Enter pH, pOH, [H⁺] or [OH⁻] and see the other three instantly — at 25 °C.
pH is the negative logarithm of the hydrogen ion concentration: pH = −log₁₀[H⁺]. It runs from 0 (strongly acidic) through 7 (neutral water) to 14 (strongly basic). This calculator converts between pH, pOH, [H⁺] and [OH⁻] using the water autoionisation constant Kw = 10⁻¹⁴ at 25 °C.
The four quantities are linked by two equations: pH + pOH = 14, and [H⁺] × [OH⁻] = 10⁻¹⁴. Knowing any one of them determines the other three. The calculator does the logarithms and the division so you do not have to reach for a scientific calculator.
The value of Kw changes with temperature — at 37 °C (body temperature) it is about 2.4 × 10⁻¹⁴, which makes neutral pH about 6.8 instead of 7.0. This tool uses the standard 25 °C value that textbooks and most lab work assume.
pH = −log₁₀[H⁺]. pOH = −log₁₀[OH⁻]. pH + pOH = 14 (at 25 °C). [H⁺] × [OH⁻] = Kw = 10⁻¹⁴.
Because the water autoionisation constant Kw = 10⁻¹⁴ holds at 25 °C. At other temperatures Kw changes, and with it the neutral pH. At 37 °C, neutral pH is about 6.8.
Yes — strong acids above 1 mol/L have a negative pH, and strong bases above 1 mol/L have pH above 14. The 0–14 scale covers the range of dilute aqueous solutions, not all chemistry.
pH measures acidity (hydrogen ions), pOH measures basicity (hydroxide ions). They always add up to 14 at 25 °C.
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