What is the concentration of hydrochloric acid needed to have a solution with the activity of the hydrogen proton = 1? I read 1.18 M somewhere but I am unable to find a corroborating source. Also, how can the activity of the hydrogen proton = 1 when it doesn't exist by itself, suggesting that its activity should = 0 (since it can't act as an independent particle)?

Also how would one get to this result? Does one use a high-grade pH meter? Don't analytical-level pH meters measure activities rather than concentrations of hydronium ion?


For almost the last 100 years, the definition of pH has been -log of hydrogen ion activity. Prior to 1920, the definition was -log of hydrogen ion concentration.

Hydrogen ion activity or concentration does not refer to bare protons, but solvated protons, as $\ce{H_3O^+, H_9O_4+}$ etc.

See Determination of Hydrogen Single Ion Activity Coefficients in Aqueous HCl Solutions at 25°C for authoritative data.

  • $\begingroup$ Would solvation of hydrogen ion decrease its activity? $\endgroup$
    – Dissenter
    Jul 24 '14 at 23:49
  • $\begingroup$ not sure I understand what you are asking. For gas phase HCl the hydrogen ion activity and concentration are many orders of magnitude lower than for the aqueous phase, so in that sense solvation greatly increases activity. See the following table where "GP" means gas phase: ut.ee/ams/… $\endgroup$
    – DavePhD
    Jul 26 '14 at 10:53

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