents the hydroxyl ion concentration, i.e.,
Considering the relationship, 
Taking log on both sides, we have
or
or 
| [H+] | [OH–] | pH | pOH | |
| Acidic solution | > 10–7 | < 10–7 | < 7 | > 7 |
| Neutral solution | 10–7 | 10–7 | 7 | 7 |
| Basic solution | < 10–7 | > 10–7 | > 7 | < 7 |
pH of some materials
| Material | pH | Material | pH |
| Gastric juice | 1.4 | Rain water | 6.5 |
| Lemon juice | 2.1 | Pure water | 7.0 |
| Vinegar | 2.9 | Human saliva | 7.0 |
| Soft drinks | 3.0 | Blood plasma | 7.4 |
| Beer | 4.5 | Tears | 7.4 |
| Black coffee | 5.0 | Egg | 7.8 |
| Cow’s milk | 6.5 | Household ammonia | 11.9 |