pH and hydrogen-ion activity
pH is an activity-based quantity realized through measurement conventions.
Key knowledge
pH is defined as the negative base-ten logarithm of hydrogen-ion activity. Individual ionic activities cannot be measured independently, so practical pH realization uses conventions and standards. Replacing activity by a concentration ratio is a dilute-solution approximation.
Conditions and scope
- State the solvent, temperature and measurement convention.
- The concentration approximation requires activity-coefficient effects to be negligible.
Sources · 1
References
Sources retain their recorded scope. A source link alone does not establish verification of every statement.
Evidence and review
IUPAC Green Book section 2.13.1(viii), pH definition and single-ion limitation.
2026-10-10 · Checked definition and measurement qualification; no clinical pH guidance.
Entry details
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- chem-ph-activity
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- university-reference.1
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- University reference