  A theory is presented which allows us to quantitatively calculate the excess
surface tension of acid solutions. The H^+, in the form of hydronium ion, is
found to be strongly adsorbed to the solution-air interface. To account for the
electrostatic potential difference measured experimentally, it is necessary to
assume that the hydronium ion is oriented with its hydrogens pointing into the
bulk water. The theory is quantitatively accurate for surface tensions and is
qualitative for electrostatic potential difference across the air-water
interface.
