Whoa now. This thread’s getting out of control. Let’s break it down.
Water spontaneously separates into [H3O] and [OH] ions on its own, therefore a certain concentration of each ion is always present, even in pure water.
The definition of pH is derived from the “equilibrium constant” – a relationship which is the product of the concentrations of the results in an equilibrium reaction divided by the product of the concentrations of the initial chemicals.
For the dissociation of water this would be
Ka (the equilibrium constant) = [H3O][OH]/[H2O]^2, so
Ka * [H2O]^2 = [H3O][OH]
Through experimentation we found that the product [H3O][OH] is a constant equal to 1.01E-14 in pure water; this is called the ion product constant. Additionally, we know that in pure water, the concentrations of [H3O] and [OH] ions have to be equal (they are mutually dependent); long story short we found that in pure water, [H3O] and [OH] are equal to each other, and are always equal to 1E-7 (at room temperature).
Now, we define pH = -log[H30]
Applying that to the value for [H3O] in pure water above, we find that the pH of pure water is 7. Life is good. By extension, at pH = 0 [H3O] = 1, at pH = 14 [H3O] = 10E-14. So far so good. So, most acids lie in that range. Not all of them, I presume.
So then what is going on with fluoroantimonic acid? A pH of -25 would mean that everything is completely fucked up, and the math is broken and oh my God everything is horrible ‘cause there are more atoms than makes sense.
Turns out, pH isn’t really a good way of measuring strength of acids, only the strength of solutions. To describe acidity we use a value called the pKa. This value is defined as
pKa = -log[Ka] (the value described above). So, fluoroantimonic acid does not in fact have a pH of -25 at all, it has a pKa of -25, which is a whole different beast.
Hope that helps?
There’s a whole bunch of math and background there that isn’t necessary, so to sum up: it’s from 0–14 ‘cause that’s way the mathematical definition of pH works. Fluoroantimonic acid does not have a pH of -25, that’s something else.