Hydrogen Ion Concentration Calculator
Find approximate hydrogen-ion concentration from pH or pOH, with an adjustable water ion product.
01 Set up your calculation
- Choose whether you know pH or pOH.
- Enter the value and, if needed, a pKw appropriate for the temperature.
- Read the concentration approximation and complementary acidity scale.
02 The relationship
For the assumptions behind these results, read the explanation below. You can also explore pH and pKa.
Follow the numbers
At pH 3 and pKw 14, [H⁺] is 0.001 mol/L, pOH is 11, and [OH⁻] is 1 × 10⁻¹¹ mol/L.
03 One pH unit changes concentration tenfold
Because pH is logarithmic, lowering pH from 4 to 3 raises the hydrogen-ion concentration estimate from 0.0001 to 0.001 mol/L. It is not an additive change of one concentration unit.
In water, H⁺ is conventional shorthand for a solvated proton; hydronium is often written H3O⁺. The calculation uses that conventional concentration representation.
04 Account for temperature and activity
The identity pH + pOH = pKw depends on the water ion product at the conditions of interest. Its familiar value of 14 is not a temperature-independent law. Neutrality occurs at equal hydrogen and hydroxide activities, corresponding to pH = pKw/2.
Strictly, pH is defined using activity. Treating 10⁻ᵖᴴ as molarity assumes that the activity coefficient can be approximated as one. Concentrated solutions need a model for nonideal behavior.
Questions from the notebook
Can pH be below zero or above fourteen?
Yes. Those values can occur; 0 to 14 is a common teaching range, not an absolute bound.
Does this determine pH from an acid bottle’s concentration?
No. That requires dissociation and equilibrium information for the acid, not just the inverse logarithm used here.
References & reading
- OpenStax, Rice University — Chemistry 2e: pH and pOH ↗
Relates pH and pOH to ion concentrations and to the temperature-dependent water ion product.
Our explanations are written for ChemistryCalc. Read how we build and check our tools. These calculations are educational estimates where the underlying model requires assumptions.