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.
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References
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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.
The pH definition uses hydrogen-ion activity with a specified convention.
For HA ⇌ H⁺ + A⁻ in a stated solvent convention, Ka relates the equilibrium activities. For a conjugate acid–base pair in water, KaKb=Kw under consistent definitions. Acid strength describes the equilibrium tendency to transfer a proton, while the fraction ionized also depends on composition.
Potentiometry measures the potential difference between an analyte-responsive indicator electrode and a reference electrode with negligible measurement current. Equilibrium response depends on activity. Ion-selective membranes and redox electrodes require their own response/selectivity models and calibration.
pH is an activity convention requiring appropriate measurement.
Amount concentration (molarity) is solute amount per solution volume; molality is solute amount per solvent mass. Mole fraction and mass fraction divide a component's amount or mass by the corresponding total. These scales are not interchangeable. Volume-based concentration can change with temperature through expansion even when composition and solute amount are unchanged.
Concentration and activity are distinct; an amount-concentration definition does not justify replacing activity in pH.