Phosphate

Phosphate Ion, PO₄^-

Acid Equilibria:

Phosphate ion is a reasonably strong base. It hydrolyzes in water to form a basic solution.

PO₄^3-(aq)+ H₂O(l) <==> HPO₄^2-(aq) + OH^-(aq) K_b = 1.0 x 10^-2

HPO₄^2-(aq) + H₂O(l) <==> H₂PO₄^-(aq) + OH^-(aq) K_b = 1.6 x 10^-7

H₂PO₄^-(aq) + H₂O(l) <==> H₃PO₄(aq) + OH^-(aq) K_b = 1.3 x 10^-12

Solubility:

Phosphates of the alkali metals are soluble. Most other phosphates, such as FePO₄, CrPO₄, BiPO₄, Ca₃(PO₄)₂, and Ag₃PO₄ are only sparingly soluble. Phosphate ion also forms a bright yellow precipitate with ammonium molybdate:

PO₄^3- + 3NH₄⁺ + 12MoO₄^2- + 24H⁺ ---> (NH₄)₃PO₄.12MoO₃ + 12H₂O

Oxidation-Reduction:

Phosphate is a very weak oxidizing agent. Since the phosphorus is in its highest oxidation state in phosphate ion, this ion cannot act as a reducing agent.

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