In contrast, if the pH is greater than pKa for the acid,
then log10 [A-] / [HA] will be positive, [A-]
/ [HA] will be greater than 1.00, and A- will be favored
Again, in physical terms, a shortage of H+ ions in
a basic solution forces more HA to dissociate and yield A-.
The equilibrium shifts to the right. For every unit of difference
between pH and pKa, the ratio of basic [A-]
to acidic [HA] forms changes by a factor of 10.
With this in mind, we can look at the pKa values for
the three phosphoric acid dissociations and say that, at physiological
pH values of around 7.0 in a living organism, any phosphate present
would be found as H2PO4- and HPO42-
in roughly equal amounts.
Since there are five pH units between the pKa of a
neutral solution and that of the first H3PO4/H2PO4-
dissociation (pKa1 = 2.12) , the ratio of undissociated
H3PO4 to H2PO4-
ion at pH = 7 will be approximately 10-5, or 1 to 100,000.
Similarly, since the third dissociation of phosphoric acid has
a pKa3 of more than 12, the ratio of HPO42-
to PO43- at pH = 7 will be more
than 100,000 to 1.
The approximate relative amounts of the four phosphate species
at pH = 7 will be
This is important physiologically because of the large number of
phosphate compounds in living organisms. Adenosine triphosphate
(ATP) was discussed as an energy-storage molecule in Chapter 10.
It has four dissociable protons. The first three dissociations
occur at pKa values around pH = 2-3, so in neutral solutions
these three dissociations are essentially complete. The fourth dissociation
has pKa = 6.5, so at pH=7 the ration of ATP4-
to ATP3- is the square root of 10 = 3.2 to 1.