Polylprotic acid

Titration curves for polyprotic acids

Sodium hydroxide reacts with phosphoric acid to remove two acidic hydrogens sequentially, as shown below. Although some PO₄^3– is present beyond the second equivalence point, OH^– is not a strong enough base to remove the last acidic hydrogen completely.  Reactions (1) and (2) are both product-favoured and therefore the amounts of the major phosphorus-containing species depend on how much OH^–  has been added.