pOH

pOH is a measure used in aqueous chemistry to express the concentration of hydroxide ions in a solution. It is defined as the negative base-10 logarithm of the hydroxide ion concentration: pOH = -log10[OH−]. Like pH, it is a logarithmic scale.

In water, pH and pOH are linked by pH + pOH = pKw, where pKw is the negative logarithm of the ionic product of water. At 25°C, Kw ≈ 1.0 × 10−14, so pKw ≈ 14 and pH + pOH ≈ 14. The exact sum depends on temperature, because Kw changes with temperature.

Calculation and interpretation are straightforward: [OH−] = 10^(−pOH). For example, if [OH−] = 1.0 × 10−7 M, then pOH = 7; pure water at 25°C has pH ≈ 7 and pOH ≈ 7. If a solution contains [OH−] = 0.01 M, then pOH = 2, and pH would be approximately 12 under standard conditions.

pOH is closely related to pH and is often used to characterize basic (alkaline) solutions. A lower pOH indicates a higher hydroxide concentration and, together with pH, determines the acidity or basicity of a solution. For practical purposes, knowing either pH or pOH allows the calculation of the other via pH = pKw − pOH (at the relevant temperature).

Limitations include the influence of temperature on Kw and, at very low ionic strengths, activity coefficients rather than concentrations govern the actual pH and pOH values.