pOH Definition
pOH = −log[OH−] is the mirror-image scale of pH, built from hydroxide-ion concentration instead of hydrogen-ion.
What it means
pOH does for hydroxide ions what pH does for hydrogen ions: it turns tiny [OH−] values into easy numbers through the negative base-10 logarithm. Each pOH unit is a tenfold change in [OH−]. At 25 °C a low pOH (below 7) means a basic solution rich in hydroxide, and a high pOH (above 7) means an acidic one poor in hydroxide.
Variables
| Symbol | Name | Unit | Meaning |
|---|---|---|---|
| Hydroxide ion concentration | mol/L (M) | The molar concentration of hydroxide ions in the solution. | |
| Potential of hydroxide | unitless | The negative base-10 logarithm of the hydroxide ion concentration; a pure number with no units. |
Assumptions
- The solution is dilute enough that concentration approximates activity.
- The value applies to aqueous solutions at the temperature of measurement.
- pOH is unitless, exactly like pH.
- At 25 °C, pOH links to pH through pH + pOH = 14.00.
Worked example
A solution has [OH⁻] = 4.00 × 10⁻⁵ M. Calculate its pOH.
Given:
- [OH⁻] = 4.00 × 10⁻⁵ M
Write pOH = −log[OH⁻] = −log(4.00 × 10⁻⁵).
Split the log: log(4.00 × 10⁻⁵) = log(4.00) + log(10⁻⁵).
log(4.00) = 0.602 and log(10⁻⁵) = −5, so the sum is −4.398.
Apply the negative sign: pOH = −(−4.398) = 4.40.
A pOH of 4.40 is below 7 at 25 °C, so hydroxide dominates — the solution is basic, with pH = 14.00 − 4.40 = 9.60.