Analytical · Free Calculator
pH and Hydrogen Ion Calculator
Convert between pH and hydrogen ion concentration, and find pOH and hydroxide concentration for aqueous solutions at 25 °C.
pH to H+H+ to pHpOH25 °C
pH3.500
H+ concentration0.0003162 mol/L
pOH10.500
OH− concentration3.162 × 10^−11 mol/L
NatureAcidic
How this result was calculated
- pH = −log₁₀[H⁺], [H⁺] = 10−pH
- pOH = 14 − pH (at 25 °C)
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pH Definitions
pH = −log₁₀[H⁺]
[H⁺] = 10−pH
pH + pOH = 14 (at 25 °C)
[H⁺] = 10−pH
pH + pOH = 14 (at 25 °C)
Worked example
A solution at pH 3.5 has [H⁺] = 10−3.5 = 3.16 × 10⁻⁴ mol/L and pOH 10.5.
Key insight: each pH unit is a factor of ten in acidity, so a change from pH 7 to pH 5 means a hundred times more hydrogen ions. Control loops on pH are highly nonlinear for this reason.
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Frequently Asked Questions
What does pH measure?
The activity of hydrogen ions in a solution on a logarithmic scale; low pH is acidic and high pH is alkaline.
Why is neutral pH 7?
At 25 °C pure water has equal H⁺ and OH⁻ concentrations of 10⁻⁷ mol/L. At other temperatures neutral pH shifts slightly.
Is concentration the same as activity?
Only in dilute solutions. In concentrated or high ionic strength solutions, activity coefficients make pH differ from −log of concentration.
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