Activities of Solutes - especially acids and bases 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.