How to use this calculator
- Select the known ion, acid, or base model.
- Enter the concentration.
- Enter the ion factor or dissociation constant when the mode requires it.
- Calculate pH, pOH, and both ion concentrations.
Formula and variables
pH = -log10[H+]; pOH = -log10[OH-]
| Symbol | Meaning | Unit |
|---|---|---|
| [H+] | hydrogen-ion activity approximation | mol/L |
| Kw | water ion product | (mol/L)² |
Worked example
A hydrogen-ion concentration of 0.010 mol/L gives pH 2.000 at the stated Kw reference.
Interpretation
pH and pOH are logarithmic. The complementary value depends on the disclosed temperature-specific water ion product.
Common mistakes
- Concentrations must be positive before taking a logarithm.
- Do not assume pH + pOH is always 14 without specifying temperature and Kw.
Frequently asked questions
Can the calculator handle weak acids and bases?
Yes. Enter Ka for a weak acid or Kb for a weak base.
Can pH be below 0 or above 14?
Yes. Concentrated solutions can give values outside that range under the concentration approximation.
Why does temperature matter for pH plus pOH?
The water ion product changes with temperature. This calculator states the Kw reference used.
Can I use scientific notation?
Yes. Enter values such as 6.022e23 or 1.0e-6 in a numeric field.
How should I round the result?
Keep extra digits during the calculation. Round the final result to the precision required by your data.