Enter your known value — Hydroxide Ion Concentration [OH⁻], pH Value, or Hydrogen Ion Concentration [H⁺] — and the pOH Calculator works out your pOH Value along with the full picture: pH, OH⁻ Concentration, H⁺ Concentration, and Solution Type (acidic, neutral, or basic).
pOH Value
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pH Value
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OH⁻ Concentration
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H⁺ Concentration
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Solution Type
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pOH is the negative logarithm of hydroxide ion concentration [OH⁻]. It measures the basicity of a solution. pH and pOH are related by the equation: pH + pOH = 14 (at 25°C). When pH is low (acidic), pOH is high, and vice versa.
Use the formula: pOH = -log([OH⁻]), where [OH⁻] is the hydroxide ion concentration in mol/L. For example, if [OH⁻] = 0.001 mol/L, then pOH = -log(0.001) = 3.
The pOH scale ranges from 0 to 14, similar to the pH scale. A pOH of 7 is neutral, values less than 7 indicate basic solutions, and values greater than 7 indicate acidic solutions. Note that this is opposite to the pH scale.
Simply use the relationship: pOH = 14 - pH (at 25°C). For example, if pH = 9, then pOH = 14 - 9 = 5. This relationship is based on the water dissociation constant Kw = 1.0 × 10⁻¹⁴.
When pOH < 7, the solution is basic (alkaline). This means there are more hydroxide ions (OH⁻) than hydrogen ions (H⁺) in the solution. The lower the pOH value, the more basic the solution.
In very concentrated solutions, pOH can theoretically be negative (very basic) or greater than 14 (very acidic), but these extreme values are rare in typical aqueous solutions and indicate highly concentrated acids or bases.
This relationship comes from the water dissociation constant (Kw = [H⁺][OH⁻] = 1.0 × 10⁻¹⁴ at 25°C). It allows you to easily convert between pH and pOH values and is fundamental to understanding acid-base chemistry.
To go from [OH⁻] to pOH: pOH = -log([OH⁻]). To go from pOH to [OH⁻]: [OH⁻] = 10^(-pOH). Similarly, you can use [H⁺] = 10^(-pH) and the relationship [H⁺][OH⁻] = 1.0 × 10⁻¹⁴ to find any missing value.