If the pH is -25 then the solution contains 10^25 moles of H+ ions in a litre. That's 10^25grams or 10000000000000000000 tons of H+ in a litre.

Do you think that's reasonable?

I think you might be talking about a Hammet function

http://en.wikipedia.org/wiki/Hammett_acidity_function

Fluoroantimonic acid for the win (as mentioned above).

Anyway, how corrosive is it?

A superacid is an acid with an acidity greater than that of 100% pure sulfuric acid.

Actually, the fluoroantimonic acid is 2 x 10^19 times stronger than H2SO4! It is the strongest superacid known (therefore the strongest acid), and its aqueous solution is about -25 in pH!

 

HSbF6 decompses with much violence in water, hydrolyzing to HF and antimony oxides.

 

10^19 times stronger just means that the proton is 10^19 less strongly attached to the rest of the molecule. pH relies on concentration of protons (hydronium ions) in water, and will never exceed the -log(conc), where conc is the molarity of the acid in solution multiplied by the number of acidic protons.

 

In water, any acid with a pKa lower than -1.7 is effectively completely dissosciated, forming a "salt" if you will of hydronium ion (analagous to ammonium)

 

How corossive acids are doesn't mean anything with regards to their strength. HSbF6 needs to be handled with all fluoropolymer equipment, simply because the fluoride is immensely corrosive to glass.

Hydroxic acid, hydroxylic acid, hydroxilic acid or water is the strongest solvent. It can dissolve almost anything over time.

its not really an acid as it has a pH of 7 and hence neutral.

For all intents and purposes, water can be classified as either a base or an acid.

no, not really. and it is far far from being the strongest acid

You're both right, but that also means you are both wrong. Water is amphoteric, meaning it can act as an acid or a base. However, its pH and pOH are each 7.00, and are thus far from being the strongest of either. This means it is not the strongest solvent, and there are a great many things, hydrophobic things, that do not dissolve in water.