Hydrogen Ion Concentration Calculator
Switch between pH, [H+], and [OH-] to describe acid and base solutions quickly.
Valid for solutions near 25 C.
Assumes Kw = 1.0e-14 (25 C). Adjust for different temperatures or ionic strengths as needed.
pH
7.000
Neutral
pOH
7.000
pH + pOH = 14 at 25 C.
[H+] (mol/L)
1.00e-7
Hydronium concentration.
[OH-] (mol/L)
1.00e-7
Hydroxide concentration.
With pH 7.00 the hydrogen ion concentration is 1.000e-7 mol/L and the hydroxide concentration is 1.000e-7 mol/L. This solution is classified as neutral for the stated assumptions.
How to Use This Calculator
Choose your starting measurement
Select whether you know the pH, the hydrogen ion concentration, or the hydroxide ion concentration. The form adapts automatically.
Enter the known value
Type the measured quantity. Scientific notation is supported (for example 3.2e-5).
Review the results
The calculator reports pH, pOH, [H+], and [OH-] along with a qualitative classification.
Apply the outputs
Copy the values into titration problems, buffer design worksheets, or lab reports.
Formula
pH = -log10 [H+]
pOH = -log10 [OH-]
[H+] * [OH-] = 1.0 x 10^-14 (at 25 C)
Because Kw = 1.0e-14 at 25 C, pH + pOH = 14. Adjust Kw for other temperatures to maintain accuracy.
Example
A solution with pH 3.40 has [H+] = 10^-3.40 = 4.0 x 10^-4 mol/L. The corresponding [OH-] is 1.0 x 10^-14 / 4.0 x 10^-4 = 2.5 x 10^-11 mol/L, giving pOH 10.60.
Full Description
Acid–base calculations often start with a single measurement such as pH or hydronium concentration. This calculator unifies those connected quantities so you can translate between pH, hydrogen ion concentration, hydroxide ion concentration, and pOH without switching tools. It assumes standard laboratory conditions (Kw = 1.0 × 10^-14 at 25 °C) yet keeps the relationships transparent so you can adapt the workflow for other temperatures or ionic strengths.
The interface supports three entry modes, validates every input, and instantly reports a qualitative classification alongside the numerical results. Copy the outputs into titration worksheets, buffer calculations, or water quality reports, knowing the conversions are handled consistently. Explanatory summaries reinforce the link between logarithmic pH values and the underlying molar concentrations.
Ideal for
- Water testing labs: Convert field pH readings into molar concentrations for compliance reports.
- Teaching acid–base fundamentals: Demonstrate how pH, pOH, and ion concentrations interrelate.
- Buffer design: Verify hydronium or hydroxide targets when preparing solutions for experiments.
Why it helps
- ✅ Multiple input paths: Work from whichever measurement you have on hand.
- ✅ Clear classifications: Instantly know whether the solution is acidic, neutral, or basic.
- ✅ Scientific notation friendly: Enter extreme concentrations without manual conversion.
- ✅ Ready for documentation: Copy formatted results directly into lab notebooks or digital reports.
Frequently Asked Questions
What temperature does this assume?
Calculations use Kw = 1.0 x 10^-14, which corresponds to 25 C (298 K). For other temperatures, supply the appropriate Kw value and recompute.
Can I input activities instead of concentrations?
Yes. As long as the value is expressed in mol/L, you can enter activities for more accurate work at higher ionic strength.
Does the calculator handle strong acids and bases?
It assumes complete dissociation, which is appropriate for strong acids and bases in dilute solutions. For weak species, compute equilibrium concentrations first.
Is it possible to enter pH values below 0 or above 14?
Yes. The calculations accept any positive concentration, so extremely acidic or basic solutions are supported if you provide the correct numbers.
Why is neutral defined at pH 7?
At 25 C, pure water has pH 7. Different temperatures shift the neutral point slightly because Kw changes.