pH calculator: pH, pOH, [H⁺] and [OH⁻]
Enter any one of pH, pOH, the hydrogen ion concentration [H⁺] or the hydroxide concentration [OH⁻] to get the other three, and see whether the solution is acidic, neutral or basic.
pH = −log[H⁺]
At 25 °C, pH + pOH = 14 and [H⁺] × [OH⁻] = 1.0 × 10⁻¹⁴. Type 1e-3 or 0.001 for 1 × 10⁻³.
- pH
- 3
- pOH
- 11
- [H⁺]
- 0.001 mol/L
- [OH⁻]
- 1 × 10⁻¹¹ mol/L
Acidic · more hydrogen ions than hydroxide ions (pH below 7).
How to use the calculator
Choose what you know from the list, type its value and the other three appear. Concentrations are in mol/L, and very small numbers can be typed as 1e-5 for 1 × 10⁻⁵.
The formulas
pH = −log₁₀[H⁺] and pOH = −log₁₀[OH⁻]. In water at 25 °C the two always add up to 14, because [H⁺] × [OH⁻] = Kw = 1.0 × 10⁻¹⁴. Going the other way, [H⁺] = 10 to the power −pH.
Each pH unit is a factor of ten: a solution at pH 3 has ten times the hydrogen ion concentration of one at pH 4.
Worked example: 0.001 M hydrochloric acid
Hydrochloric acid is a strong acid, so 0.001 mol/L HCl gives [H⁺] = 1 × 10⁻³ mol/L. Then pH = −log(10⁻³) = 3, pOH = 14 − 3 = 11, and [OH⁻] = 1 × 10⁻¹¹ mol/L.
For a weak acid such as acetic acid, [H⁺] is much smaller than the acid concentration, because only a little of the acid splits into ions.
FAQ
What pH is neutral?
Pure water at 25 °C has pH 7, with equal concentrations of H⁺ and OH⁻ ions, 1 × 10⁻⁷ mol/L each. Below 7 is acidic and above 7 is basic.
Can pH be negative?
Yes. Very concentrated strong acids, with [H⁺] above 1 mol/L, have a pH below 0, and very concentrated bases can go above 14.
Does temperature change pH?
Yes. Kw changes with temperature, so neutral water is exactly pH 7 only at 25 °C. This calculator uses the 25 °C value.