The pH scale
The pH of a solution measures how acidic it is. It is based on the molar concentration of hydrogen ions, written [H+].
pH = -log [H+]
| pH | Solution |
|---|---|
| Below 7 | Acidic |
| 7 | Neutral |
| Above 7 | Basic |
The scale is logarithmic. Each step of 1 pH unit is a factor of 10 in [H+]. A solution at pH 3 has ten times the hydrogen ion concentration of one at pH 4.
Water itself ionizes very slightly, which links the two ions. At 25 °C:
[H+] × [OH−] = 1.0 × 10−14
In pure water both are 1.0 × 10−7 M, so the pH is 7. A matching scale for hydroxide gives pH + pOH = 14.
Worked example: find the pH
A solution has [H+] = 1.0 × 10−3 M.
pH = -log(1.0 × 10−3) = 3.00
When [H+] is exactly a power of ten, the pH is that exponent with the sign changed.
Worked example: find the hydrogen ion concentration
A solution has pH 9. Reverse the logarithm:
[H+] = 10−9 M
The pH is above 7, so the solution is basic.
Common mistake
Thinking a higher pH means more acidic. It is the reverse: the lower the pH, the higher [H+].