log10H3O

Log10H3O refers to the base-10 logarithm of the hydronium ion concentration in an aqueous solution, commonly written as log10[H3O+]. It is a convenient way to express acidity and is directly related to the pH scale, since pH = -log10[H3O+]. Therefore, log10[H3O+] = -pH. In rigorous thermodynamics, pH is defined using the activity of hydronium, a(H3O+), rather than simply concentration; the relation becomes pH = -log10 a(H3O+). In practice, log10[H3O+] is often used as an approximation when activities are close to the activity of the solution behaving like an ideal dilute solution.

Examples illustrate the relationship. At 25°C, pure water has [H3O+] = 1.0×10^-7 M, so log10[H3O+] = -7 and pH = 7. A solution with pH 2 has [H3O+] = 1.0×10^-2 M, giving log10[H3O+] = -2. Conversely, pH 12 corresponds to log10[H3O+] = -12 ( [H3O+] = 1.0×10^-12 M ). Thus, more acidic solutions have less negative log10[H3O+] values, while highly basic solutions yield more negative values (in typical aqueous ranges).

Notes and limitations include the distinction between concentration and activity, and the temperature dependence of the pH scale. The standard pH scale is defined at 25°C; deviations in temperature alter water autoprotolysis and the exact relationship between log10[H3O+] and pH. Despite its ease of use, log10[H3O+] is less commonly tabulated than pH, but it remains a useful way to quantify acidity when working directly with hydronium concentrations.