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pH and pOH Calculator
Convert between pH, pOH, and hydrogen and hydroxide ion concentrations at 25°C.
pH
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How it works
pOH = 14 − pH; [H⁺] = 10^(−pH); [OH⁻] = 10^(−pOH)
At 25°C, pH and pOH always add to 14. pH is the negative log of the hydrogen-ion concentration, so each pH unit is a tenfold change in acidity. Below 7 is acidic, above 7 is basic.
Worked example
pH 3.5 gives pOH 10.5 and [H⁺] = 3.16 × 10⁻⁴ M — acidic.
Frequently asked questions
Why does pH + pOH = 14?
Because water’s ion product Kw is 10⁻¹⁴ at 25°C. At other temperatures the sum shifts slightly.
What does each pH step mean?
A tenfold change in hydrogen-ion concentration — pH 4 is ten times more acidic than pH 5.
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Assumes 25°C. For educational use.