[Organishe]

Hello all,

Something I have wondered for a while is why is there such variation in the reported values of the pK_as for strong acids? For example, in one of my organic books, it is reported that the pK_a  of HI in water is -10. In another book, it is reported as -7, and in yet another it is -11. I have found the same thing for HCl and HBr in various books.  For a weak acid such as acetic, though, the value listed is nearly always the same to a couple of decimals.  So why is there such variation? If I had to guess, I would say because it is a strong acid, the amount that remains in the unionized form is so small that experimental error has a significant effect on the result.

In many cases its not like it would matter, since it is so strong to begin with. Just curious....

Thanks all,

Eric

[Borek]

If I had to guess, I would say because it is a strong acid, the amount that remains in the unionized form is so small that experimental error has a significant effect on the result.

This, plus fact that some of these constants are not determined experimentally, but calculated using approach similar to Born-Haber cycle.

[dls]

Take into consideration that the reading of the pH electrode, when less than about 1 and when more than about 13, is more semantic than real.  See also a book by the late Prof. J Ricci "Hydrogen Ion Concentration."

dls

[Mitch]

Also check out our "pKa values" link to your left in the menu