pOH is the negative base-10 logarithm of the hydroxide ion concentration: pOH = -log10([OH-]). At 25 degrees C, pOH and pH sum to 14 (the pKw of water). A pOH below 7 indicates a basic (alkaline) solution; above 7 indicates an acidic solution.

How do you convert pH to pOH?

At 25 degrees C, pOH = 14 minus pH. If a solution has a pH of 3, its pOH is 11. If a solution has a pH of 11, its pOH is 3. The relationship comes from the water autoionisation constant Kw = [H+][OH-] = 1 x 10 to the power of negative 14 at 25 degrees C.

What is the hydroxide ion concentration at pOH 11?

If pOH = 11, then [OH-] = 10 to the power of negative 11 mol per litre = 1 x 10 to the power of negative 11 mol/L. This corresponds to a pH of 3, which is an acidic solution.

pOH Calculator

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.

Input mode

pH value

[OH-] concentration (mol/L)

pOH value

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.
Molar Mass Calculator: find the molar mass of acids, bases and other compounds.
Percent Error Calculator: compare a measured pH or pOH with the expected value.

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