Negative pH values are possible on an activity-based pH scale, but concentrated strong acids cannot be evaluated accurately by inserting molar concentration into pH = -log10(cH+). pH is defined in terms of hydrogen-ion activity, and activity coefficients, acid dissociation, water activity, junction potentials and glass-electrode errors become important in concentrated media.
Can a glass pH electrode display a negative value?
Some electrode/meter systems can display a negative value, but a display range is not proof of valid measurement accuracy. The electrode glass, reference/junction, calibration/standardization method, temperature and acid matrix must support the intended measurement.
A general-purpose probe calibrated with ordinary aqueous buffers should not be assumed accurate in concentrated strong acid. Acid error, liquid-junction potential, dehydration/chemical attack and activity-scale limitations can dominate.
Why -log10(12 M HCl) is not a valid pH measurement
The simple concentration calculation assumes that concentration equals activity. That approximation is unsuitable for a concentrated acid such as 12 mol/L HCl. It also ignores matrix-dependent activity coefficients and the operational definition used to realize pH.
Therefore, do not publish pH = -1.08 as the measured pH of 12 M HCl based only on concentration. A defensible value requires an appropriate thermodynamic/operational method and stated assumptions.
What affects very-low-pH measurement?
- glass composition and acid-error behavior;
- reference electrode and liquid-junction potential;
- calibration/standardization outside ordinary buffer coverage;
- acid concentration, composition and water activity;
- temperature;
- electrode chemical compatibility and conditioning;
- method-specific activity conventions or models.
Practical decision
If a process may be below the validated range of a standard pH system, define the decision first. Obtain an electrode and method validated for that acid/matrix, confirm reference/junction compatibility and use an independent acid-concentration or process-chemistry method where required. Do not extrapolate an ordinary two-buffer calibration without evidence.
For routine aqueous measurements within the supported range, see the pH sensor guide and pH calibration guide.