Editing PH (section)

=== pOH ===
[[File:PHscalenolang.svg|thumb|Relation between pH and pOH. Red represents the acidic region. Blue represents the basic region.]]
pOH is sometimes used as a measure of the concentration of hydroxide ions, {{chem2|OH−}}. By definition, pOH is the negative logarithm (to the base 10) of the hydroxide ion concentration (mol/L). pOH values can be derived from pH measurements and vice-versa. The concentration of hydroxide ions in water is related to the concentration of hydrogen cations by
: <math chem="">[\ce{OH^-}] = \frac{K_\ce{W}}{[\ce{H^+}]}</math>
where ''K''<sub>W</sub> is the [[Self-ionization of water|self-ionization]] constant of water. Taking [[logarithm]]s,
: <math chem="">\ce{pOH} = \ce{p}K_\ce{W} - \ce{pH}.</math>

So, at room temperature, pOH ≈ 14 − pH. However this relationship is not strictly valid in other circumstances, such as in measurements of [[Alkaline soils|soil alkalinity]].