pH Calculator - Acidity, [H⁺], and pOH Converter
Science & Engineering
Convert between pH, hydrogen ion concentration [H⁺], and pOH. Determine whether a solution is acidic, neutral, or basic.
25 °C assumption
This converter uses Kw = 1×10⁻¹⁴ mol²/L² and pH + pOH = 14 at 25 °C. Calculations run in your browser.
pH and ion converter
Edit any field to update the other three values. Concentrations must be finite and greater than zero; pH and pOH may be outside 0–14.
Negative values and values above 14 are accepted.
This value can also fall outside 0–14.
Enter a finite concentration greater than zero.
Enter a finite concentration greater than zero.
Neutral solution. Editing PH.
Formulas: pH = −log₁₀([H⁺]); pOH = −log₁₀([OH⁻]); pH + pOH = 14; Kw = [H⁺][OH⁻] = 1×10⁻¹⁴ at 25 °C.
Visual pH scale
The scale shows 0–14. Values outside that visual range are accepted, with the marker pinned to the nearest edge.
Current pH is 7.0000 on the 0–14 visual scale.
Typical examples (approximate values). Select one to fill the unified converter:
How to use this tool
- Edit pH, pOH, [H⁺], or [OH⁻]; the other three values and the acidic, neutral, or basic label update together.
- For a monoprotic strong acid or base, choose the strong concentration mode and enter its positive mol/L concentration. The model assumes full dissociation and includes water autoionization at 25 °C.
- pH and pOH values outside 0–14 are accepted; the visual scale pins its marker to the nearest edge. The converter does not solve weak-acid or buffer equilibria.
pH Scale Reference
| pH | Example |
|---|---|
| 0 | Battery acid |
| 2 | Lemon juice |
| 4 | Coffee |
| 7 | Pure water (neutral) |
| 8 | Baking soda |
| 11 | Ammonia |
| 14 | Lye (NaOH) |
The logarithmic scale explained
pH is defined as −log₁₀[H⁺], where [H⁺] is the hydrogen ion concentration in moles per liter. Each unit change on the pH scale represents a 10-fold change in acidity:
- pH 3 is 10× more acidic than pH 4
- pH 3 is 100× more acidic than pH 5
- pH 3 is 10,000× more acidic than pH 7 (neutral water)
This means small pH differences in, for example, ocean chemistry (the ocean has acidified from pH 8.2 to 8.1 since pre-industrial times) represent a 26% increase in ocean acidity - a significant change despite the small numerical difference.
pH in chemistry and biology
pH profoundly affects chemical reaction rates, enzyme activity, and cell function:
- Human blood: maintained at pH 7.35–7.45. A drop below 7.35 (acidosis) or rise above 7.45 (alkalosis) is a medical emergency affecting enzyme and protein function.
- Stomach acid: pH 1.5–3.5, necessary for pepsin activity and killing pathogens.
- Soil pH: most crops grow best at pH 6–7. Acidic soil (below 5.5) limits phosphorus availability; alkaline soil (above 7.5) limits iron and manganese uptake.
- Swimming pools: maintained at pH 7.2–7.8 for effective chlorine disinfection and swimmer comfort.
Buffers and pH stability
A buffer is a solution that resists pH changes when acid or base is added. Blood is buffered by the carbonate/bicarbonate system: CO₂ + H₂O ⇌ H₂CO₃ ⇌ H⁺ + HCO₃⁻. The kidneys and lungs regulate this equilibrium to maintain blood pH within its narrow range. In the lab, common buffers include phosphate buffer saline (PBS) at pH 7.4 and acetate buffer at pH 4–5.
This page converts pH and ion concentrations; it does not calculate buffer composition or weak-acid equilibrium.
Frequently asked questions
What is pH?
pH = −log₁₀([H⁺]). At 25 °C, this converter uses Kw = [H⁺][OH⁻] = 1×10⁻¹⁴ mol²/L², so pH + pOH = 14 and pH 7 is neutral. The 0–14 scale is a visual reference, not a limit on accepted pH values.
What is pOH?
pOH = −log₁₀([OH⁻]); at 25 °C, pOH = 14 − pH because Kw is 1×10⁻¹⁴. Kw changes with temperature, so this 25 °C relationship should not be applied unchanged at other temperatures.
Can pH be below 0 or above 14?
Yes. Those values can occur in concentrated solutions, so this converter accepts pH and pOH outside 0–14 when the corresponding values fit its numeric range. Its color scale only visualizes 0–14 and pins an outside value to the nearest edge.
Does this calculator solve weak acids or buffers?
No. The concentration mode is only for fully dissociated monoprotic strong acids and bases, with water autoionization included. This page does not calculate weak-acid dissociation, buffer recipes, Henderson–Hasselbalch equilibria, or activity corrections.
Related tools
- MolarityCalculate solution concentration before relating it to hydrogen-ion activity.
- Scientific Notation ConverterNormalize very small ion concentrations written with powers of ten.
- Log CalculatorInspect the base-10 logarithm underlying the pH definition.
- Periodic TableReference the elements in acids, bases, or dissolved compounds.