pOH Calculator

pOH is the negative base-10 logarithm of the hydroxide ion concentration. It mirrors pH on the other end of the acid-base scale.

pOH = -log₁₀[OH⁻]

At 25°C, water autoionizes so that the product of hydrogen and hydroxide ion concentrations is always 1.0 × 10⁻¹⁴:

Kw = [H⁺][OH⁻] = 1.0 × 10⁻¹⁴

Taking logarithms of both sides:

pH + pOH = 14

This relationship lets you convert instantly between pH and pOH. A solution with pH = 11 has pOH = 3. A solution with pOH = 5 has pH = 9.

The pOH scale:

• pOH 0-3: Strongly basic (concentrated NaOH, KOH)
• pOH 4-6: Moderately basic (baking soda solution ~pOH 5.7)
• pOH 7: Neutral at 25°C
• pOH 8-10: Mildly acidic
• pOH 11-14: Strongly acidic

The Kw relationship pH + pOH = 14 is only exact at 25°C. At higher temperatures, Kw increases — water ionizes more readily. At 37°C (body temperature), Kw = 2.4 × 10⁻¹⁴, so neutral pH is about 6.81, not 7.00. Blood sits about 0.6 pH units above neutral even at body temperature, so it really is slightly alkaline. The 37 °C correction narrows the gap against the room-temperature scale, it does not erase it.

This calculator works in three modes: from [OH⁻] concentration, from a given pOH, or from a known pH.

Why bother with pOH at all

Anything pOH tells you, pH could tell you, so the scale exists mostly for convenience. When a problem starts from a base, you get [OH⁻] first and pOH follows in one step, where reaching pH means going through the water constant. Titration and buffer work with bases reads more naturally in pOH for the same reason.

The relationship pH + pOH = 14 makes converting trivial, with one caveat worth remembering: that 14 is pKw at 25 °C and nothing else. Kw rises with temperature, so at 37 °C the sum is 13.62 and at 100 °C it is 12.

The neutral point moves with temperature

This is the part that surprises people. Neutral does not mean pH 7, it means [H⁺] equals [OH⁻], which lands on 7.00 only at 25 °C. At body temperature neutral is 6.81, and in boiling water it is 6.14. Hot pure water is not acidic despite reading below 7 on a room-temperature scale; it has simply moved its own zero.

Handheld pH meters correct for this if they have a temperature probe attached and are set to use it. Meters used without one, or with the compensation switched off, quietly report the room-temperature figure for a sample that is not at room temperature.