Phosphoric acid is , an oxyacid with three successively removable protons. Its conjugate-base forms include dihydrogen phosphate, hydrogen phosphate, and phosphate.
Their proportions depend on pH. A phosphate-containing buffer therefore represents a mixture of protonation states rather than one universally triply charged ion; this is also relevant to interpreting an acid-base titration.
[ref]
Chemistry 2e by Paul Flowers and others. Chapter14, Acid-Base Equilibria: polyprotic acids and phosphate protonation
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