Phosphate Ion, PO4-


Acid Equilibria:

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

PO43-(aq)+ H2O(l) <==> HPO42-(aq) + OH-(aq) Kb = 1.0 x 10-2

HPO42-(aq) + H2O(l) <==> H2PO4-(aq) + OH-(aq) Kb = 1.6 x 10-7

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


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:

PO43- + 3NH4+ + 12MoO42- + 24H+ ---> (NH4)3PO4.12MoO3 + 12H2O


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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