Oxalate ion
Oxalate (IUPAC: ethanedioate) is the dianion with the formula C
2O2−
4, also written (COO)2−
2. Either name is often used for derivatives, such as salts of oxalic acid, for example sodium oxalate Na2C2O4, or dimethyl oxalate ((CH3)2C2O4). Oxalate also forms coordination compounds where it is sometimes abbreviated as ox.
Many metal ions form insoluble precipitates with oxalate, a prominent example being calcium oxalate, the primary constituent of the most common kind of kidney stones.
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 HC
2O−
4. 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 5.37 × 10−2 (pKa = 1.27). The loss of the second proton, which yields the oxalate ion has an equilibrium constant of 5.25 × 10−5 (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, HC
2O−
4, and C
2O2−
4, and the collection of species is referred to as oxalic acid.
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