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Dilute-Water pH, pOH & Hydrogen-Ion Estimate

Convert pH, pOH and approximate hydrogen-ion concentration in dilute water using pKw = 14 near 25°C.

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Calculator inputs

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How to use this calculator

  1. Enter the known values in the units shown. Results update as you type.
  2. Where results are editable, change one to solve backwards. Lock a value to hold it fixed.
  3. Use the worked example to check the method. Reset restores the starting fields.

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Formula and method

At pH 7, the concentration approximation is 10⁻⁷ mol/L, or 0.1 μM. With pKw = 14, the corresponding pOH is 7.

Approximate [H⁺] in mol/L = 10^(−pH); pOH = 14 − pH

Worked example

Enter these known values and leave the other values blank.

pH within this model’s 0–14 range
7
pOH using pKw = 14
7
Approximate hydrogen-ion concentration
1e-7 M

Assumptions and limitations

  • This model fixes pKw at 14, an approximation for water near 25°C. Other temperatures and solvents require an appropriate equilibrium constant.
  • Thermodynamic pH is based on hydrogen-ion activity. Replacing activity with numerical molar concentration assumes a suitable dilute solution and unit activity coefficient.
  • The retained input range is pH and pOH from 0 to 14. Real solutions can have pH outside this interval; this tool does not model them.

Common questions

Is neutral pH always seven?

No. Neutrality means equal hydrogen-ion and hydroxide-ion activities. The corresponding pH varies with temperature because the water equilibrium constant changes.

Why is this a concentration estimate?

The defining pH quantity is activity. Concentration is a useful approximation only when the activity correction is suitable for the solution.

References

The calculation equations, inverse formulas, units, and input rules were imported from this source. Bookify provides the interface and equation solver.

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