Diprotic acids are those acid which consist of two protons. Out of citric acid, chromic acid, oxalic acid, pyrosulphuric acid and sulphurous acid, only citric acid is not a diprotic acid. Citric acid −C6H8O7 Chromic acid - H2CrO4 Oxalic acid −H2C2O4 Pyrosulphuric acid −H2S2O7 Sulphurous acid −H2SO3