Question

Phosphoric acid is a polyprotic acid. Which form is the dominant species in the chromatography system? Hint – calculate the pH of the eluent and look up the pKas for phosphate.

Answer 1

It is it is basically that the most deprotonated form will predominate at the least acidic pH (PO₄^3- at pH = 11), the most protonated ion form will predominate at the most acidic pH (H₂PO₄^- at PH = 4), and the monoprotic form will predominate at the near-neutral pH (HPO₄^2- at pH = 8).

H₃PO₄(s) + H₂O(l) ⇌ H₃O⁺(aq) + H₂PO₄^-(aq)       K_a1 = 7.5×10⁻³

H₂PO₄^-(aq) + H₂O(l) ⇌ H₃O⁺(aq) + HPO₄^2-(aq)       K_a2 = 6.2×10⁻⁸

HPO₄^2-(aq) + H₂O(l) ⇌ H₃O⁺(aq) + PO₄^3-(aq)        K_a3 = 2.2×10⁻¹³

Now after knowing the pH and pKa value of the eluent you can easily confirm the dominant form of the phosphate.​