PO43

Meaning: PO₄³⁻ is the phosphate ion, the fully deprotonated form of triprotic phosphoric acid. The sequence is H₃PO₄, H₂PO₄⁻, HPO₄²⁻, PO₄³⁻, losing one H⁺ at each step. Phosphorus is in the +5 oxidation state, and the ion is tetrahedral (bond angles near 109.5°), with resonance spreading the 3− charge over the four oxygens.

Equation: pH = pKa + log([HPO₄²⁻]/[H₂PO₄⁻]), where pKa ≈ 7.2 (unitless) for H₂PO₄⁻ ⇌ H⁺ + HPO₄²⁻, brackets are molar concentrations (mol/L), and pH is unitless.

On the MCAT: Expect a buffer or titration question. The three pKa values are roughly 2, 7 and 12, so phosphate buffers best near pH 7. At pH 7.4 the main species are H₂PO₄⁻ and HPO₄²⁻, and PO₄³⁻ is a tiny fraction. Other questions ask you to write the formula of a salt such as Ca₃(PO₄)₂ by charge balance, or use the solubility rule that phosphates of most metals, apart from group 1 cations and NH₄⁺, are poorly soluble.

Common confusion: Students name PO₄³⁻ as the buffer base near physiological pH. The buffer pair there is H₂PO₄⁻/HPO₄²⁻. PO₄³⁻ dominates only when the pH is well above roughly 12, so match the pH to the nearest pKa.