pH Definition
An introduction to pH
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pH can be viewed as an abbreviation for power of Hydrogen - or more completely, power of the concentration of the Hydrogen ion.
The mathematical definition of pH is a bit less intuitive but in general more useful. It says that the pH is equal to to the negative logarithmic value of the Hydrogen ion (H+) concentration, or
pH = -log [H+]
pH can alternatively be defined mathematically as the negative logarithmic value of the Hydroxonium ion (H3O+) concentration. Using the Bronsted-Lowry approach
pH = -log [H3O+]
pH values are calculated in powers of 10. The hydrogen ion concentration of a solution with pH 1.0 is 10 times larger than the hydrogen concentration in a solution with pH 2.0. The larger the hydrogen ion concentration, the smaller the pH.
- when the pH is above 7 the solution is basic (alkaline)
- when the pH is below 7 the solution is acidic
In pure neutral water the concentration of hydrogen and hydroxide ions are both 10-7 equivalents per liter.
| pH | Ion Concentration (gram equivalent per liter) | Type of Solution |
| 0 | 1.0 | Acid Solution - Hydrogen ions - H+ |
| 1 | 0.1 | |
| 2 | 0.01 | |
| 3 | 0.001 | |
| 4 | 0.0001 | |
| 5 | 0.00001 | |
| 6 | 0.000001 | |
| 7 | 0.0000001 | Neutral Solution |
| 8 | 0.000001 | Basic (alkaline) Solution - Hydroxide ions - OH- |
| 9 | 0.00001 | |
| 10 | 0.0001 | |
| 11 | 0.001 | |
| 12 | 0.01 | |
| 13 | 0.1 | |
| 14 | 1.0 |
Some common Products and their pH Values
pH values in some common products:
| Product | pH |
| Battery Acid | 0 |
| HCl in stomach acid | 1 |
| Lemon juice, vinegar | 2-3 |
| Orange juice | 3-4 |
| Acid rain | 4 |
| Black coffee | 5 |
| Urine, salvia | 6 |
| Pure water | 7 |
| Sea water | 8 |
| Baking soda | 9 |
| Ammonia solution | 10-11 |
| Soapy water | 12 |
| Bleach | 13 |
| Oven cleaner | 13-14 |
| Drain cleaner | 14 |
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