Phosphate Ion (PO₄³⁻)
- Page ID
- 97283
Acid Equilibria
Phosphate ion is a reasonably strong base. It hydrolyzes in water to form a basic solution.
\[\ce{PO4^{3-}(aq) + H2O(l) <=> HPO4^{2-}(aq) + OH^{-}(aq)} \nonumber \]
with \(K_b = 1.0 \times 10^{-2}\)
\[\ce{HPO4^{2-}(aq) + H2O(l) <=> H2PO4^{-}(aq) + OH^{-}(aq)} \nonumber \]
with \(K_b = 1.6 \times 10^{-7}\)
\[\ce{H2PO4^{-}(aq) + H2O(l) <=> H3PO4(aq) + OH^{-}(aq)} \nonumber \]
with \(K_b = 1.3 \times 10^{-12}\)
Solubility
Phosphates of the alkali metals are soluble. Most other phosphates, such as \(\ce{FePO4}\), \(\ce{CrPO4}\), \(\ce{BiPO4}\), \(\ce{Ca3(PO4)2}\), and \(\ce{Ag3PO4}\) are only sparingly soluble. Phosphate ion also forms a bright yellow precipitate with ammonium molybdate:
\[\ce{PO4^{3-} + 3NH4^{+} + 12MoO4^{2-} + 24H^+ -> (NH4)3PO4 \cdot 12MoO3 + 12H2O} \nonumber \]
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.