Fig 6E.2 - Fractional Composition of Triprotic Acids

For a triprotic acid in aqueous solution the following equilibria must be considered:

H₃A(aq) + H₂O(l) ⇄ H₂A^-(aq) + H₃O⁺(aq)     K_a1

H₂A^-(aq) + H₂O(l) ⇄ HA^2-(aq) + H₃O⁺(aq)     K_a2

HA^2-(aq) + H₂O(l) ⇄ A^3-(aq) + H₃O⁺(aq)     K_a3

The fractional composition of the protonated and deprotonated forms of the acid are given by

f(H₂A^-) = [H₃O⁺]²K_a1 / H

f(HA^2-) = [H₃O⁺]K_a1K_a2 / H

f(A^3-) = K_a1K_a2K_a3 / H

with

H = [H₃O⁺]³ + [H₃O⁺]²K_a1 + [H₃O⁺]K_a1K_a2 + K_a1K_a2K_a3

Note that conjugate pairs are present at equal concentrations when the pH is equal to the pK_a of the acid member of the pair.

To graph the fractional composition enter the pK_a's for the triprotic acid and click Plot. Values from the table below can be input by clicking the name of the acid.

The Clear button will remove all plots. To see the parameters for each form click the Legend button. The Redraw button will refresh the graph. This is useful when the function domain (i.e., pH ) has been changed. To show a value on the graph, move your cursor to the desired location then click.