logH3O

LogH3O is the base-10 logarithm of the hydronium ion concentration in an aqueous solution, commonly written as log10([H3O+]). It is a compact way to express acidity, since [H3O+] spans many orders of magnitude. For example, a solution with [H3O+] = 1×10−3 M has logH3O = −3, while a neutral solution near 25 °C with [H3O+] ≈ 1×10−7 M has logH3O = −7.

In practice, logH3O is closely related to the pH scale. The pH of a solution is defined as pH = −log10(a(H3O+)), where a(H3O+) is the activity of the hydronium ion. Under dilute, ideal conditions, activity is well approximated by concentration, so pH ≈ −log10([H3O+]). Thus, logH3O and pH provide inverse measures of acidity.

Hydronium concentration can be tied to activity, especially at higher ionic strengths where non-ideal behavior is significant. Therefore, logH3O can be computed from either concentration or activity, depending on the context and the level of approximation required. Accurate thermodynamic calculations often prefer activities (a(H3O+)) rather than concentrations ([H3O+]), and thus use pH expressions based on activities.

Notes: logH3O is a convenient shorthand in data tables and calculations involving acid–base equilibria. Its interpretation assumes aqueous solution conditions, standard states near 1 M for reference, and, when precision is needed, consideration of temperature and ionic strength that affect activity coefficients.