Oxalic acid ($\mathrm{HOOC}-\mathrm{COOH}$)

Oxalic acid is the simplest dicarboxylic acid, with structure $\mathrm{HOOC}-\mathrm{COOH}$. Stepwise loss of two protons gives hydrogen oxalate and then oxalate.

Oxalic acid combines acid-base chemistry with metal-ion binding. Some oxalate salts have low solubility, so the behaviour of an oxalate-containing system depends on both pH and the available metal ions.

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.