Hydrogen Ion Activity
I want to add a slight clarification of hydrogen activity versus hydrogen concentration. Hydrogen activity can be thought of as applying a correction factor to the hydrogen concentration. Activity is related to concentration by the Hydrogen activity coefficient:
a(H) = K(a) x c(H)
a(H) = Hydrogen activity in the solution
K(a) = Hydrogen activity coefficient
c(H) = Hydrogen concentration in the solution
Hydrogen activity indicates how many hydrogen ions seem to be present in the solution and is different from how many actually are present (concentration).
Most pH measurements are in extremely dilute water solutions where the hydrogen ion concentration is the same as the hydrogen ion activity, K(a) is thus equal to 1.