Question

Using a pH number line or a potentiometric curve, dtermine the predominant specie/s at pH 4.5 and 10.5. Look up pKa3 of phosphoric acid in the literature

Answer 1

So the standard equation for Phosphoric acid are as below given in the literature.

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

H₂PO4(aq) + H₂O(l) ⇌ H₃O⁺(aq) + HPO2 4(aq),   K_a2 = 6.2×10⁻⁸

HPO2−4(aq) + H₂O(l) ⇌ H₃O⁺(aq) + PO3−4(aq)        K_a3 = 2.2×10⁻¹³

Based on above equations pK_a1 = 2.12, pK_a2 = 7.21, and pK_a3 = 12.67

Now the question, predominant species at pH 4.5 will be H₂PO₄^(-) Dihydrogen phosphate anion, as it does not dissociate at this high acidic condition. .

and at pH 10.5, which is high basic condition, the predominant species will be HPO₄^(2-) Hydrogen Phoshphate anion

^{Hope this helps. Thanks}