pOH is the measure of the hydroxide ion concentration in an aqueous solution, defined as the negative base-10 logarithm of the molar hydroxide ion concentration: pOH equals negative log10 of [OH minus]. Just as pH quantifies acidity through the hydrogen ion concentration, pOH quantifies basicity through the hydroxide ion concentration. The two are linked at 25 degrees Celsius by the water autoionisation constant Kw, which equals 1 times 10 to the power of negative 14: because pH plus pOH equals 14, knowing one gives you the other immediately. A pOH of 7 indicates a neutral solution, a value below 7 indicates a basic solution (high hydroxide concentration), and a value above 7 indicates an acidic solution. This calculator is fully bidirectional. You can start from a known pH value to get pOH, [OH minus] and [H plus]. You can start from a known hydroxide ion concentration to get pOH, pH and [H plus]. Or you can enter a pOH directly and get pH and both ion concentrations. All four values are returned simultaneously. The relationship pOH = 14 minus pH holds strictly at 25 degrees Celsius; at other temperatures, pKw differs and the midpoint shifts away from 7. This tool assumes 25 degrees Celsius throughout and is intended for educational use in chemistry at secondary and university level.
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11.00
pOH
pH3.00
[OH-]1.00e-11 mol/L
[H+]1.00e-3 mol/L
Solution typeAcidic
Relationship pOH = 14 minus pH holds at 25 degrees C (Kw = 1 x 10⁻¹⁴). At other temperatures pKw differs and the neutral point shifts.
How it works
The key relationships are pOH = -log₁₀([OH-]), pH = 14 - pOH (at 25 degC), [OH-] = 10^(-pOH) and [H+] = 10^(-pH). From pH: pOH = 14 minus pH, [OH-] = 10^(pOH) negative. From [OH-]: pOH = -log10([OH-]), pH = 14 minus pOH. From pOH: pH = 14 minus pOH directly. Solution type: pH below 7 is acidic; pH = 7 is neutral; pH above 7 is basic.
Worked example
A solution has pH = 3. pOH = 14 minus 3 = 11.00. The hydroxide ion concentration [OH-] = 10 to the power of negative 11 = 1.00 x 10 to the power of negative 11 mol/L. The hydrogen ion concentration [H+] = 10 to the power of negative 3 = 1.00 x 10 to the power of negative 3 mol/L. The solution is acidic (pH below 7). These match the default values pre-filled above.
Related calculators
pH Calculator: calculate pH from hydrogen ion concentration or acid/base data.
Molality Calculator: find solution concentration in moles per kilogram of solvent.
