The formula
pH = −log₁₀[H⁺] · pH + pOH = 14
| Symbol | Quantity | Unit |
|---|---|---|
| pH | pH | |
| pOH | pOH | |
| H | Concentration of H⁺ | mol/l |
| OH | Concentration of OH⁻ | mol/l |
What it means
pH is the negative base-ten logarithm of the hydrogen ion concentration. Being logarithmic has a consequence people constantly forget: each unit of pH is a factor of ten. A pH of 4 is not "slightly more acidic" than 5, it is ten times more acidic, and 3 is a hundred times. That is why acid rain at pH 4.5 against normal rain at 5.6 is a large change, even though the numbers look close.
How to work it out by hand
- Take the H⁺ concentration in moles per litre
- Work out its base-ten logarithm
- Change the sign: that is the pH
- Subtract from fourteen for the pOH
What is worth knowing
The fourteen in pH + pOH = 14 is not a round number picked for convenience: it comes from the ionic product of water, which is 10⁻¹⁴ at 25 °C. And there lies the nuance almost no table mentions: **that value depends on temperature**. At 50 °C the ionic product rises and neutral water has a pH of 6.63, not 7. The water has not turned acidic, since it still holds as many H⁺ as OH⁻; what has moved is where neutral sits. That is why a serious pH meter compensates for temperature.