磷酸
A polyprotic acid is a compound which may lose more than 1 proton. Stepwise dissociation constants are each defined for the loss of a single proton. The constant for dissociation of the first proton may be denoted as "K"a1 and the constants for dissociation of successive protons as "K"a2, etc. Phosphoric acid, H3PO4, is an example of a polyprotic acid as it can lose three protons.