Identified content errors were corrected on 2026-10-25. Sources and scope for this page: IUPAC Gold Book — pH (accessed 2026-10-25), covering activity definition of pH. The activity table is calculated from the definition; no product-specific concentrations are asserted. What the 2026-10-25 correction changed is stated with the source above; nothing beyond that scope was re-checked. Full sources, values and limits on this page.
Page last changed 2026-10-25 · based on Public reference accessed 2026-10-25; scope stated on page
Quick Answer
Hydrogen-ion activity corresponding to pH
| pH[2] | a(H⁺), dimensionless[1] |
|---|---|
| 0 # | 1 |
| 1 # | 10^−1 |
| 2 # | 10^−2 |
| 3 # | 10^−3 |
| 4 # | 10^−4 |
| 5 # | 10^−5 |
| 6 # | 10^−6 |
| 7 # | 10^−7 |
| 8 # | 10^−8 |
| 9 # | 10^−9 |
| 10 # | 10^−10 |
| 11 # | 10^−11 |
| 12 # | 10^−12 |
| 13 # | 10^−13 |
| 14 # | 10^−14 |
Calculated from a(H⁺) = 10^−pH. These are activities, not exact mol/L concentrations. No pH is assigned to a household product without concentration, formulation and measurement conditions.
When a concentration approximation is useful
For an appropriate dilute solution, activity can be approximated from concentration relative to its standard state when the activity coefficient is near one. A tenfold dilution raises pH by approximately one only for a suitable strong-acid regime where dissociation is effectively complete, activity effects are small and water autoionisation is negligible. Weak acids and buffers do not follow that shortcut.
Neutrality and temperature
Neutrality corresponds to equal hydrogen- and hydroxide-ion activities. Neutral pH is approximately 7 at 25 °C and varies with temperature. Report temperature and measurement conditions; pH 0–14 is a familiar range, not a universal bound.
Basis and scope
IUPAC Gold Book — pH: Activity definition of pH. The activity table is calculated from the definition; no product-specific concentrations are asserted.
Frequently Asked Questions
- Does pH directly give an exact molar concentration?
- No. The definition uses activity. A concentration estimate needs an activity model and the solution conditions.
- Does diluting any acid tenfold raise its pH by one?
- No. That approximation needs a suitable dilute strong-acid regime. It is not a general rule for weak acids, buffers or solutions near neutral.
- Can pH be outside 0–14?
- Yes. The definition is not limited to that range. Concentrated solutions require particular care with activity and measurement conventions.
Related
Value Sources
References and calculation methods used for this page are listed below. Numbered markers connect table columns where present. A reference listing does not itself establish that every value has been independently checked.
| # | Source | Type | Revision / method |
|---|---|---|---|
| [1] | Activity derived from definition | derived | 10^(-pH), dimensionless |
| [2] | IUPAC Gold Book — pH | reference | Public reference accessed 2026-10-25 — source |
Data Sources
| Standard | Revision | What it covers on this page |
|---|---|---|
| IUPAC Gold Book — pH | Public reference accessed 2026-10-25; scope stated on page | Activity definition of pH. The activity table is calculated from the definition; no product-specific concentrations are asserted. |
Verification scope
- 2026-10-25 — Content corrected 2026-10-25. Activity definition of pH. The activity table is calculated from the definition; no product-specific concentrations are asserted. Evidence
Content corrected 2026-10-25. Activity definition of pH. The activity table is calculated from the definition; no product-specific concentrations are asserted.
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