Polyprotic Acids
Polyprotic acids have more than one proton that may be removed by reaction with a base. These acids are often referred to as diprotic, supplying two protons, triprotic, supplying three protons, etc. Consider a generic triprotic acid H3A. In aqueous solution, this acid will be distributed in four different species; the acid and the three conjugate bases.
Each acid equilibrium has a separate equilibrium equation. Each equation, in turn, has a different constant. To distinguish these acid equilibrium constants, they are numbered 1 through 3. Ka1 is the constant for the first proton ionization, Ka2 is the second, etc. The acid equilibria are expressed by the three equilibrium equations
Beta Formation Constants
One can also find the formation constant, or b equations. These are found by taking products of the equilibrium equations
Notice that the Ka1 and b equations give concentrations of the conjugate bases in terms of only the Ka's, [H3A], and [H3O+].
Conjugate Bases
The conjugate base of a weak, monoprotic acid, acts as a base, extracting a proton from water. Similarly, the three conjugate bases of the triprotic acid can extract protons through the chemical equilibria
There are 3 equilibrium equations for the conjugate bases. The base equilibrium constants, Kb, are also numbered. In this case, the numbering starts from the base with the fewest protons
The base equilibrium constants are related to those of the acid through Kw, the disproportionation constant of water
using
Notice the reverse order of Ka relative to Kb numbering.
Relative Strength of Acid and Base Forms
The magnitude of the acid equilibrium constants, relative to the concentration of H3O+ and OH- in pure water (10-7 M) gives an indication of the relative strength of each of the acids and conjugate bases. The relative magnitudes are often compared using the pKa. These are defined similar to the pH
If the pKa is less than 7, then the proton is said to be an "acidic proton." On the other hand, if the pKa is greater than 7, then the conjugate base is strong, and the the proton is said to be a "basic proton"
For example, the equilibrium constants for phosphoric acid are; Ka1=7.11´ 10-3, Ka2=6.32´ 10-8, and Ka3=7.1´ 10-13. The corresponding pKa.are pKa1=2.15, pKa2=7.12, and pKa3=12.15. Because pKa2 and pKa3 are greater than 7, we say that phosphoric acid is a triprotic, dibasic acid.
This page was created by Professor Stephen Bialkowski, Utah State University.
Monday, August 28, 2006