The hydrogen phthalate ion, C₈H₅O₄⁻, is a weak acid with K_a = 3.91 × 10⁻⁶.

What is the pH of a 0.050 M solution of potassium hydrogen phthalate, KC₈H₅O₄? Note: To find the pH for a solution of the anion, we must take into account that the ion is amphiprotic. It can be shown that, for most cases of amphiprotic ions, the H₃O⁺ concentration is

For phthalic acid, C₈H₆O₄, K_a1 is 1.12 × 10⁻³, and K_a2 is 3.91 × 10⁻⁶.

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C8H5O4 (aq) + H₂O(l) CH4O42-(aq) + H3O+ (aq)