Oxalate (IUPAC: ethanedioate) is an anion with the formula C2O42−. This dianion is colorless. It occurs naturally, including in some foods. It forms a variety of salts, for example sodium oxalate (Na2C2O4), and several esters such as dimethyl oxalate (C2O4(CH3)2). It is a conjugate base of oxalic acid. At neutral pH in aqueous solution, oxalic acid converts completely to oxalate. Relationship to oxalic acid The dissociation of protons from oxalic acid proceeds in a stepwise manner; as for other polyprotic acids, loss of a single proton results in the monovalent hydrogenoxalate anion . A salt with this anion is sometimes called an acid oxalate, monobasic oxalate, or hydrogen oxalate. The equilibrium constant (Ka) for loss of the first proton is (pKa = 1.27). The loss of the second proton, which yields the oxalate ion, has an equilibrium constant of (pKa = 4.28). These values imply, in solutions with neutral pH, no oxalic acid and only trace amounts of hydrogen oxalate exist. The literature is often unclear on the distinction between H2C2O4, , and , and the collection of species is referred to as oxalic acid. Structure The oxalate anion exists in a nonplanar conformation where the O–C–C–O dihedrals approach 90° with approximate D2d symmetry. When chelated to cations, oxalate adopts the planar, D2h conformation. However, in the structure of Cs2C2O4 the O–C–C–O dihedral angle is 81(1)°. Therefore, Cs2C2O4 is more closely approximated by a D2d symmetry structure because the two CO2 planes are staggered. Two structural forms of Rb2C2O4 have been identified by single-crystal X-ray diffraction: one contains a planar and the other a staggered oxalate. The barrier to rotation about this bond is calculated to be roughly 2–6 kcal/mol for the free dianion, . Such results are consistent with the interpretation that the central carbon–carbon bond is regarded as a single bond with minimal π interactions between the two units. This barrier to rotation about the C−C bond (which formally corresponds to the difference in energy between the planar and staggered forms) may be attributed to electrostatic interactions as unfavorable O−O repulsion is maximized in the planar form. Occurrence in nature Oxalate occurs in many plants, where it is synthesized by the incomplete oxidation of saccharides. Several plant foods such as the root and/or leaves of spinach, rhubarb, and buckwheat are high in oxalic acid and can contribute to the formation of kidney stones in some individuals. Other oxalate-rich