pOH Calculator
Calculate pOH, pH, and hydroxide ion concentration for any solution.
Enter an OH concentration or pH value to find all related quantities at 25 degrees C.
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 (household ammonia ~pOH 5.5, baking soda ~pOH 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. The blood is “neutral” at physiological conditions even though its pH (7.35-7.45) appears basic by the room-temperature scale.
This calculator works in three modes: from [OH⁻] concentration, from a given pOH, or from a known pH.
How we build and check this calculator
This calculator runs entirely in your browser, so the numbers you enter stay on your device. The math behind it is written by hand and tested against worked examples and standard references before the page goes live.
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