pH and pOH calculator
pH from hydrogen ion concentration and back, with pOH and the medium.
Fill in the fields and the result will appear here automatically.
An educational calculation from H⁺ concentration in mol/L or from a specified pH. Strictly, pH is defined by hydrogen-ion activity; here activity is approximated by concentration relative to 1 mol/L, commonly used for sufficiently dilute solutions. pOH uses the assumed pKw = 14 at 25 °C. Temperature and activity coefficients are not inputs.
How it works
Formula and logic
pH = −log₁₀ a(H⁺). The model uses a(H⁺) ≈ [H⁺]/c°, with c° = 1 mol/L; inversely [H⁺] ≈ c° × 10⁻pH. pOH = 14 − pH. The model’s neutral point is pH 7. The product accepts pH from 0 to 14 and corresponding positive concentrations; these are calculator limits, not universal limits of the pH scale.
Example
For [H⁺] = 10⁻³ mol/L, the model gives pH 3.00 and pOH 11.00. At pH 8.4 it gives approximately 3.981 × 10⁻⁹ mol/L and pOH 5.60.
Fields and units
- What is known — list option
- Concentration of H⁺, mol/L — mol/L
- pH — unitless
How to use
- — Choose H⁺ concentration or pH.
- — Enter a finite positive concentration in mol/L or pH from 0 to 14.
- — Interpret the result within the approximation at 25 °C.
Method and limitations
- Calculation method
- Formula and logic
- Data or methodology source
- IUPAC: pH is defined through H⁺ activity, not uncorrected concentration
- Limitation
- Activity is approximated by concentration; pKw = 14 at 25 °C. The product range 0–14 is not a physical pH limit. Ionic strength, temperature and measurement corrections are not modelled.
FAQ
Does pH + pOH always equal 14?
No. The sum is pKw, which depends on temperature and medium. This model fixes the educational assumption pKw = 14 at 25 °C; other temperatures are not calculated.
Is concentration the exact definition of pH?
No. The definition uses dimensionless activity. The concentration approximation omits activity coefficients and can be inaccurate in concentrated solutions.
Can pH be outside 0–14?
Yes, the scale remains meaningful beyond those bounds. This calculator deliberately accepts only 0–14 and does not model such solutions.
Why is zero concentration rejected?
The logarithm of zero has no finite real value. Blank or malformed input also gives an error instead of being replaced with zero.