Oxalate (${\mathrm{C}}_2{\mathrm{O}}_4^{2-}$)

Oxalate is the dianion ${\mathrm{C}}_2{\mathrm{O}}_4^{2-}$ formed by loss of two protons from oxalic acid. Its oxygen donors can bind a metal centre as a chelate.

Precipitation of calcium oxalate illustrates the difference between a dissolved molecular ion and a solid salt. Changing the metal counterion or the pH can change solubility and hence whether oxalate remains in chemical solution.