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What does this calculator estimate?
A pH calculator converts hydrogen ion concentration into pH, pOH, and hydroxide concentration. Enter [H⁺] in mol/L to see the acidity or basicity of the solution.
- pH = −log₁₀[H⁺]
- pOH = 14 − pH
- pH < 7 acidic, 7 neutral, > 7 basic
What pH measures
pH expresses hydrogen ion concentration on a logarithmic scale: each whole number is a 10× change in acidity. A pH of 3 is ten times more acidic than pH 4, and a hundred times more than pH 5.
Limitations to watch for
The formula assumes ideal dilute solutions at 25°C — concentrated acids and bases deviate. pH below 0 or above 14 is possible for very strong solutions. The scale measures activity of H⁺, not total acid content.
How to use it in practice
Use pH to characterize solutions in chemistry, biology, and water testing. Remember the log scale: small pH changes mean large concentration changes. For buffers and precise work, calibrate meters with standards.
['Enter the hydrogen ion concentration [H⁺] in mol/L.', 'The tool computes pH, pOH, and [OH⁻].', 'It also classifies the solution as acidic, neutral, or basic.']
How this calculator works
Formula
pH = −log₁₀[H⁺], where [H⁺] is the hydrogen ion concentration in mol/L. pOH = 14 − pH; [OH⁻] = 10^(−pOH). pH < 7 is acidic, pH = 7 neutral, pH > 7 basic.
Worked example
With [H⁺] = 1.0×10⁻³ mol/L: pH = −log₁₀(10⁻³) = 3.00, pOH = 11.0, and the solution is acidic.
Assumptions to verify
- The solution is dilute and aqueous at ~25°C.
- [H⁺] is the activity of hydrogen ions.
- Kw = 1.0×10⁻¹⁴ at 25°C (so pH + pOH = 14).
Frequently asked questions
What is pH?
A logarithmic measure of hydrogen ion concentration: pH = −log₁₀[H⁺]. Lower pH means more acidic.
How do I calculate pH from concentration?
Take the negative base-10 log of the hydrogen ion concentration. [H⁺] = 1×10⁻³ gives pH 3.
What is the pH scale?
0–14 is the common range: 0–6.9 acidic, 7 neutral, 7.1–14 basic. Each step is a 10× change in acidity.
What is pOH?
The hydroxide counterpart: pOH = 14 − pH at 25°C. A solution with pH 3 has pOH 11.
Can pH be negative or above 14?
Yes, for very strong acids or bases — the 0–14 range is just the common window for aqueous solutions.
What does pH measure exactly?
The concentration (activity) of hydrogen ions, which determines how acidic or basic a solution is. It's not a measure of total acid present.
Why is the log scale used?
Because H⁺ concentrations span many orders of magnitude — from ~10⁰ to 10⁻¹⁴ mol/L. The log compresses that range into a usable 0–14 scale.
Cite this tool
BoringToolsKit. “pH Calculator.” boringtoolskit.com/ph-calculator/ (reviewed 2026-08-25). Free to reference in articles, syllabi, and answer posts with a link.
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