pOH from Hydroxide Concentration

pOH from hydroxide concentration

pOH tells you how basic an aqueous solution is, working from the hydroxide-ion concentration: pOH = -log₁₀[OH⁻]. Think of it as pH seen from the basic side. At 25 °C the ion product of water ties the two together, so pH + pOH = 14. Example: a 0.01 mol/L NaOH solution dissociates completely, which puts [OH⁻] at 0.01, so pOH = -log(0.01) = 2 and pH = 14 − 2 = 12 (strongly basic). That 14 only applies at 25 °C. Change the temperature and Kw changes with it, nudging the sum pH + pOH along. At 50 °C, for instance, Kw is about 5.5×10⁻¹⁴, so pH + pOH lands near 13.26.

Applications

It shows up in acid-base titration whenever you track the basic side of the curve. Analysts reach for it in analytical chemistry to characterize alkaline solutions like NaOH, KOH or ammonia. Industrial water treatment uses it to keep boiler and cooling-tower alkalinity in check. And in high-school chemistry it's the natural stepping stone from [OH⁻] to pOH to pH.

FAQ

Why use pOH instead of pH directly? If you already know the hydroxide concentration, as with strong bases or alkaline buffers, pOH comes straight off [OH⁻]. You skip the trip through Kw and the rounding it adds.

Does pH + pOH = 14 always hold? Only at 25 °C. Kw depends on temperature, so the sum drifts once you move off that mark. What never breaks is the underlying pH + pOH = pKw.

What is pOH for pure water? At 25 °C the hydroxide concentration is 10⁻⁷, which gives pOH = 7. That's neutral, and it lines up with pH = 7.