In contrast, if the pH is greater than pK_a for the acid, then log₁₀ [A^-] / [HA] will be positive, [A^-] / [HA] will be greater than 1.00, and A^- will be favored over HA.

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 pK_a, the ratio of basic [A^-] to acidic [HA] forms changes by a factor of 10.

With this in mind, we can look at the pK_a 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 HPO₄^2- in roughly equal amounts.

Since there are five pH units between the pK_a of a neutral solution and that of the first H₃PO₄/H₂PO₄^- dissociation (pK_a1 = 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 pK_a3 of more than 12, the ratio of HPO₄^2- to PO₄^3- 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 pK_a values around pH = 2-3, so in neutral solutions these three dissociations are essentially complete. The fourth dissociation has pK_a = 6.5, so at pH=7 the ration of ATP^4- to ATP^3- is the square root of 10 = 3.2 to 1.